Light by Iwona Jablonowska

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about light

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a ray of light?” he asked. “What if one were riding on the beam? … If one were to run fast enough, would it no longer move at all?” Einstein, though, is getting ahead of the story. To appreciate how light works, we have to put it in its proper historical context. Our first stop is the ancient world, where some of the earliest scientists and philosophers pondered the true nature of this mysterious substance that stimulates sight and makes things visible. Greeks argued over whether light rays emanated from a person’s eye or the object being viewed. Visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has a wavelength in the range of about 380 nanometres to about 740 nm – between the invisible infrared, with longer wavelengths and the invisible ultraviolet, with shorter wavelengths.

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Light is at once both obvious and mysterious. We are bathed in yellow warmth every day and stave off the darkness with incandescent and fluorescent bulbs. But what exactly is light? We catch glimpses of its nature when a sunbeam angles through a dust-filled room, when a rainbow appears after a storm or when a drinking straw in a glass of water looks disjointed. These glimpses, however, only lead to more questions. Does light travel as a wave, a ray or a stream of particles? Is it a single color or many colors mixed together? Does it have a frequency like sound? And what are some of the common properties of light, such as absorption, reflection, refraction and diffraction? You might think scientists know all the answers, but light continues to surprise them.

Here’s an example: We’ve always taken for granted that light travels faster than anything in the universe. Then, in 1999, researchers at Harvard University were able to slow a beam of light down to 38 miles an hour (61 kilometers per hour) by passing it through a state of matter known as a Bose-Einstein condensate. That’s almost 18 million times slower than normal! No one would have thought such a feat possible just a few years ago, yet this is the capricious way of light. Just when you think you have it figured out, it defies your efforts and seems to change its nature. Still, we’ve come a long way in our understanding. Some of the brightest minds in the history of science have focused their powerful intellects on the subject. Albert Einstein tried to imagine what it would be like to ride on a beam of light. “What if one were to run after

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Introduction

What Is Light? Over the centuries, our view of light has changed dramatically. The first real theories about light came from the ancient Greeks. Many of these theories sought to describe light as a ray -- a straight line moving from one point to another. Pythagoras, best known for the theorem of the right-angled triangle, proposed that vision resulted from light rays emerging from a person’s eye and striking an object. Epicurus argued the opposite: Objects produce light rays, which then travel to the eye. Other Greek philosophers -- most notably Euclid and Ptolemy -- used ray diagrams quite successfully to show how light bounces off a smooth surface or bends as it passes from one transparent medium to another. Arab scholars took these ideas and honed them

even further, developing what is now known as geometrical optics -- applying geometrical methods to the optics of lenses, mirrors and prisms. The most famous practitioner of geometrical optics was Ibn al-Haytham, who lived in present-day Iraq between A.D. 965 and 1039. Ibn al-Haytham identified the optical components of the human eye and correctly described vision as a process involving light rays bouncing from an object to a person’s eye. The Arab scientist also invented the pinhole camera, discovered the laws of refraction and studied a number of light-based phenomena, such as rainbows and eclipses. By the 17th century, some prominent European scientists began to think differently about light. One key figure was the Dutch mathematician-astronomer Christiaan Huygens. In 1690, Huygens published his “Treatise on Light,” in

which he described the undulatory theory. In this theory, he speculated on the existence of some invisible medium -- an ether -- filling all empty space between objects. He further speculated that light forms when a luminous body causes a series of waves or vibrations in this ether. Those waves then advance forward until they encounter an object. If that object is an eye, the waves stimulate vision. This stood as one of the earliest, and most eloquent, wave theories of light. Not everyone embraced it. Isaac Newton was one of those people. In 1704, Newton proposed a different take -- one describing light as corpuscles, or particles. After all, light travels in straight lines and bounces off a mirror much like a ball bouncing off a wall. No one had actually seen particles of light, but even now, it’s easy to explain why that might be. The particles

