Question

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?

Answer 1

i) The mirror used by Sunita is Concave mirror as she gets her image of face   as erect and enlarged.

Only concave mirrors can produce virtual  and erect image when object is placed in between Pole and Focus.

ii) As a distant object which is regarded as placed At infinity,  image is formed well defined and sharp at 35cm.

hence focal length of the mirror is 35cm.[ Because concave mirror will form image at focus when object is placed at infinity.]

Answer 2

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 .