Lecture #9: Properties of Light: wave and particle


I. Wave nature of light
--Light follows the motion of waves, and has a meaningful frequency, which is the frequency of the wiggling electron that emitted the light. Since many electrons are wiggling and at different frequencies, in general we see light at all frequencies (i.e., all colors), as in a rainbow.
--Waves do interesting things, like diffraction, reflection, and refraction. Refraction is what happens in a lens, that allows our eyes to separate the light coming from different directions.
--Diffraction may be used to separate light at different frequencies, rather than light coming from different directions. When we do the former, we form what is called a spectrum, when we do the latter, it's called an image.
--The connection between frequency and wavelength is that they are inversely proportional to each other. The reason for this is that the wave speed is the same for all the frequencies (or very nearly so).
II. Particle nature of light
--Einstein discovered via the photoelectric effect that light must come in tiny packets of energy called photons. The frequency that governs the wave behavior of the photon is simply proportional to the energy of that photon. The wave mechanics is what tells the photons where they will enjoy constructive interference and be very likely to go, and where they will suffer destructive interference and be very unlikely to go. This is how you get a diffraction pattern of bright and dark bands, for example.