Question

1. An object of is placed at 30cm from a convex lens of focal length 15cm,
the image distance will be
O a. 10cm
b. 20cm
C. 30cm
d. 40cm

Answer 1

Explanation:

An object is placed 30 cm in front of convex lens of focal length 10 cm :

Given :  u1=−30 cm        f1=10 cm

From lens formula,   v11−u11=f11

∴    v11−−301=101       ⟹v=15 cm

Thus image is formed at a distance of 15 cm behind the lens.

Now a concave lens of focal length f2 is placed in contact with convex lens. so the screen has to be shifted by 45 cm further away.

Now the new image distance   v2=15+45=60 cm

Let the focal length of combination of lens be F.

Using lens formula,  v21−u11=F1

∴    601−−301=F1             ⟹F=20 cm