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 best result he should focus(A) a distant tree or an electric pole(B) a well-illuminated distant building(C) well-lit grills of the nearest window(D) a burning candle laced at the distant edge of the laboratory table

Answer 1

Focal length can be only obtained when we know the image distance and object distance. Therefore as the distance of the candle from the mirror can be found out (more practically) the correct answer is (D)

Answer 2

Explanation:

Option B is correct

As focal length can only be determined if the object is kept very far away and is illuminated

An object kept at infinity forms image on focus