Imagining light as a ray makes it easy to describe, with great accuracy, three well-known phenomena: reflection, refraction and scattering. Let’s take a second to discuss each one. In reflection, at light rayse strikes a smooth surface, such as a mirror, and bounces off. A reflected rays always comes off the surface of a material at an angle equal to the angle at which the incoming ray hit the surface. In physics, you’ll hear this called the law of reflection. You’ve probably heard this law stated

light ray bends, either toward or away from what we call the normal line, an imaginary straight line that runs perpendicular to the surface of the object. The amount of bending, or angle of refraction, of the light wave depends on how much the material slows down the light. Diamonds wouldn’t be so glittery if they didn’t slow down incoming light much more than, say, water does. Diamonds have a higher index of refraction than water, which is to say that those sparkly, costly light traps slow down light to a greater degree. Lenses, like those in a telescope or in a pair of glasses, take advantage of refraction. A lens is a piece of glass or other transparent substance with curved sides for concentrating or dispersing light rays. Lenses serve to refract light at each boundary. As a ray of light enters the transparent material, it is refracted. As the

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Light as Rays

as “the angle of incidence equals the angle of reflection.” Of course, we live in an imperfect world and not all surfaces are smooth. When light strikes a rough surface, incoming light rays reflect at all sorts of angles because the surface is uneven. This scattering occurs in many of the objects we encounter every day. The surface of paper is a good example. You can see just how rough it is if you peer at it under a microscope. When light hits paper, the waves are reflected in all directions. This is what makes paper so incredibly useful -- you can read the words on a printed page regardless of the angle at which your eyes view the surface. Refraction occurs when a ray of light passes from one transparent medium (air, let’s say) to a second transparent medium (water). When this happens, light changes speed and the

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could be too small, or moving too fast, to be seen, or perhaps our eyes see right through them. As it turns out, all of these theories are both right and wrong at once. And they’re all useful in describing certain behaviors of light.

same ray exits, it’s refracted again. The net effect of the refraction at these two boundaries is that the light ray has changed directions. We take advantage of this effect to correct a person’s vision or enhance it by making distant objects appear closer or small objects appear bigger. Unfortunately, a ray theory can’t explain all of the behaviors exhibited by light.

Light as Waves dawned, no real evidence had accumulated to prove the wave theory of light. That changed in 1801 when Thomas Young, an English physician and physicist, designed and ran one of the most famous experiments in the history of science. It’s known today as the double-slit ex-

in a straight line to the screen, where it would form two bright spots. This isn’t what Young observed. Instead, he saw a bar code pattern of alternating light and dark bands on the screen. To explain this unexpected pattern, he imagined light traveling through space like a water wave, with crests and troughs. Thinking this way, he concluded that light waves traveled

periment and requires simple equipment -- a light source, a thin card with two holes cut side by side and a screen. To run the experiment, Young allowed a beam of light to pass through a pinhole and strike the card. If light contained particles or simple straight-line rays, he reasoned, light not blocked by the opaque card would pass through the slits and travel

through each of the slits, creating two separate wave fronts. As these wave fronts arrived at the screen, they interfered with each other. Bright bands formed where two wave crests overlapped and added together. Dark bands formed where crests and troughs lined up and canceled each other out completely. Young’s work sparked a new way of thinking

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about light. Scientists began referring to light waves and reshaped their descriptions of reflection and refraction accordingly, noting that light waves still obey the laws of reflection and refraction. Incidentally, the bending of a light wave accounts for some of the visual phenomena we often encounter, such as mirages. A mirage is an optical illusion caused when light

at right angles to the direction of movement of the wave, and at right angles to each other. Because light has both electric and magnetic fields, it’s also referred to as electromagnetic radiation. Electromagnetic radiation doesn’t need a medium to travel through, and, when it’s traveling in a vacuum, moves at 186,000 miles per second (300,000 kilometers per

Light Frequencies

waves moving from the sky toward the ground are bent by the heated air. In the 1860s, Scottish physicist James Clerk Maxwell put the cherry on top of the lightwave model when he formulated the theory of electromagnetism. Maxwell described light as a very special kind of wave -- one composed of electric and magnetic fields. The fields vibrate

second). Scientists refer to this as the speed of light, one of the most important numbers in physics. Light waves come in a continuous variety of sizes, frequencies and energies, a continuum known as the electromagnetic spectrum.

