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Combinations of Lenses & Mirrors

A wire gauge ...

Question

A wire gauge is illuminated with blue light and a convex lens forms a sharp image of it on a screen. If now the wire gauge is illuminated with red light, to get a sharp image again the screen must be :

A

moved towards the lens

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B

moved away from the lens

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C

placed there itself

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D

moved to infinity

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Solution

The correct option is B moved away from the lens
Blue light has less wavelength than red light and we know that as wavelength increases deviation decreases. so, for red light, image will be formed at a larger distance. So, screen should be moved a way from lens. Hence correct option is B.

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