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Question:

A convex lens, of focal length 30cm, a concave lens of focal length 120cm, and a plane mirror are arranged as shown. For an object kept at a distance of 60cm from the convex lens, the final image, formed by the combination, is a real image, at a distance of: 60cm from the convex lens, 60cm from the concave lens, 70cm from the convex lens, 70cm from the concave lens

60cm from the convex lens

70cm from the concave lens

70cm from the convex lens

60cm from the concave lens

Solution:

Location of image formation after first refraction from the lens,
1/v + 1/60 = 1/30.
This gives v = 60cm
Next refraction from concave lens,
1/v2 - 1/40 = 1/120
Hence v2 = 60cm
Since this will be formed behind the mirror, this will be the final image. It is at a distance of 60cm from the concave lens.