According to that model, light waves come in many sizes. The size of a wave is measured as its wavelength, which is the distance between any two corresponding points on successive waves, usually peak to peak or trough to trough. The wavelengths of the light we can see range from 400 to 700 nanometers (or billionths of a meter). But the full range

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Once Maxwell introduced the concept of electromagnetic waves, everything clicked into place. Scientists now could develop a complete working model of light using terms and concepts, such as wavelength and frequency, based on the structure and function of waves.

of wavelengths included in the definition of electromagnetic radiation extends from 0.1 nanometers, as in gamma rays, to centimeters and meters, as in radio waves.

full range of frequencies extends beyond the visible portion, from less than 3 billion hertz, as in radio waves, to greater than 3 billion billion hertz (3 x 1019), as in gamma rays.

quencies and energies, shown in the accompanying figure, is known as the electromagnetic spectrum. Note that the figure isnâ&#x20AC;&#x2122;t drawn to scale and that visible light occupies only one-

Light waves also come in many frequencies. The frequency is the number of waves that pass a point in space during any time interval, usually one second. We measure it in units of

The amount of energy in a light wave is proportionally related to its frequency: High frequency light has high energy; low frequency light has low energy. So, gamma rays have the

thousandth of a percent of the spectrum. This might be the end of the discussion, except that Albert Einstein couldnâ&#x20AC;&#x2122;t let speeding light waves lie. His work in the early 20th century

cycles (waves) per second, or hertz. The frequency of visible light is referred to as color, and ranges from 430 trillion hertz, seen as red, to 750 trillion hertz, seen as violet. Again, the

most energy (part of what makes them so dangerous to humans), and radio waves have the least. Of visible light, violet has the most energy and red the least. The whole range of fre-

resurrected the old idea that light, just maybe, was a particle after all.

of light waves, was so elegant and predictive that many physicists in the 1890s thought that there was nothing more to say about light and how it worked. Then, on Dec. 14, 1900, Max

Albert Einstein advanced Planck’s theory in 1905 when he studied the photoelectric effect. First, he began by shining ultraviolet light on the surface of a metal. When he did this, he

Once freed, the electrons move along the metal or get ejected from the surface. The particle theory of light had returned -- with a vengeance. Next, Niels Bohr applied Planck’s

Planck came along and introduced a stunningly simple, yet strangely unsettling, concept: that light must carry energy in discrete quantities. Those quantities, he proposed,

was able to detect electrons being emitted from the surface. This was Einstein’s explanation: If the energy in light comes in bundles, then one can think of light as containing tiny

ideas to refine the model of an atom. Earlier scientists had demonstrated that atoms consist of positively charged nuclei surrounded by electrons orbiting like planets, but they

lumps, or photons. When these photons strike a metal surface, they act like billiard balls, transferring their energy to electrons, which become dislodged from their “parent” atoms.

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Maxwell’s theoretical treatment of electromagnetic radiation, including its description

must be units of the basic energy increment, hf, where h is a universal constant now known as Planck’s constant and f is the frequency of the radiation.

