1.

An object placed in front of the concave lens of focal length 20 cm and magnification is found to be 1/2. Find the location of the object.

Answer» Here object distance =u?-

Image distance is v= u/2, since magnification is 1/2

Focal length f=-20cm (concave lens)

Using lens formula we get

1/v-1/u=1/f

=>2/u-1/u=-1/20

=>u=-20 cm (real object distance)

m = 1/2
so -v/u = 1/2

Similarly, 

1/v + 1/u = 1/f

so 1/v - 1/u = 1/-20

v/v - v/u = v/-20 (multiply throughout by v)

1 +1/2 = -v/20

3/2 = -v/20

v = -30

1/2 = -v/u

1/2 = 30/u

u = 60

so the object must be placed 60 cm from the mirror. 


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