In an experiment a convex lens of focal length 15 cm is placed coaxially on an optical bench in front of a convex mirror at a distance of 5 cm from it. It is found that an object and its image coincide, if the object is placed at a distance of 20 cm from the lens. The focal length of the convex mirror is :
← JEE Main 2017 · 9 Apr · Shift 1
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JEE Main 2017 · 9 Apr · Shift 1, Physics question 22
Answer and worked solution
Official answer: A
For the image to coincide with the object the rays must hit the mirror normally, i.e. the lens image must be at the mirror's centre of curvature. The lens gives , cm, which is 55 cm behind the mirror. So cm and cm.