Light, Sound and Waves

Shallow pools

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

Refraction can be deceiving: This activity poses a problem that can be explained in terms of refraction.

Teacher: Why does the swimming pool look shallower than it really is?

What to Prepare

  • A means of sharing a large view of a computer screen
  • the interactive presentation (see below)

What Happens During this Activity

This activity is an interactive teacher demonstration in which you introduce the problem of the swimming pool and talk through the explanation with the class.

Have you noticed that whenever you stand on the side of a swimming pool the water looks to be shallower than it really is?

When you dive in, you expect to be able to touch the bottom straight away, but it takes two or three strokes to get there.

Suppose you are standing on the side of the pool and you notice a red disc at the bottom. Once again it seems to be closer than it really is.

How can we explain this?

You need to imagine that you are leaning out over the pool looking down on the red disc.

A possible commentary to go with the presentation:

  • First of all, if you are able to see the disc, then light must travel from the disc up through the water to your eye. Here we have drawn just two rays drawn up from the disc.
  • Rays from the block meet the surface of the water. When the rays meet the surface, they do not carry straight on, but are refracted. Since the rays are passing from water to air (from a low speed medium to a high speed medium) they are refracted away from the normal.
  • Rays appear to come from higher up in the water.
  • The refracted rays leave the water. You are able to see the disc as rays from every point on its surface (not just the ones shown here) predict that beams will travel to your eye from all parts of the disc.
  • Where do the rays predict that the light will appear to be coming from? The rays do not seem to be coming from the disc at the bottom of the pool but from a point somewhere above the bottom.
  • This point can be found by extending the refracted rays back into the water. The depth at which the disc appears to be is referred to as the apparent depth of the pool.


Download the software for this activity.

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

Support our manifesto for change

The IOP wants to support young people to fulfil their potential by doing physics. Please sign the manifesto today so that we can show our politicians there is widespread support for improving equity and inclusion across the education sector.

Sign today