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Light as Particles

couldn’t explain why electrons didn’t simply spiral into the nucleus. In 1913, Bohr proposed that electrons exist in discrete orbits based on their energy. When an electron jumps from

Wave-Particle Duality At first, physicists were reluctant to accept the dual nature of light. After all, many of us

one orbit to a lower orbit, it gives off energy in the form of a photon. The quantum theory of light - the idea that light exists as tiny packets, or particles, called photons - slowly began

to describe light as a photon. Later that year, however, he added a twist to the story in a paper introducing special relativity. In this paper, Einstein treated light as a continuous field of

waves - an apparent contradiction to his description of light as a stream of particles. Yet that was part of his genius. He willingly accepted the strange nature of light and chose

to emerge. Our understanding of the physical world would no longer be the same. Today, physicists accept the dual nature definition.

humans like to have one right answer. But Einstein paved the way in 1905 by embracing wave-particle duality. We’ve already discussed the photoelectric effect, which led Einstein

whichever attribute best addressed the problem he was trying to solve.Today, physicists accept the dual nature of light. In this modern view, they define light as a collection of one or

double-slit experiment in this way: Light travels away from a source as an electromagnetic wave. When it encounters the slits, it passes through and divides into two wave fronts.

jects. If you look around you right now, there is probably a light source in the room producing photons, and objects in the room that reflect those photons. Your eyes absorb some of the

photons flowing through the room, and that’s how you see a source as an electromagnetic it. But wait. What makes a light source produce photons? We’ll get to that. Next. If you looking.

These wave fronts overlap and approach the screen. At the moment of impact, however, the entire wave field disappears and a photon appears. Quantum physicists often describe this

eyes are actually able to sense single photons, but generally what we see in our daily lives comes to us in the form of zillions of photons produced by light sources and reflected off ob-

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by saying the spread-out wave “collapses” into a small point. At the moment of impact, source similarly, photons make it possible for us to see the world around us. In total darkness, our

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more photons propagating through space as electromagnetic waves. This definition, which combines light’s wave and particle nature, makes it possible to rethink Thomas Young’s

Producing a Photon There are many different ways to produce photons, but all of them use the same mecha-

trons in some detail. For example, hydrogen atoms have one electron orbiting the nucleus. Helium atoms have two electrons orbiting the nucleus. Aluminum atoms have 13 electrons

simplified way to think about it is to imagine how satellites orbit the Earth. Thereâ&#x20AC;&#x2122;s a huge amount of theory around electron orbitals, but to understand light there is just one key fact

nism inside an atom to do it. This mechanism involves the energizing of electrons orbiting each atomâ&#x20AC;&#x2122;s nucleus. How Nuclear Radiation Works describes protons, neutrons and elec-

circling the nucleus. Each atom has a preferred number of electrons zipping around its nucleus. Examples of this kind of electric light Electrons circle the nucleus in fixed orbits - a

to understand: An electron has a natural orbit that it occupies, but if you energize an atom, you can move its electrons to higher orbitals. A photon is produced whenever an electron in a

lamps, neon signs and sodium-vapor lamps are common examples of this kind of electric lighting, which passes an electric current through a gas to make the gas emit light. The colors of

lots, you often see sodium vapor lights. You can tell a sodium vapor light because it’s really yellow when you look at it. A sodium vapor light energizes sodium atoms to generate

teristics. The photon has a frequency, or color, that exactly matches the distance the electron falls. You can see this phenomenon quite clearly in gas-discharge lamps. Fluorescent

gas-discharge lamps vary widely depending on the identity of the gas and the construction of the lamp. For example, along highways and in parking

photons. A sodium atom has 11 electrons, and because of the way they’re stacked in orbitals one of those electrons is most likely to accept and emit energy. The energy packets that this

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higher-than-normal orbit falls back to its normal orbit. During the fall from high energy to normal energy, the electron emits a photon - a packet of energy -- with very specific charac-

electron is most likely to emit fall right around a wavelength of 590 nanometers. This wavelength corresponds to yellow light. If you run sodium light through a prism, you donâ&#x20AC;&#x2122;t see a

Bioluminescence: How Organisms Light Things Up

and deep-sea fishes, the process is known as bioluminescence. At least two chemicals are required to make light. Chemists use the generic term luciferin to describe the one pro-

rainbow -- you see a pair of yellow linesion ito. What else do you think of when you think of bioluminescence? Our friend the firefly ocourse.

