Question

A concave lens forms an erect image of $\frac{1}{3}$rd size of the object which is placed at a distance 30 cm in front of the lens. Find :

(a) the position of image, and

(b) the focal length of the lens.

Answer 1

Using Real Is Positive RIP Convention, u=30 and
M=-1/3

(a) M=v/u
v=-1/3*30=-10
v=-10
10 cm from the lens, (between lens and object)

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

1/30 + 1/-10 = 1/f

1/f=1/30-1/10
1/f=1-3/30
1/f=-2/30
1/f=-1/15
f=-15