Page 171 - Science Class 08

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Optical Lenses and Devices
• Development of Lens
• Op cal Devices
DEVELOPMENT OF LENS
When most of the facts about refrac on has
been discovered, the scien sts began to
research it further so that the refrac on of light
may be controlled and u lised.
Two facts already well known to the scien sts
were as under:
v A light-ray entering a prism from its side is
refracted to turn towards the base of the
prism.
v A light-ray entering a slab parallel to its
base does not refract (deflect) to any side
and goes straight.
Based on these two facts, the scien st went
through the following two arrangements:
v Two prism of different shapes were placed
on both sides of a rectangular slab with
their bases towards the slab. It resulted in
a regular shape that was bulged outside on
both sides. This shape was given the name-
convex lens or converging lens.
v The same two prisms were placed on both
sides of the rectangular slab with their
Science-8
• Images Through Lenses
• The Human Eye
bases away from the slab, i.e. outwards. It
resulted in a regular shape bulged inside on
both the sides. This shape was given the
name – concave lens or diverging lens.
Thus, two types of lenses came into being
about which we shall study.
Convex Lenses
In a convex lens (some mes called a posi ve
lens), the glass (or plas c) surfaces bulge
outwards in the center giving the classic len l-
like shape. A convex lens is also called a
converging lens because it makes parallel light
rays passing through it bend inward and meet
(converge) at a spot just beyond the lens
known as the focal point.
F
Focal
Point
Focal Length
Convex Lens
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