The physics of light – phenomona part II

Part I


Remember the diagram of the wave hitting the wall? It caused interference where the two waves overlapped. Here is a picture of interference on the surface of a lake:

Interference on lake

Interference happens with all waves, even sound. Interference happens when different parts of wave coincide, like this:


The waveform at the top shows what the resulting wave is. The two waveforms underneath show what waves produced it. As you can see, when two waves coincide they reinforce each other — called ‘constructive interference’. When the don’t, they cancel each other out — called ‘destructive interference’.

Interference is very important because it helped us understand the nature of light in the famous double-slit experiment. If light shines through two slits and onto a screen, a pattern of lines can be seen. The dark patches are where the light has destructively interfered, and the light patches are where it has constructively interfered.

Double slit interference

Note – the light spreads out the way it does because of diffraction, which will be discussed later.

The trouble with the double slit

Interference is one of the most convincing proofs that light is a wave. However, there is a problem. A wave on the ocean can be made to pass through to gaps in a harbour wall and interfere with itself. No problem. But a wave on the ocean is not made up of a single particle.

The double slit experiment was carried out with, instead of a steady beam of light being shone at the slits, a single photon was used. A single particle can only pass through one slit at a time, right?

Turns out, even if you use a single photon, the interference pattern still emerges.

An interference pattern only emerges when light rays from two sources overlap. How can a single photon have two sources?

You already know that light behaves like a wave or a particle and which one it behaves like depends on how you measure it. If you don’t measure its position, there is an uncertainty as to where it is. Turns out the photon is everywhere within that uncertainty. In the case of the double slit experiment, you don’t know which slit it will go through, so it goes through both at the same time. Because it goes through both, it manages to interfere with itself.

This runs against everything classical physics says about light, but it happens, so it’s true.


Diffraction is another property of a wave that causes it to spread out when it meets and obstruction or passes through a gap. For example, if you speak into a cardboard tube, people to either side can hear you because the sound waves spread out when they pass through the end of the tube, rather than carry on in a narrow beam.

The same thing happens with light, but on a smaller scale. The diagram below shows diffraction.


As you can see, waves spread out when they reach a boundary (left picture). If there is a gap that is about equal in size to the wavelength of the wave, semi-circular wavefronts form.

Because the wavelength of visible light is so small, diffraction is minimal. The slit needs to be roughly the same size as the wavelegth of light to cause semi-circular wavefronts — the wavelength of visible light ranges from 400 (violet light) to 700 (red light) nanometres, or 400 to 700 billionths of a metre.


Remember the diagram of light (The physics of light – Introduction) which showed the magnetic and electric fields vibrating at right angles? These fields can vibrate at angle as long as the two are at right angles. We normally talk about the angle of vibration of the electric field, because we know the magnetic field will always be at right angles to this.

Ordinary light contains photons vibrating in all directions. A polariser only allows through light that is vibrating in a certain direction. This is the principle of LCDs (liquid crystal displays). Light enters the diplay and is polarised. It reflects off a mirror at the back and through another polariser. If the two polarisers are aligned the light goes back and and you see a whit patch. If they are not the light cannot get through and you see a black patch.


The more mass something has, the bigger the force gravity exerts on it. However, if it is more massive it needs a bigger force to accelerate it by the same amount. These two effects cancel each other out and mean that heavy objects fall just as fast as light ones. The mass makes no difference.

Photons have no mass, but they are still affected by gravity. Light falls just as fast as anything else, but is moving so fast that you don’t notice it. In fact, light is bent by stars and galaxies to produce gravitational lensing. A black hole is something not even light can escape from. See black holes for beginners.

Summary of light

  • Colour is dependant on wavelength.
  • There are wavelengths that we can’t see — some other animals can.
  • Frequency and enrgy are both related to wavelength.
  • Light can be both a wave and a particle, depending on how it is measured.
  • Light exhibits all the characteristics of waves.
  • Light is made up of little particles called photons.

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