Page 166 - Science Class 08

water obliquely. The light rays reaching the
light at the opposite parallel faces which are
PQ (air-glass interface) and SR (glass-air observer from the pencil outside the water (i.e.
in the air) bend towards the normal while
interface) of the rectangular glass slab is
equal and opposite. entering the water. But, the light-rays from the
immersed part of the pencil bend away from
In the figure i is the angle of incidence, r is the normal while emerging from the water. So,
the angle of refrac on and e is the angle of
they appear to be coming from a different
emergence.
direc on to the observer. Hence, the rod
Angle of incidence and angle of emergence appears to be bent.
are equal as emergent ray and incident ray
REFRACTION THROUGH A PRISM
are parallel to each other.
What is a Prism?
When a light ray is incident normally to the
interface of two media then there is no A prism is a solid block of transparent material
bending of light ray and it goes straight with two parallel bases and three or more
through the medium. lateral surfaces that are parallelograms.
Given below are different types of prisms:
So, we can conclude the following facts from
this experiment:

v When light-rays travel from a rarer medium
to a denser medium, they deflect towards
the normal.
v When light-rays travel from a denser
medium to a rarer medium, they deflect
Triangular Prism Rectangular Prism
away from the normal.
v No deflec on takes place when light-rays
meet perpendicularly to the new medium
as in this case the angle of incidence is 0°.

v The emergent ray is parallel to the incident
ray.
v The emergent ray is laterally displaced

from the incident ray.
Quadrangular Prism Hexagoal Prism
v The lateral displacement increases if the
angle of incident increases. Let us do an ac vity to follow how light gets

v The lateral displacement decreases if the refracted through a prism:
angle of incidence decreases. Take a triangular prism ABC and place it on its
v The lateral displacement increases, if the base BC on a white sheet of paper fixed on a

density of the new medium increases. drawing-board. Draw the outline of the prism.
Draw a line PE that meets a face of the prism
The explana ons given above clarifies why a
obliquely. Fix two pins P, Q on this line to form
straight pencil appeared bent when placed in

166 Science-8