Having covered a couple of real-world observations of the behaviour of light it seems appropriate to look at some of the optics theory that explains how lenses are able to manipulate light to form images.

Our starting point is the simple observation that light “bends” when it travels across the boundary between two different transparent materials. This is the reason why a straight bar or piece of wood looks bent when it is seen half-immersed in water: it is not the object that is bent but rather the light that is reflected from the parts of the object that are underwater.

Various models, one of which is illustrated in the above diagram, are used to explain why light “bends” in this way.

We can imagine that a beam of light has a certain width and when it strikes a surface at an oblique angle one side of the beam will hit the surface before the other. Now let’s assume that the light beam travels slower in the material on the far side of the boundary. This means that in the time it takes for the rest of the light beam to reach the surface (the distance C in the diagram above) the first side of the light beam will have travelled a shorter distance (shown as D in the diagram) in the slower material.

This in turn means that a line drawn across the light beam, which was vertical before the beam hit the surface, has now become rotated – and that in turn is simply another way of saying that the light beam itself is now moving in a different direction. In everyday language we say that the light has been “bent”.

There are several flaws in this model. One is the fact that light beams are simply a human invention: light itself does not have any physical width! Another is the implication that the lower side of the light beam (A) actually bends some distance inside the slower material whereas the upper side of the beam (B) bends at the surface itself.

It is tempting to find other ways of drawing this model (as can be done) but the bottom line is that this model isn’t correct no matter how it is refined. This is because it is based entirely on a wave model for light when in fact we know that light can behave as if it is either a wave or a particle. Nevertheless, the model shown above is enough to provide a simple explanation and can be adopted here on the grounds that it is sufficient for our purposes.

All we need to do is start using the word refraction, rather than “bending of light” and we are then making progress towards creating a useful model for the way in which lenses are able to form images of the objects that are placed in front of them.

In the next instalment we’ll look at the effect that is produced when the surface is curved rather than flat.

  • Jon Tarrant

    You’re right: that was a sweeping assumption to state so boldly! In fact, the speed of light in free space is a constant but light slows down when it moves through a different medium. There are even materials in which the speed of light is different in different directions within the same material.

  • Graham Duro

    “Now let’s assume that the light beam travels slower in the material on the far side of the boundary”

    From the little I remember of my Physics lessons I thought that the speed of light was a universal constant, and therefore could not be speeded up or slowed down.

    Illumination please!