Size of the image of an object by a concave lens of focal length 20 cm is observed reduced to 1/3rd if it’s size find distance of the object from the lens
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m=f-u/f and for concave lens f is negative sign where m is 1/3 then you will find -40 cm mark this is brainlist ans
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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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