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.


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?

Reflection on water

(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.

Reflection on a mirror

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).

Diffuse reflection

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.

Coloured reflection

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.


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.

Absorption emission

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:

Refraction equation 1

Rearranging this gives:

Refraction equation 2

Part II


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  1. * tyler lelea says:

    Thanks for the info.

    If I use transparency film of the various colors of the light spectrum to cover a small solar panel for a science experiment and the solar panel is connected to a motor that spins a disk and I count the # of RPM’s of my disk to test the effects of the filter on the solar panel, then exactly what are the filters doing? For instance, if I use the red filter is it filtering out all the other colors and only allowing red to reach the solar panel? I’m not exactly sure what I’m proving and I need to know for my project.


    Tyler 7th grade

    | Reply Posted 7 years, 10 months ago
  2. * steve says:

    Hi Tyler.
    You’re right, the red filter only lets through the red light, and blocks all the other colours. For instance, if you hold it up to your eye, the whole world looks red because only the red light is getting through to your eye.
    Good luck with your project,

    | Reply Posted 7 years, 10 months ago
    • * Andrew says:

      Thanks for giving him the tip. He’s my little bro. He’s in 11th grade now. He won Grand Prize out of his whole middle school on his solar powered Ferris wheel project.

      | Reply Posted 4 years ago
  3. * Ian says:

    ithank you very intrtesting site. I am working with rainbows and sound vibrations or oscillations and an energy difference

    | Reply Posted 7 years, 3 months ago
  4. * Brice Kelsch says:


    Amateur photographer, professional dork here.

    A project I want to pull is similar to clandlin lines.

    What I hope to display is a 3-d model of sound.

    How I hope to get there is affixing (some form of rubberized material) a sound generator (speaker) to the bottom of a transparent container (likely glass); spraying some sort of either matte black or gloss black paint over it, tossing in some water, raking a continuous light over it to inform of shape (and using an effective camera angle, with continuous capture), and use (as close as I can) sine waves to show active/nodal areas. Ideal would be high speed capture to give a good description of it taking up shape.

    My concerns would be-

    refractive index of water is different than air, and the rebound from the bottom may be an issue- will that alter the output/information?

    In my experience, water tends to kill higher frequencies, lows travel better; is there a better medium, or a way to compensate?

    Would there be any appreciable alteration of light or sound from the mutual reaction/where could I read up on that?

    (I understand that what we call sound (in earth’s atmosphere) is a pattern of compression and rarefaction, and that those can disturb light’s path; therefore it might be an issue in water; though, water is a little tricky to compress)

    | Reply Posted 6 years, 6 months ago
  5. * M.Huth says:

    Thanks for one’s marvelous posting! I genuinely enjoyed reading it, you could be a great author.I will make sure to bookmark your blog and definitely will come back from now on. I want to encourage that you continue your great work, have a nice evening!

    | Reply Posted 4 years, 8 months ago
  6. Pretty! This was an extremely wonderful article. Thanks
    for providing this information.

    | Reply Posted 4 years, 6 months ago

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