Light, Sound and Waves

Refraction - a simple pattern

Physics Narrative for 11-14 Supporting Physics Teaching

The consequences of light travelling quickly in some things and slowly in others

When the beam of light changes from a fast medium (say air) to a slow medium (say glass), it changes direction so that it bends towards the normal.

The incident beam does not travel straight on, but is refracted towards the normal line. In drawing rays to predict beam behaviour, we need to draw the ray in the same way.

When the beam of light changes from a slow medium (say water) to a fast medium (say air), it changes direction such that it bends away from the normal.

The incident beam does not travel straight on, but is refracted away from the normal line. Again we need to draw the rays to show this.

Parallel rays

The overall effect is that the light beam leaving the glass block (the emergent beam) is parallel to the beam entering but shifted slightly to one side.

Why does light bend?

Why is light refracted as it passes between different media? To offer a satisfactory answer to this question we need to think about the wave properties of light. The explanation for refraction is not needed for the 11–14 age range. It turns out that the properties of light lead to the path taken by the beam being the shortest travel time possible. (There is more on this in the SPT: Radiations and radiating topic.)

Like all good answers, that immediately leads to another question. How does the beam know which the shortest path is? That is a good question! Quantum physics can answer that one, but not right here.

is formalised by Law of Reflection
can be exhibited by Progressive Wave
has the special case Total Internal Reflection
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