Question

if the image formed by a convex mirror is one third of the object ,when it is placed at 40cm distance from the mirror ,then the focal lenght of mirror will be

Answer 1

m=1/3
u= -40cm
so, m= -v/u
1/3=-(v/-40)
1/3= v/40
v = 40/3cm
Now, 1/f =1/v + 1/u
1/f = 3/40 + (-1/40)
1/f = 3/40 - 1/40
1/f = 2/40 = 1/20
So, f = 20 cm