Blackbody Radiation and Planck’s Constant Edward Wei (Year 10, Peel)
By the late 19th century, it was the general consensus that nothing more could be discovered in physics. Physicists could calculate the motion of material objects using Newton’s laws of classical mechanics, and describe the properties of radiant energy using Maxwell’s mathematical relationships. The universe appeared to be a simple and orderly place, containing matter (which consisted of particles that had mass and whose location and motion could be accurately described) and light (which was viewed as having no mass and whose exact position in space cannot be fixed). Matter and energy were considered distinct and unrelated phenomena. However, there were several contradictions that continued to puzzle classical physicists. The one we will be looking at is known as the “ultraviolet catastrophe”.
Figure 1: A diagram of an electromagnetic wave (Source: Clinuvel)
Light or electromagnetic radiation is a form of energy which travels in a wave, as shown in Figure 1 (Note: this is a simple depiction, but for our purposes it is enough). The crest is the tip of a wave; the trough is the bottom. The wavelength is the distance between the crests of 2 waves (measured in meters), and the amplitude is the distance between the crest and the trough divided by 2 (measured in meters). The wave period or frequency is the time taken to complete one cycle - as shown in Figure 1. It is measured in 1/time in seconds or Hertz. The frequency and wavelength of a wave are inversely proportional to each other.
Figure 2: Diagram of the electromagnetic spectrum. (Source: Mini Physics)
The electromagnetic spectrum categorizes light into several categories as its frequency/wavelength varies. Higher frequencies or shorter wavelengths equate to higher energy and vice versa. The relevant part is the visible section, which is the only part of the entire spectrum that we can see. Our eyes are not sensitive enough to detect infrared whilst the lenses of our eyes block out ultraviolet (it is harmful to cells). We are able to see objects because they either emit light themselves, or they absorb light and reflect it in all directions, and some of the light reflected just so happens to enter our eyes.
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