THE HUMAN EYE AND THE COLOURFUL WORLD
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).
