Sunita takes a mirror which is depressed at the centre and mounts it on a mirror stand. An erect and enlarged image of her face is formed. She places the mirror on a stand along a meter scale at 15 cm mark. In front of this mirror, she mounts a white screen and moves it back and forth along the meter scale till a sharp, well-defined inverted image of a distant tree is formed on the screen at 35 cm mark.
(i) Name the mirror and find its focal length.
(ii) Why does Sunita get sharp image of the distant building at 35 cm mark?
Answers
(I). Sunita takes a mirror which is depressed at the centre and mounts it on a mirror stand. An erect and enlarged image of her face is formed. After reading this statement we conclude that mirror is concave mirror , because concave mirror is depressed at the centre and also it can form erect and elarged image, while convex mirror or plane mirror can’t .
Let the focal length of concave mirror is f.
Sunita places the mirror on the stand along a metre scale at 15cm mark. In front of this mirror , she mounts a white screen and moves it back and forth along the metre scale till sharp. Well defined inverted image of distant tree is formed on the screen at 35cm mark.
We know, parallel light rays will meet at a point , known as focus.
so, focal length = distant image formed of tree with respect to end of scale - location of mirror with respect to origin
= 35cm - 15cm = 20
Hence, focal length = 20cm
(ii) because incident rays parallel to each other , after reflection from concave mirror meets at a point called focus and produce sharp image.
Answer:
( i )
Sunita used concave mirror.
Focal length of mirror is :
= > 35 cm - 15 cm
= > 20 cm .
( ii )
As we know concave mirror always form image at focus when object is placed at infinity. Hence image is form sharp image at it was placed at 35 cm .