Refraction Of Light Through A Prism

Refraction Of Light Through A Prism

You have learnt how the light gets refracted through a rectangular glass slab. For parallel refracting surfaces, as in a glass slab, the emergent ray is parallel to the incident ray. However, it is slightly displaced laterally. How would light get refracted through a transparent prism? Consider a triangular glass prism. It has two triangular bases and three rectangular lateral surfaces. These surfaces are inclined to each other. The angle between its two lateral faces is called the angle of the prism. Let us now do an activity to study the refraction of light through a triangular glass prism.

Activity 11.1

Fix a sheet of white paper on a drawing board using drawing pins.

Place a glass prism on it in such a way that it rests on its triangular base. Trace the outline of the prism using a pencil.

Draw a straight line PE inclined to one of the refracting surfaces, say AB, of the prism.

Fix two pins, say at points P and Q, on the line PE as shown in  Fig. 11.4.

Look for the images of the pins, fixed at P and Q, through the other face AC.

Fix two more pins, at points R and S, such that the pins at R and S and the images of the pins at P and Q lie on the same straight line.

Remove the pins and the glass prism.

The line PE meets the boundary of the prism at point E  (see Fig. 11.4).

Figure 11.4 Refraction of light through a triangular glass prism

PE – Incident ray                   Ði – Angle of incidence

EF – Refracted ray                 Ðr – Angle of refraction

FS – Emergent ray                 Ðe – Angle of emergence

ÐA – Angle of the prism       ÐD – Angle of deviation

Similarly, join and produce the points R and S. Let these lines meet the boundary of the prism at E and F, respectively. Join E and F.

Draw perpendiculars to the refracting surfaces AB and AC of the prism at points E and F, respectively.

Mark the angle of incidence (Ði), the angle of refraction (Ðr) and the angle of emergence (Ðe) as shown in Fig. 11.4.

Here PE is the incident ray, EF is the refracted ray and FS is the emergent ray. You may note that a ray of light is entering from air to glass at the first surface AB. The light ray on refraction has bent towards the normal. At the second surface AC, the light ray has entered from glass to air. Hence it has bent away from normal. Compare the angle of incidence and the angle of refraction at each refracting surface of the prism. Is this similar to the kind of bending that occurs in a glass slab? The peculiar shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation. In this case ÐD is the angle of deviation. Mark the angle of deviation in the above activity and measure it.