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← JEE Main 2016 · 9 Apr · Shift 1
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JEE Main 2016 · 9 Apr · Shift 1, Physics question 19

A convex lens, of focal length 30 cm, a concave lens of focal length 120 cm, and a plane mirror are arranged as shown. For an object kept at a distance of 60 cm from the convex lens, the final image, formed by the combination, is a real image, at a distance of :

On one axis: an object 60 cm left of a convex lens (|f| = 30 cm), a concave lens (|f| = 120 cm) 20 cm to its right, and a plane mirror 70 cm to the right of the convex lens.
Choose one option
Answer and worked solution

Official answer: A

The convex lens images the object (60 cm, at 2f) at 60 cm on the other side, i.e. 40 cm beyond the concave lens. For the concave lens (u = +40, f = −120): 1v=−1120+140\frac1v=-\frac1{120}+\frac1{40}, v = 60 cm, a point 10 cm behind the mirror. The mirror turns this into a real image 10 cm in front of it, i.e. 60 cm from the convex lens.

This derived key takes that real image as the one meant. Tracing the light back through both lenses would place the image 75 cm on the far side of the convex lens, which is not an option.

Topics:Combination of lenses and mirrorsThin lenses and lens maker's formula
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