Another way to make photons, known as chemiluminescence, involves chemical reactions. When these reactions occur in living organisms such as bacteria, fireflies, squid

ducing the light. They use the term luciferase to describe the enzyme that drives, or catalyzes, the reaction. The basic reaction follows a straightforward sequence. First, the luciferase

the photoprotein, it oxidizes the luciferin, resulting in light and inactive oxyluciferin. In marine organisms, the blue light produced by bioluminescence is most helpful because

longer (yellow, red) or shorter (indigo, ultraviolet) wavelengths. One exception can be found in the Malacosteid family of fishes, also known as loosejaws. These animals can both produce

gion of the spectrum. Sometimes, the luciferin binds with a catalyzing protein and oxygen in a large structure known as a photoprotein. When an ion -- typically calcium -- is added to

the wavelength of the light, around 470 nanometers, transmits much farther in water. Also, most organisms don’t have pigments in their visual organs that enable them to see

red light and detect it when other organisms can’t a catalyzing protein and oxygen in a large structure known as a photoprotein. The reaction also produces light and inactive oxylucife.

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catalyzes the oxidation of luciferin. In other words, luciferin combines chemically with oxygen to produce oxyluciferin. The reaction also produces light, usually in the blue or green re-

Making Colors Visible light is light that the human eye can perceive. When you look at the sun’s visible

single color but instead many colors. When sunlight passes through a glass of water to land on a wall, we see a rainbow on the wall. This wouldn’t happen unless white light

into a rainbow spectrum. He then passed sunlight through a second glass prism and combined the two rainbows. The combination produced white light. His simple experiment

light, it appears to be colorless, which we call white. Although we can see this light, white isn’t considered part of the visible spectrum. That’s because white light isn’t the light of a

were a mixture of all of the colors of the visible spectrum. Isaac Newton was the first person to demonstrate this. Newton passed sunlight through a glass prism to separate the colors

proved conclusively that white light is a mixture of colors. You can do a similar experiment with three flashlights and three different colors of cel-

flashlights on and shine them against a wall so that the beams overlap, as shown in the figure. Where red and blue light overlap, you will see magenta. Where red and green light overlap,

green, cyan with red, and by mixing all of the colors together. By adding various combinations of these socalled additive colors -- red, green and blue

many layers or you’ll block the light from the flashlight). Cover another flashlight with blue cellophane and a third flashlight with green cellophane. Go into a darkened room, turn the

you will see yellow. Where green and blue light overlap, you will see cyan. You will notice that white light can be made by various combinations, such as yellow with blue, magenta with

light -- you can make all the colors of the visible spectrum. This is how computer monitors (RGB monitors) generate colors.

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lophane -- red, green and blue (commonly referred to as RGB). Cover one flashlight with one to two layers of red cellophane and fasten the cellophane with a rubber band (don’t use too

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Light sources Marck Been There are many sources of light. The most common light sources are thermal: a body at a given temperature emits a characteristic spectrum of black-body radiation. A simple thermal source is sunlight, the radiation emitted by the chromosphere of the Sun at around 6,000 Kelvin peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units and roughly 44% of sunlight energy that reaches the ground is visible. Another example is incandescent light bulbs, which emit only around 10% of their energy as visible light and the remainder as infrared. A common thermal light source in history is the glowing solid particles in flames, but these also emit most of their radiation in the infrared, and only a fraction in the visible spectrum. The peak of the blackbody spectrum is in the deep infrared, at about 10 micrometer wavelength, for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue-white colour as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colours can be seen when metal is heated to “red hot” or “white hot”. Blue-white thermal emission is not often seen, except in stars (the commonly seen pure-blue colour in a gas flame or a welder’s torch is in fact due to molecular emission, notably by CH radicals (emitting a wavelength band around 425 nm, and is not seen in stars or pure thermal radiation)). The peak of the blackbody spectrum is in the deep infrared altho.

