Combined Science
Physics
Cambridge IGCSE
Physics
Cambridge IGCSE
TOPIC 3: WAVES
Refraction is defined as the bending of the direction of travel of a wave as it moves from one substance, or 'medium', into another. Many of you will be familiar with the refraction of light, but refraction works with all types of waves.
In this section, we will be learning about the refraction of light. To understand how refraction works, we will use the example of a rectangular glass block. Have a look at figure 1 to see how the path of the light ray is affected by the glass:
Figure 1. The path of a light ray through a glass block
As you can see from the diagram. a ray that hits the side of the glass at 90° - perpendicular to the surface - is not refracted. However at angles of incidence between 0 and 90°, the light ray bends towards the normal line as it enters the glass, and away from the normal line as it leaves. This happens with water, clear plastic, or any other transparent medium denser than air. Note that the ray does not travel along the normal line, just closer to it inside the glass.
Why does the light ray change direction like this? The answer lies in the fact that glass is denser than air, and that this typically makes light travel more slowly. The change in speed causes a change in direction.
The best way to try to understand why refraction happens is to draw a wavefront diagram. (See section 3.1 for more on wavefronts). In figure 2, we look at a horizontal block of glass, and compare it to a line of people walking holding hands.
Figure 2. Refraction of wavefronts
As you can see from the diagram, the wavefronts change direction as they slow down, and also become closer together. (This means the wavelength is shorter). The diagram on the right is a good way of trying to picture why this happens - if a line of people walk into mud as shown, the first to enter the mud slow down, causing the line to bend, just a shown in the first diagram.
Remember that wavefronts are always drawn perpendicular to the ray which shows the direction the wave is travelling.
A standard refraction experiment that you need to learn involves semi-circular 'D-shaped' blocks. These blocks work well in this context because - as shown in figure 3 - the light ray is not refracted as it leaves the glass, making it easy to measure angles and draw the emerging ray. Note that, as for reflection, the angles of incidence (i) and refraction (r) are measured from the normal line as shown here:
Figure 3. Refraction through a semi-circular block
You will be expected to be able to add a normal line to any refraction diagram, as well as measure the angles of incidence (i) and refraction (r) ftom the diagram. You will need a protractor to do this.
A triangular glass or plastic block is called a prism. In the arrangement shown in figure 4, the white light is refracted 'downwards' at both surfaces. This magnifies the effect called dispersion - the fact that different colours actually refract by slightly different angles. The refraction of light depends on the frequency of the light. As white light is made of many frequencies of light, the prism splits the light. We see the familiar traditional 7 colours of the spectrum: Red, orange, yellow, green, blue, indigo and violet. Note that the violet coloured light is refracted more than the red colour.
Figure 4. Dispersion in a prism
Questions:
1. A fish is swimming in a shallow lake. The sun shines on the fish, and then light reflected from the fish moves upwards. One ray is shown in the following diagram:
a) The normal line should be perpendicular to the water surface. It is usually drawn as a dotted line, but as long as it is labelled N, that is fine.
b) The refracted line labelled R bends away from the normal line, so it is shifted towards the horizontal.
c) The reflected line labelled L should be (approximately) the same angle to the normal line as the incident ray already given.
Example solution:
