11. The Human Eye and the Colourful World | Class 10 CBSE | Web Notes | Part 3: Refraction of Light through a Prism



A triangular glass prism 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.

Refraction of light through a triangular glass prism

Fix a white paper on a drawing board using drawing pins. Place a glass prism on it such that it rests on triangular base. Trace the outline of the prism.

Draw a straight-line PE inclined to one of the refracting surfaces (AB). Fix two pins, at points P & Q on the line PE.

Look for the images of the pins through the other face AC.

Fix two pins, at points R & S, such that these pins and the images of the pins at P & 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. 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), angle of refraction (r) and angle of emergence (e).


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

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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