Question

a concave lens has focal length of 20 cm. at what distance from the lens a 5cm tall object be placed so that it forms an image at 15cm from the lens? also calculate the size of the image formed.

Answer 1

Given,

For Concave lens ,

Focal lenght (f)= -20cm

Height of object(O) = 5cm

distance of image from the lens (v) = -15cm

distance of object from the lens (u) = ?

Height of image (I) = ?

We know that,

1/f = 1/v - 1/u

1/(-20) = 1/(-15) - 1/u

1/u = 1/15 - 1/20

1/u = (3-4)/60

1/u = -1/60

u = -60

Again ,

According to formula of Linear Magnification

I/O = v//u

I/5 = 15/60

I/5 = 1/4

I = 5/4

I = 1.25cm

Answer 2

Answer:

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