Atoms emit and absorb light at characteristic energies. This produces “emission lines” in the spectrum of each atom. Emission can be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, etc.), and flames (light from the hot gas itself—so, for example, sodium in a gas flame emits characteristic yellow light). Emission can also be stimulated, as in a laser or a microwave maser. Deceleration of a free charged particle, such as an electron, can produce visible radiation: cyclotron radiation, synchrotron radiation, and bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation. Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats

Certain other mechanisms can produce light:

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• Bioluminescence • Cherenkov radiati • Electroluminescence • Scintillation • Sonoluminescence • Triboluminescence ‹‹

moving through water can disturb plankton which produce a glowing wake. Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence. Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example. This mechanism is used in cathode ray tube television sets and computer monitors. When the concept of light is intended to include very-high-energy photons, additional generation.

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Other properties of a star by observing itsspectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined. A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star’s interior carries energy away from the core through a combination of radiative and convective processes. The Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun expands to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves other properties of a star by observing itsspectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined. A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star’s interior carries energy away from the core through a combination of radiative and convective processes. The Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun expands to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves other properties of a star by observing itsspectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined. A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star’s interior carries energy away from the core through a combination of radiative and convective processes. The Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun expands to become a red giant, in some cases fusing. light 33

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The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined. A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star’s interior carries energy away from the core through a combination of radiative and convective processes. The Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun expands to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves other properties of a star by observing itsspectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined. A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star’s interior carries energy away from the core through a combination of radiative and convective processes. The Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun expands to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves other properties of a star by observing itsspectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined. A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star’s interior carries energy away from the core through a combination of radiative and convective processes. The Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun expands to become a red giant, in some cases fusing. heavier elements at the core or in shells around the core. The star

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temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined. A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star’s interior carries energy away from the core through a combination of radiative and convective processes. The Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun expands to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves other properties of a star by observing itsspectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined. A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star’s interior carries energy away from the core through a combination of radiative and convective processes.

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Sell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined. A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star’s interior carries energy away from the core through a combination of radiative and convective processes. The Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun expands to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves other properties of a star by observing itsspectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined. A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star’s interior carries energy away from the core through a combination of radiative and convective processes. The Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun expands to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves other properties of a star by observing itsspectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined. A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star’s interior carries energy away from the core through a combination of radiative and convective processes. The Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun expands to become a red giant, in some cases fusing. heavier elements at the core or in shells around the core. The star then evolves other properties of a star by observing itsspectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature plot of. light 41

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Aurora

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An aurora (plural: aurorae or auroras; from the Latin word aurora, “dawn”) is a natural light display in the sky particularly in the high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere (thermosphere). The charged particles originate in the magnetosphere and solar wind and, on Earth, are directed by the Earth’s magnetic fieldinto the atmosphere. Aurora is classified as diffuse or discrete aurora. Most aurorae occur in a band known as the auroral zone, which is typically 3° to 6° in latitudinal extent and at all

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local times or longitudes. The auroral zone is typically 10Â° to 20Â° from the magnetic pole defined by the axis of the Earthâ&#x20AC;&#x2122;s magnetic dipole. During a geomagnetic storm, the auroral zone will expand to lower latitudes. The diffuse aurora is a featureless glow in the sky which may not be visible to the naked eye even on a dark night and defines the extent of the auroral zone. The discrete aurorae are sharply defined features within the diffuse aurora which vary in brightness from just barely visible to the naked eye to bright enough to read a newspaper at night. Discrete aurorae are usually observed Pono, Norway, 1998, fot. by Elizabeth Grey

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the northern horizon as a greenish glow or sometimes a faint red, as if the Sun were rising from an unusual direction. Discrete aurorae often display magnetic field lines or curtain-like structures, and can change within seconds or glow unchanging for hours, most often in fluorescent green. The au-

australis (or the southern lights), has almost identical features to the aurora borealis and changes simultaneously with changes in the northern auroral zone and is visible from high southern latitudes in Antarctica, South America, New Zealand, and Australia. Aurorae occur on other planets. Similar to

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rora borealis most often occurs near the equinoctes. The northern lights have had a number of names throughout history. The Cree call this phenomenon the “Dance of the Spirits”. In Europe, in the Middle Ages, the auroras were commonly believed a sign from God. Its southern counterpart, the aurora

the Earth’s aurora, they are visible close to the planet’s magnetic poles. Modern style guides recommend that the names of meteorological phenomena, such as aurora borealis, be uncapitalized. The northern lights have had a number of names throughout history. The Cree call this phenomenon but it is.

