In order to determine the focal length of a concave mirror, a student obtained a sharp image of the grill of a window in the laboratory wall on a screen. His teacher suggested that to get better result for focal length, he should focus a distant tree instead of the window grill. In which direction should the mirror be moved for this purpose so as to get a sharp image of the tree on the screen?
(1) behind the screen
(2) away from the screen
(3) towards the screen
(4) very far away from the screen

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Solution

The grill of the window is a nearby object for the concave mirror, hence the mirror will always form its image beyond its focal point. But if the object is a distant tree then the concave mirror will converge all the incident rays on its focus and will form a sharp image. So, in order to get the image of a distant tree, screens should be at the focal point of the mirror. That's why the student needs to move the mirror towards the screen.

Hence, the correct answer is option 3.

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