Page 172 - Science Class 08

When parallel rays of light pass through a 0° refrac ve angle. But, as the outer pieces are
convex lens, they deflect inwards and appear
to meet at a point called the focus. The
distance between the op cal centre and the
focus in called focal length.
Concave Lenses
A concave lens is exactly the opposite with the
outer surfaces curving inward, so it makes
parallel light rays curve outward or diverge.
That’s why concave lenses are some mes
called diverging lenses. (One easy way to
remember the difference between concave and
convex lenses is to think of concave lenses
caving inwards.)
F
Focal
point (virtual) Focal length
Concave Lens
When parallel rays pass through a concave
lens, they deflect outwards. If you look through
the lens at these parallel rays, they will appear
to diverge (start and spread outwards) from a
single point called the focus and the op cal
centre of the lens is called focal length.
Why Rays Converge?
We have read that parallel rays passing through
a convex lens converge to a point.
Let us try to understand why it happens:
Observe the convex lens and the parallel rays
passing through it. The lens is made up of five
pieces joined together. The central piece is like a
slab while the other pieces resemble prisms.
The ray that falls on the slab falls ver cally and
so it passes through it without refrac on with
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prisms, the rays passing through them deflect
towards their bases, i.e., inwards to meet at a
point called their focus.
This is how parallel rays passing through a
convex lens converge inwards to meet at a
point.
Why Rays Diverge?
We have read that parallel rays passing through
a concave lens diverge (spread outwards).
Let us try to follow why it happens:
Observe the concave lens carefully. Its central
piece is a slab just like that of a convex lens.
The outer four pieces are prisms with their
bases outwards. So, the ray that falls on the
slab pass through it without any refrac on but
the rays passing through the outer pieces
deflect towards their bases, i.e, outwards.
This is how the parallel rays passing through a
concave lens diverge, i.e., spread outwards.
IMAGES THROUGH LENSES
We have followed how and why light-rays
converge of diverge a er refrac on. So, it is not
now difficult to locate the posi on of an image
formed through a lens. We can do that by
drawing ray-diagrams.
Image through a Convex Lens
Image formed through convex lenses can be
real or unreal. The nature, size and loca on of
the images formed through these lenses
depend on the distance of the object from the
lens and its focal length.
Let us study the various types of images formed
through a convex lens:
v If the object is beyond double the focal
length (beyond 2F) of the lens, the image
will be real, inverted and smaller in size.
v If the object is at double the focal length (at
2F) from the lens, the image is real,
inverted and equal to the object in size, but
on the opposite side of the lens.
Science-8