Badioma, Alaska, 1987, fot. John Mood

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Forms and magnetism

direction of the magnetic field lines, suggesting that auroras are shaped by Earthâ&#x20AC;&#x2122;s magnetic field. Indeed, satellites show electrons to be guided by magnetic field lines, spiraling around them while moving toward Earth. The similarity to curtains is often enhanced by folds called â&#x20AC;&#x153;striationsâ&#x20AC;?. When

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Typically the aurora appears either as a diffuse glow or as “curtains” that approximately extend in the east-west direction. At some times, they form “quiet arcs”; at others (“active aurora”), they evolve and change constantly. Each curtain consists of many parallel rays, each lined up with the local

Anisamania, Finland, 2003, fot.Tori Mirniok

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the field line guiding a bright auroral patch leads to a point directly above the observer, the aurora may appear as a “corona” of diverging rays, an effect of perspective. Although it was first mentioned by Ancient Greek explorer/geographer Pytheas, Hiorter and Celsius first described in 1741 evidence for

magnetic control, namely, large magnetic fluctuations occurred whenever the aurora was observed overhead. This indicates (it was later realized) that large electric currents were associated with the aurora, flowing in the region where auroral light originated. Kristian Birkeland (1908) deduced that the

currents flowed in the east-west directions along the auroral arc, and such currents, flowing from the dayside toward (approximately) midnight were later named “auroral electrojets” (see also Birkeland currents).

prior to auroral intensification. Dr. Vassilis Angelopoulos of the University of California, Los Angeles, the principal investigator for the THEMIS mission, claimed, “Our data show clearly and for the first time that magnetic reconnection is the trigger. ”Still more evidence for a magnetic connection are the Helsinki, Finland, 2001, fot. Tori Mirniok

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In traditional and popular culture

On 26 February 2008, THEMIS probes were able to determine, for the first time, the triggering event for the onset of magnetospheric substorms. Two of the five probes, positioned approximately one third the distance to the moon, measured events suggesting a magnetic reconnection event 96 seconds

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statistics of auroral observations. Elias Loomis (1860) and later in more detail Hermann Fritz (1881) and S. Tromholt (1882) established that the aurora appeared mainly in the “auroral zone”, a ring-shaped region with a radius of approximately 2500 km around Earth’s magnetic pole. It was hardly ever

seen near the geographic pole, which is about 2000 km away from the magnetic pole. The instantaneous distribution of auroras (“auroral oval”) is slightly different, centered about 3–5 degrees nightward of the magnetic pole, so that auroral arcs reach furthest toward the equator about an hour before.

arcs”; at others (“active aurora”), they evolve and change constantly. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that auroras are shaped by Earth’s magnetic field. Indeed, satellites show electrons to be guided by magnetic field on. Fanima, Greenland, 2010, Catherine May

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Buble, fot. Moblish press. The hot filament is protected from oxidation glass.

Buble, fot. Moblish press

An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light which produces light with a filament wire heated to a high temperature by an electric current passing through it, until it glows (see Incandescence). The hot filament is protected from oxidation with a glass bulb that is filled with inert gas (or evacuated). In a halogen lamp, filament evaporation is prevented by a chemical process that redeposits metal vapor onto the filament, extending its life. The light bulb is supplied with electrical current by feed-through terminals or wires embedded in the glass. Most bulbs are used in a sock-

Buble, fot. Moblish press. The hot filament is protected from oxidation witha glass. Such applications include incubators, brooding boxes for poultrysini.

