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 after refraction from the first convex lens should form at the secondary focus of the second convex lens since the rays emerging from the second lens would again be parallel.

Hence the distance between the two lenses should be $20cm+10cm=30cm$