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This is the Physics notes from September 2000. It is a free notes for revision.
Physics Factsheet
September 2000
Number 01
The Quantum Nature of Light
This Factsheet covers one of the fundamental - and hard to grasp - concepts of modern physics; wave-particle duality. To ensure this important topic is clear, the Factsheet is longer than usual.
What is light?
Well, there's no simple answer, because it behaves differently in different situations. Usually it behaves as a wave-it can be diffracted, reflected and refracted -so we consider it to be a wave. However, sometimes this 'wave model' does not explain what we see-for example, when we shine light onto a metal surface, electrons can be released (the photoelectric effect). This requires a new explanation; that light comes in lumps (quanta), or particles called photons. So depending on what experiment we do, light behaves as either a 'wave' or a 'particle'. Neither explanation by itself can describe all that we observe.
'Wave-particle duality' means:
the ability of something to show both wave-like and particle-like properties, depending on how we look at it (i.e. what experiment is performed). It is just one aspect of quantum physics.
Light is part of the electromagnetic spectrum, which we commonly explain as consisting of waves with different frequencies (e.g. gamma rays, X-rays, radio waves, infra-red radiation and microwaves). We find that all parts of the electromagnetic spectrum show the same 'wave-particle duality' as light, so we commonly speak of 'electromagnetic radiation' (EM radiation) rather than 'light'.
What is the particle model of EM radiation?
By Plank's theory, EM radiation (e.g. light) is considered to be composed of a stream of small lumps or packets (quanta), which are called photons. Each photon travels at the speed of light and has no mass and no charge. Each photon has a fixed amount of energy (E) which depends on the frequency (f) of the EM radiation: the higher the frequency, the higher the energy of each photon:
E: Photon energy ( $J$ )
$\mathbf{E}=\mathbf{h} \boldsymbol{f} \quad h$ : Planck's constant ( $6.63 \times 10^{-34} \mathrm{Js}$ )
f: frequence of EM radiation (Hz)
The more photons (i.e. packets of energy) there are, then the more energy there is arriving at a point each second. Thus, the power (defined as energy per second) of the EM radiation depends on the number of photons arriving per second. If the power is measured over an area (e.g. $1 \mathrm{~m}^{2}$ ), then we call it intensity ( $\mathrm{Wm}^{-2}$ ), which in the case of light is …