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The image of the needle placed `10 cm` from a lens is formed on a wall `20 cm` on the other side of the lens. Find focal length of the lens and size of image formed, if the size of object needle is `2.5 cm`.

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Correct Answer - `6.67 cm ; - 5 cm`
Here, `u = -10 cm, v = 20 cm, f = ?, h = ?, h_(1) = 2.5 cm`
As `(1)/(f) = (1)/(v) - 1/u, (1)/(f) = 1/20 + 1/10 = (1 + 2)/20 = 3/20 or f = 20/3 = 6.67 cm`.
Also, as `m = (h_(2))/(h_(1)) = v/u, (h_(2))/(2.5) = 20/(-10), h_(2) = -5 cm`. As `h_(2)` is negative, image is inverted.

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