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The distance between a convex lens and a plane mirror is 10 cm. The parallel rays incident on the convex lens after reflection from the mirror form an

image at the optical centre of the lens. Find the focal length of the lens.



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The parallel rays (to principal axes)  after being incident on the convex lens tend to converge at the focus of the lens. 

There a plane mirror between the lens and its focus.  The rays get reflected with the same angles of incidence and reflection.  The plane mirror does not change the converging distance of the rays.

The path travelled by the rays after refraction from the lens = 10 cm + 10 cm = 20 cm

Focal length = 20 cm
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