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

Answer:

(b) a well illuminated distant building

Answer 2

(A) a distant tree or an electric pole

Explanation:

The focal length of a concave mirror can be obtained by focusing an object at infinity. An illuminated object can have undesirable effects on the experiment. So it is best to use a distant tree or an electric pole to get a defined image. When light rays from infinity fall on a concave mirror, image will be formed at the focus. So once we get a sharp image of the tree or electric pole, we can measure the distance of the image and obtain the focal length.