Question

a ) A student measure the focal length of a concave mirror by focussing a distant object on the screen as 20 cm. What is the distance between mirror and screen ) Why white paper cannot be choosen as a screen, if student selected the sun as distant object to find the focal length of a concave mirror?​

Answer 1

a)10 cm

 when the image of a very distant  object is formed using a concave  mirror, image is formed at focus.  so, the screen is at a distance of  10 cm from the mirror. 10 cm is  also the focal length of the mirror. 

b)

The ray diagram of a distant object kept in front of a concave mirror is as shown in the diagram above. The rays from the object kept at a large distance meet near the focus of the object, depicting the focal length of the mirror.

The image is real, inverted and diminished as seen above.

Hence correct answer is option C.

Answer 2

A student measures the focal length of a concave mirror by focusing a distant object on the screen as 20 cm. So, the distance between the mirror and the screen is also 20 cm.

White paper cannot be chosen as a screen if the student selected the sun as distant object to find the focal length of a concave mirror.  

(a)

• When a distant object is focused on a screen, the parallel rays of the object are supposed to be coming from infinity.
• When the object is at infinity, the concave mirror produces the image at the focus.
• So, the image distance and the focal length are the same.
• Since the focal length is 20 cm, the distance of the screen from the mirror also must be 20 cm.

(b)

• When the sun is selected as the distant object to find the focal length, the rays from it are almost parallel to the mirror and they move through the focus of the mirror.
• The paper is placed as a screen at the focus of the mirror.
• Power of a mirror is inversely proportional to the focal length of the mirror.
• Since the distance between the concave mirror and the focus is very less, the power produced by the mirror is very high.
• Due to this, the paper can be burnt by the sun's rays. That is why paper cannot be chosen as a screen.