The size of an object by a concave lens of focal length 20cm is observed to be reduced to 1/3rd of its size .Find the distance of the object from the optical centre of the lens.
Answers
Answer:
The distance of the object from the optical centre of the lens will be 20 cms
Explanation:
Given data,
Focal length = 20 cm
magnification = 1/3
We know that magnification is given by the formula,
m = v/u
where v = image distance
u = object distance
=> 1/3 = v/u
=> v = u/3
From the lens formula for concave lens
1/v - 1/u = 1/f
=> 1/(u/3) - 1/u = 1/f
=> 3/u - 1/u = 1/f
=> 2/u = 1/f
=> u = 2f = 2 x 20 = 40 cm
Hence the distance of the object from the optical centre of the lens will be 20 cms
hi mate,
Solution: object distance from the optical centre of the lens is 40cm.
explanation : focal length of concave lens , f = -20cm
a/c to question,
size of image = 1/3 × size of object.
so, magnification , m = 1/3
we know, magnification of lens , m = v/u
so, v/u = 1/3
so, v = u/3 ........(1)
now applying lens maker formula,
1/v - 1/u = 1/f
or, 1/(u/3) - 1/u = 1/-20
or, 3/u - 1/u = 1/20
or, 2/u = 1/-20 cm
or, u = -40 cm
hence, object is placed 40cm from the pole of concave lens.
i hope it helps you.......
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