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.

(i) Concave mirror; f = 35 – 15 = 20 cm
(ii) Sharp Image of Distant Tree at 35 cm Mark:
The fact that Sunita gets a sharp, well-defined inverted image of a distant tree at the 35 cm mark indicates that the image is formed at the focus of the concave mirror. Because the incident rays parallel to each other after reflection from concave mirror meets at focus and produce sharp image at focus. This is because, for a concave mirror, when the object is at infinity (distant object), the image is formed at the focal point.
In this case, the white screen is adjusted until the image of the distant tree is formed at the 35 cm mark, suggesting that the screen is placed at the focal point of the concave mirror.