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Standard XII

Physics

Combinations of Lenses & Mirrors

Two thin bico...

Question

Two thin biconvex lenses $L_{1}$ and $L_{2}$ having focal lengths 10 cm and 15 cm respectively are placed coaxially.What distance (d) should be maintained between them so that a parallel beam of light parallel beam of light parallel to their axis, incident on the lens $L_{1}$ emerges from $L_{2}$ undeviated? If the distance between them is increased by 'd' at what distance from $L_{1}$ , on the axis, does the emergent beam converge?

28 cm, 74 cm

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25 cm, 74 cm

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25 cm, 54 cm

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25 cm, 25 cm

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Solution

The correct option is B 25 cm, 74 cm
When parallel beam of light is incident of $L_{1}$ , it will converge at its focal point $10 c m$ away from the lens. If this point is the focal point of the second lens then after refraction from $L_{2}$ , the beam of light will become parallel to the axis. So, the distance between two lens must be the combined focal lengths of two lens i.e. $10 c m + 15 c m = 25 c m .$

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