Atoms of Light Acting as Particles

The energy of light comes in quantum units of value hf. In this chapter we go a step further, and look at evidence that it comes as a bundle, concentrated at a point, and behaves just like a particle. We will describe experiments that show that the photon has not only energy but also momentum.

First, we will show that the photon can behave like a bullet causing considerable disruption to surfaces of materials, particularly metals, whose atoms have 'loose' outer orbit electrons which are vulnerable to such bombardment. In the photoelectric effect an electron is knocked out from the surface of a metal by a photon. Because this happens instantaneously, and from the energy of the ejected electron, we can deduce that the collision is a 'one-on-one' encounter. There are many practical applications; we can use the photoelectric effect, whenever we want to translate a light signal into an electric current.

Next, we will look at an experiment which demonstrates that the photon has another characteristic of a particle, namely momentum. In the Compton effect the photon bounces off an electron, and the two then go off at different angles. We can apply the laws of conservation of energy and momentum to calculate angles and energies just as we do for a collision on a snooker table. In this experiment we look at the photon after it has been scattered in the collision.

There is one interesting difference between a scattered photon and a scattered snooker ball. The photon cannot slow down;

and always travels at the speed of light. It loses energy and momentum by decreasing its frequency and correspondingly increasing its wavelength, without changing its speed. The exact way in which this should happen is calculated in Appendix 13.1, while experimental evidence that it does happen is presented below.

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