A real image of an object is formed at a distance of `20 cm` from a lens. On putting another lens in contact with it, the image is shifted `10 cm` towards the combination, Determine the power of the second lens.
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Here, as image formed by the lens is real, the lens must be convex, `v = 20 cm`. If `f_(1)` is focal length of this lens, them from `(1)/(v) - (1)/(u) = (1)/(f_(1))` `(1)/(20) - (1)/(u) = (1)/(f_(1))` …(i) For the combination (of focal length `f`), `v = (20 - 10) cm = 10 cm` `:. (1)/(10) - (1)/(u) = (1)/(f) = (1)/(f_(1)) + (1)/(f_(2))` using (i), `(1)/(10) - (1)/(u) = ((1)/(20) - (1)/(u)) + (1)/(f_(2))` or `(1)/(10) - (1)/(20) = (1)/(f_(2))` or `f_(2) = 20 cm` `P = (100)/(20) = 5 dioptre`
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