Refraction

Figure 137.

Light can reflect also from irregular surfaces; however, the light will strike and bounce off at different angles, making the light appear more diffused. Diffused light is what allows a person to see what’s written on a piece of paper from many different angles. Diffusion happens with many things, with the exception of mirrors. According to the law of reflection, the angle of reflection equals the angle of incidence, as is seen in figure 137.

REFRACTION

Refraction can be seen when looking into a fish tank. There is distortion of the image because light will change directions or bend when it passes through the water. This bending of water is referred to as refraction. Refraction accounts for many phenomena seen when looking at light images and is the process light goes through in a lens, in water, and in optical fibers or fiber optics.

The speed of light, defined by the letter c, is a key concept in Einstein’s theory of relativity. It turns out that light will change speed when traveling from one medium to another. The traditional speed of light is that which it travels in a vacuum. It does not travel at the same speed through water, glass, and other mediums. This leads to the index of refraction, called by the small letter n, which is specific to the material and is defined as the speed of light divided by the velocity of light in the material. Because of this ratio and the fact that light is slower in all media, the value of n will always be greater than one.

Some light will be reflected as it reaches a different medium but, unless the medium is opaque, light will get through and will be refracted according to the law of refraction. The larger the index of refraction, the slower light will travel through it. The angle of the bending of light will depend on its index of refraction.

The law of refraction called “Snell’s law”, stated in equation form, is that the index of refraction for one medium multiplied by the sine of theta one is equal to the index of refraction of the second medium multiplied by the sine of theta two. This is seen in figure 138:

Figure 138.

The incoming ray is referred to as the incident ray and the outgoing ray is called the refracted ray, with the two angles called the incident angle and the refracted angle. There is a list of the index of refraction for various substances that you can look up for these types of equations.

A good quality mirror may reflect more than 90 percent of the light that originally falls on it but it would absorb the rest. Total reflection can be achieved using certain aspects of refraction. If the index of refraction for medium two is less than the index of refraction for medium one, the ray will bend away from the perpendicular. The largest angle of refraction for angle two is 90 degrees. The critical angle theta c for a combination of material is the angle at which the angle of refraction will be 90 degrees. If angle one is greater than this critical angle, all of the light is reflected back into the first medium. This is called total internal reflection. This critical angle is seen in figure 139:

Figure 139.

There can only be total internal reflection when the index of refraction in the second medium is less than the index of refraction than the first.

Fiber optics makes use of total internal reflection and is widely used in the transmission of cable TV signals, the internet, and telephone signals. This involves the transmission of light down fibers made from plastic or glass. The fibers are thin so that light strikes the inside surface at an angle greater than the critical angle, allowing the light rays to be totally reflected. Fibers are coated with a substance having an appropriate refractive index. This allows the transmission and “bending” of light across large distances and over angulated fibers. Cladding or “coating” of the fibers prevents light from being transmitted between fibers in a bundle.

A light ray that strikes an object that has two mutually perpendicular reflecting surfaces, will come back exactly parallel in the direction from which it came. This is independent of the angle of incidence. This type of object is referred to as a corner reflector because light bounces from its inside corner. Simple safety reflectors operate as corner reflectors. These are seen in periscopes and in binoculars as well. Two perpendicular mirrors will be typical corner reflectors.

Diamonds sparkle because of total internal reflection. The critical angle for diamonds in air is only 24.4 degrees, making it difficult for light to exit the diamond. The facets on diamonds are particularly good at enhancing this property. Light will exit only at specific places, leading to the sparkle seen in diamonds. Zircon is a gemstone that has a