a student takes a mirror which is depressed at the centre and mounts it on a mirror stand. An erect and enlarged image of his face is formed. He places the mirror on a stand along a meter scale at 10 cm mark. In front of this mirror,he mounts a white screen and moves it back and forth along the meter scale till a highly sharp well defined image of a distant building is formed on the screen at 25.5cm mark 

what is meant by depressed at centre?
what is meant by highly sharp image?
name the mirror and find its focal length
why does the student get sharp image of distant building at 25.5 mark?

Dear Student ,
Depressed at the centre means that it is bent towards the centre or compressed inwards .
Highly sharp image means that the image is highly magnified .
The mirror is a concave mirror and it's focal length is 25.5 cm
the image of the distant building is formed at 25.5 cm mark because the light rays coming from the building are parallel to each other and after reflection they converge at the focus point and hence meet at 25.5 cm mark.
hence an image is formed at that mark .
Hope this helps you .

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