Notes on Light and Photons

It is very difficult to communicate the authentic nature of a photon without incorporating it into a quantum (wave-like) picture of the atom. (Or more generally, without incorporating it into a quantum view of atoms, molecules and solids --things which contain electrons that interact with photons.)

On Monday we will begin our treatment of quantum physics. We will focus in particular on the simplest atom, the hydrogen atom, and on related --even simpler-- models of confined electrons, which are similar to atoms. Simple does not mean easy. It only means we will strip away any unnecessary complexity so that we can focus in on the essential nature of quantum physics.

Here is a summary of what we should understand so far regarding light:

1) Light is a wave which involves an oscillating electric field. The oscillation has a frequency f and a wavelength, lamda.

2) Frequency and wavelength are related through the wave-like relation f=c/lamda, where c is the wave speed (3 x 10^8 m/s).

3a) Visible light refers to the portion of the frequency range of oscillating electric field waves that can be seen by the eye. This corresponds to frequencies between about 4 x 10^14 Hz and 8 x 10^14 Hz. The lowest frequency part of the visible range is perceived as red light; the highest part is violet. The frequency order of colors is: red, orange, yellow, green, blue, violet.

3b) Frequencies below those of visible light are called infrared. The infrared range extends from about 10^11 Hz to 4 x 10^14 Hz. Below that is the microwave range. [Note that the infrared range covers more than 3 decades; the visible range covers only factor of 2, which less than 1 decade. (Decade means 10x.)

3c) Frequencies higher than visible are called ultra-violet(UV). Higher frequencies than UV are called xray. They are all waves involving oscillating electric fields. A UV photon has more energy a visible light or infrared photon.

3d) electromagnetic spectrum (partial) includes:
...; infrared-red; red-orange-yellow-green-blue-violet; ultraviolet;...

4) A photon is a quantum unit of light energy. The size of the basic unit of light energy depends on the frequency of the light. For light of frequency f, one quantum of light energy is, hf, where h=6.6 x 10^-34 J/Hz. (Note that Joules/Hz is the same as Joule-seconds.)

5) The basic quantum unit of energy cannot be broken. You cannot have amounts of energy in a light wave that involve a fraction of hf. For light at a frequency f, the energy of the oscillating electric field must always be an integer multiple of hf.

6) The quantization of light energy becomes increasingly important and relevant to physical phenomena at high frequencies. That is because the basic quantum unit of energy, hf, is big when f is big. (A UV photon has more energy than any visible or infrared photon.)