most incandescent bulbs convert less than 5% of the energy they use into visible light (with the remaining energy being converted into heat). Some applications of the incandescent bulb deliberately use the heat generated by the filament. Such applications include incubators, brooding boxes for poultry, heat lights for reptile tanks, infrared heating for industrial heating and drying processes, and the Easy-Bake Oven toy. But waste heat can also significantly increase the energy required by a buildingâ&#x20AC;&#x2122;s air conditioning system. Because of their inefficiency, incandescent light bulbs are gradually being replaced in many applications by

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et which provides mechanical support and electrical connections. Incandescent bulbs are manufactured in a wide range of sizes, light output, and voltage ratings, from 1.5 volts to about 300 volts. They require no external regulating equipment, have low manufacturing costs, and work equally well on either alternating current or direct current. As a result, the incandescent lamp is widely used in household and commercial lighting, for portable lighting such as table lamps, car headlamps, and flashlights, and for decorative and advertising lighting. Incandescent bulbs are less efficient than several other modern types of light bulbs;

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In 1840, British Astronomer Chemist, Warren de la Rue, enclosed a platinum coil in a vacuum tube and passed an electric current through it, thus creating the world’s first

Buble, fot. Moblish press

other types of electric lights, such as fluorescent lamps, compact fluorescent lamps (CFL), cold cathode fluorescent lamps (CCFL), high-intensity discharge lamps, and light-emitting diodes (LEDs). Some jurisdictions, such as the European Union, are in the process of phasing out the use of incandescent light bulbs in favor of more energy-efficient lighting. History of the light bulb

light bulb – a full 40 years before Edison was issued a patent for creating it . In addressing the question of who invented the incandescent lamp, historians Robert Friedel and Paul Israel list 22 inventors of incandescent lamps prior to Joseph Swan and Thomas Edison. They conclude that Edison’s version was able to outstrip the others because of a combination of three factors: an effective incandescent material, a higher vacuum than others were able to achieve (by use of the Sprengel pump) and a high resistance that made power distribution from a cen-

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tralized source economically viable. Other historian, Thomas Hughes, has attributed Edisonâ&#x20AC;&#x2122;s success to his development of an entire, integrated system of electric lighting. The lamp was a small component in his system of electric lighting, and no more critical to its effective functioning than the Edison Jumbo generator, the Edison main and feeder, and the parallel-distribution system. Other inventors with generators and incandescent lamps, and with comparable ingenuity and excellence, have long been forgotten because their creators did not preside over their introduction in a

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Fluorescent, fot. Moblish

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; this two-photon absorption can lead to emission of radiation having a shorter wavelength than the absorbed radiation. Them emitted radiation may also be of the same wavelength

as the absorbed radiation, termed â&#x20AC;&#x153;resonance fluorescenceâ&#x20AC;?. The most striking examples of fluorescence occur when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye, and the emitted light is in the visible region. Fluorescience has many practical applications, including mineralogy, gemology, chemical sensors (fluorescence spectroscopy), fluorescent labelling, dyes, biological detectors, and, most commonly, fluorescient lamps. However, when the absorbed electromagnetic radiation is not.

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History An early observation of fluorescence was described in 1560 by Bernardino de Sahagún and in 1565 by Nicolás Monardes in the infusion known as lignum nephriticum (Latin for “kidney wood”). It was derived from the wood of two tree species, Pterocairptus indicsus and Eysenhardtia polystacha. Them chemical compound responsible for this fluorescence is matlaline, which is the oxidation product of one of the flavonoids found in this wood. In 1819 Edward D. Clarke and in 1822 René Juste Haüy described fluorescence in fluorites, Sir in David Brewster.

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A lamp is a replaceable component such as an incandescent light bulb. Which is designed to produce light from electricity. These components usually have a base of ceramic, metal, glass or plastic, which makes an electrical connection in the socket of a light fixture. This connection may be.

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Itum inequam. Habem Rompopu bliis, Ti. Rumus fac rebat culica; enimus actorid con vil vir habus estem cludeperet