Two thin lenses are placed 5cm apart along the same axis and illuminted with a beam of light parallel to that axis. The first lens in the path of the beam is a converging lens of focal length 10cm whereas the second is a diverging lens of focal length 5cm. If the second lens if now moved toward the first, the emergent light

Options

A. remains parallel
B. remains convergent
C. remains divergent
D. changes form parallel to divergent

Answer

d.

Explanation

At 5cm from the lens, the second lens has a virtual object (image of the first lens) at its focal length. The emergent rays are therefore parallel.
When the second lens is closer then 5 cm to the first lens, its object is outside the focal length of the diverging second lens. This produces a virtual image outside 2f of th esecond lens. The emergent rays are therefore divergent.