Question

How far should an object be from the pole of a concave mirror of focal length 20 CM to form a real image whose size is 1/6 of the size of object

Answer 1

Given ;

f = -20 cm

h2 = (1/5)h1

Magnification :

m = h2/h1 = v/u

1/5 = -v/u

v = -5u

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Now use this relation between v ,& u in the mirror formula to find u ;

(1/f) = (1/v) + (1/u)

(1/-20) = (1/-5u) + (1/u)

-(1/20) = (1/u) - (1/5u)

-(1/20) = (4/5u)

u = -(20×4)/5

u = -80/5

u = -16

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Hence the object should be placed at a distance of 16m in front of a concave Mirror.