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An object is kept at a distance of 16 cm from a thin lens and the image formed is real. If the object is kept at a distance of 6 cm from the same lens, the image formed is virtual. If the size of the image formed are equal, the focal length of the lens will beA. 15cmB. 17cmC. 21cmD. 11cm |
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Answer» Correct Answer - d. Only convex lens can form a real well as virtual image. So, the given lens is a convex lens. Let f is the focal length of the lens and n the magniture of magnification. In the first case, when the image is real: `muv=-16` `v=+16n` So, applying `(1)/(v)-(1)/(u)=(1)/(f)` `(1)/(16)+(1)/(16)=(1)/(f)` or `1+(1)/(n)=(16)/(f)` In the second case, when image is virtual: `u=-6` `v=-6n` `f=+f` `:. (1)/(-6n)+(1)/(6)=(1)/(f)` `:.1-(1)/(n)=(6)/(f)` Adding (i) and (ii), we get `2=(22)/(f) or f=11cm` |
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