A level revision

Quantum behaviour

There is evidence that light acts like a wave as well as evidence light acts like a particle, this is known as the 'wave-particle duality' of light. Light (photons) together with other particles such as electrons share what is known as 'quantum behaviour'. Quantum behaviour is often hard to get your head around as it typically challenges what you've been already been taught.

How photons move

If you shine a light onto a wall you'll see a small circle of light appear. One might assume that the photons from the torch have simply travelled in a straight line to the wall. In fact photons explore all paths:

Photons do not obey many rules, none of the above paths are forbidden and there is a chance that a photon will use either of the paths or any one of a limitless number of others.

Phasors

Photons have phasors and each photon may have a different amplitude. As a result the effect each path has on the net effect is what needs to be considered. This best demonstrated by explaining the idea of a mirror using photons which explore all paths:

Photons from the light source can make their way to the detect by a number of paths. As some paths are longer than others the photons arrive in different phases, all the phasors can be added together like vectors to get the resultant phasor which is related to the amplitude of the final wave.

If you were to do this you'd find that the longer paths cancel each other out as they are out of phase with one another. The majority of the resultant phasor comes from the short and central paths which is inline with what we see - reflected light travels in a narrow straight line.

Probability and intensity

The probability that a photon will arrive at a point is obtained by calculating the resultant phasor amplitude - adding the phasor amplitudes for all possible paths.

A high probability means a high intensity - more photons will arrive at that point

Probability is proportional to the square root of the resultant phasor amplitude.