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Question:

A student has to determine the focal length of a concave mirror by obtaining the image of a distant object on a screen. For getting the best result, he should focus on:
a distant tree or an electric pole
a well-illuminated distant building
a burning candle placed at the distant edge of the laboratory table
well-lit grills of the nearest window

a well-illuminated distant building

well-lit grills of the nearest window

a distant tree or an electric pole

a burning candle placed at the distant edge of the laboratory table

Solution:

The student needs a parallel beam of light falling on the concave mirror so that after reflection the reflected rays meet at a point indicating the position of the focus. We know that the light rays which are coming from a distant object of smaller (comparatively) size are parallel to each other and parallel to the principal axis as well. Hence the student should choose a distant tree or an electric pole. The image hence formed will be the location of the focus. Hence correct answer is option A.