Question

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

• A. $60\text{cm}$
• B. $30\text{cm}$
• C. $90\text{cm}$
• D. $40\text{cm}$

Answer 1

The correct option is B $30\text{cm}$


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 $20\text{cm}$ from the first lens and the second lens should lie at $10\text{cm}$ from this point. So, the total distance $20+10=30\text{cm}$