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 placed at the distant edge of the laboratory table

Answer 1

Hint: For finding the better value of focal length of the concave mirror, the image obtained should be obtained close to the mirror near the focus. To obtain an image close to the mirror, objects should be placed at a larger distance from the mirror.

Complete step by step answer:
A concave mirror is the one which is curved inward in the middle. Concave mirror can produce both types of images, real and virtual. The image can be upright, if virtual, and inverted, if real. Also, the image can be behind the mirror, if virtual, and in front of the mirror, if real. The images can also be magnified, diminished or of the same size depending on the position of the object.
When the object is kept at a large distance from the mirror, the rays coming from the object meet near the focus of the mirror, depicting the focal length of the mirror. The image obtained is real, inverted, diminished and well illuminated.

For determining the focal length of a mirror, the screen should be placed in front of the reflecting surface of the mirror. To obtain a clear and sharp image, the screen should be adjusted accordingly. To get a well illuminated image, objects should be placed at a large distance from the mirror.
Therefore,
To determine the focal length of a concave mirror by obtaining the image of a distant object on the screen, students should focus on a distant tree or an electric pole. A distant tree can be considered at infinity. So, when the light rays coming from infinity fall on the surface of a concave mirror, the image obtained at the focus of the mirror and focal length of the mirror can be calculated by getting best results.
Hence, the correct option is A.

Note:
When the object is shifted away from the concave mirror, the image moves towards the mirror close to the focus. A well illuminated image is obtained at the focus of the concave mirror. For getting best results, objects should be placed at infinity to get a well-defined image on the focus of the mirror.