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 best result he should focus:

a distant tree or an electric pole

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a well-illuminated distant building

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well-lit grills of the nearest window

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

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Solution

The correct option is A a distant tree or an electric pole
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.

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To determine focal length of a concave mirror a student obtains the image of a well lit distant object on a screen. To determine the focal length of the given concave mirror he needs to measures the distance between:

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