# The Human Eye and the Colourful World | Activities in Text Book with Solution

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). 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 (i), the angle of refraction (r) and the angle of emergence (e) as shown in Fig. 11.4.

Refraction of light through a triangular glass prism

PE – Incident ray                     i – angle of incidence

EF – Refracted ray                   r – Angle of refraction

FS – Emergent ray                   e – Angle of emergence

A – Angle of the prism          D – Angle of deviation

✅Answer: This activity should be done by students.

A light ray is entering from air to glass at the first surface AB. The light ray, on refraction, bends towards the normal. At the second surface AC, the light ray enters from glass to air. Hence it bends away from normal. Compare angle of incidence and angle of refraction at each refracting surface of the prism. 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 (D).

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