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  4. A convex lens is used to form real image of an object on a screen. It is observed that even when the positions of the object and the screen are fixed there are two positions of the lens to form real images. If the heights of the images are 4 cm and 9cm respectively, the height of the objective is;

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Q. A convex lens is used to form real image of an object on a screen. It is observed that even when the positions of the object and the screen are fixed there are two positions of the lens to form real images. If the heights of the images are 4 cm and 9cm respectively, the height of the objective is;

AMUAMU 1999

Solution:

: According to displacement method for determining focal length of a convex lens, we know Height of object $ =\sqrt{Heights\text{ }of\text{ (}first\times second)\text{ }images} $ or $ h=\sqrt{{{I}_{1}}{{I}_{2}}} $ $ \Rightarrow $ $ h=\sqrt{4\times 9} $ $ \Rightarrow $ $ h=6\,cm $