Question

A parallel beam of light is incident on the system of two convex lenses of focal lengths $f_1=20cm$ and $f_2=10cm$. What should be the distance between the two lenses so that rays after refraction from both the lenses pass undeviated.

• A. $60cm$
• B. $30cm$
• C. $90cm$
• D. $40cm$

Answer 1

The correct option is B $30cm$

For the rays to pass undeviated, the image formed by the first lens should lie at the focus of the second lens.
The first image is formed at a distance 20 cm from the first lens. and the second lens should lie at 10 cm from this point. So, the total distance = 20 +10 = 30