The distance between a convex lens and a plan | vedclass

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(B) When parallel rays are incident on a convex lens,they converge at its focal point at a distance $f$ from the lens.Since the rays are reflected by

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$20$

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(B) When parallel rays are incident on a convex lens,they converge at its focal point at a distance $f$ from the lens.Since the rays are reflected by a plane mirror placed at a distance of $10 \, cm$ from the lens,the rays appear to come from an object point behind the mirror.According to the property of a plane mirror,the image formed by the mirror is at the same distance behind the mirror as the object is in front of it.Here,the rays are converging towards a point at distance $f$ from the lens. Since the mirror is at $10 \, cm$,the distance of this convergence point from the mirror is $(f - 10) \, cm$.For the rays to form an image at the optical centre of the lens after reflection,the reflected rays must appear to originate from a virtual object located at the optical centre.Thus,the distance of the virtual object from the mirror must be equal to the distance of the image from the mirror.Therefore,$f - 10 = 10$,which gives $f = 20 \, cm$.

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. The focal length of the lens will be.....$cm$.

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