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

Solution

The correct option is A 20 cmFocal length of convex lens  $$f_1 = 15 \ cm$$Focal length of concave lens   $$f_2 = -5 \ cm$$Since the convex lens forms the image at its focus for the incident parallel beam. Thus image is formed at a distance of $$15 \ cm$$ 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 $$5 \ cm$$ in front of concave lens.Hence, separation between the lenses   $$d = 15+5 = 20 \ cm$$Physics

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