A collimated beam of light of diameter 2 mm is propagating along x-axis. The beam is required to be expanded in a collimated beam of diameter 14 mm using a system of two convex lenses. If first lens has focal length 40 mm, then the focal length of second lens is mm.

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280. Two convex lenses with a common focal point make a beam expander (Keplerian telescope). Beam width scales as the ratio of focal lengths: (D₂)/(D₁) = (f₂)/(f₁). f₂ = 40 × (14)/2 = 280 mm → 280

A collimated beam of light of diameter 2 mm is propagating along x-axis. The beam is required to be expanded in a collimated beam of diameter 14 mm using a system of two convex lenses. If first lens has focal length 40 mm, then the focal length of second lens is ______ mm.

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