Question

A parallel beam of monochromatic light falls on a combination of a convex lens and concave lens of focal lengths 15 cm and 5 cm respectively. What is the distance between the two lenses to obtain a parallel beam of light from the concave lens?

• A. 20 cm
• B. 3 cm
• C. 10 cm
• D. 45 cm

Answer 1

The correct option is A 20 cm

Focal length of convex lens $f_1=15cm$
Focal length of concave lens $f_2=-5cm$
Since the convex lens forms the image at its focus for the incident parallel beam. Thus image is formed at a distance of $15cm$ from the convex lens.
Now the rays emerge parallel from the concave lens, thus the object has to be at the focus of the concave lens. So the object is at a distance $5cm$ in front of concave lens.
Hence, separation between the lenses $d=15+5=20cm$