The phyiscs of light – phenomena part I
Well, it seems some images in this post have stopped working. However, I’m too lazy to sort them out, so live with it.
Reflection
Imagine a wave heading towards the shore at an angle. At the shore there is a sea wall. When the wave hits the sea wall, what angle does it bounce off at?
(Click to enlarge)
As you can see, the angle the wave bounces back from a perfectly smooth wall is the same as the angle it hits it at i.e. Angle 1 = Angle 2. Note the funny shapes where the two waves overlap. This is called interference and will be discussed later.
It is exactly the same with a ray of light. This diagram will introduce some terminology.
In this case also, Angle of incidence = Angle of reflection.
But a mirror is a smooth surface. What happens with a matte one? A matte surface still reflects light that falls on it, but on a microscopic level is bumpy (even if it looks smooth, it is still bumpy on a tiny scale).
This is why it is still impossible to see a proper reflection in a matte surface — the image is broken up when the light is reflected.
Objects have a colour because only particular wavelengths of light are reflected. The diagrams below should explain.
Because only red light reflects off a red object, we see it as red. In fact, the colour of an object is defined by what wavelengths of light reflect off it.
But why is light reflected?
This is complicated. When a ray of light strikes a surface, the photons are absorbed by the atoms of the material. They are then re-emitted at a different angle. Why is this angle the same angle they came in at? The answer to that is advanced and complicated and involves quantum electrodynamics.
Refraction
When light travels from one medium (which just means a material it travels through) to another it can refract. This happens because the ligth slows down when it passes into a more ‘optially dense‘ medium.
If the light hits a more optically dense medium at an angle, on side slows down before the other. This causes the light ray to bend towards the side that slows down first, as shown in the diagram.
The diagram shows a rod in a bowl of water. Red arrows represent the path of the light, X represents the end of rod, and Y represents where you think the end of the rod is.
This is why things look bent under water. It is also why the bottom of a swimming pool looks closer than it actually is.
Why does light slow down?
When light enters a medium, it is constantly being absorbed by electrons that hang around atoms. The electrons gain a little energy and ‘jump up’ to a higher energy state. But they don’t want to be there. They ‘drop down’ back to a lower energy state, but the energy lost when they do this has to go somewhere, so they emit a photon that is identical to the one it just absorbed. This process takes time.
If light is travelling through a medium where there are lots of absorptions and emissions, it will take longer to get through the material than if it didn’t get absorbed much. This is why light travels slower through glass than water, and slower through water than through air. Note that each photon is always travelling at he speed of light, c. They only apparently slow down.
Also, the photons are sometimes not emitted in the direction they came. This leads to scattering, which is a whole new topic. It is why a sheet of galss looks green when viewed end-on, and why the sky and the ocean look blue.
How much is light refracted?
The angle that light is refracted at depends on the refractive index of two materials. The higher the refractive index, the more light is refracted. The refractive index of a pair of materials is given by:
Rearranging this gives:
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