Two adjacent walls and the ceiling of a rectangular room are mirror surfaced. The number of images of himself that an observer sees is

What happens to the image formed by a convex lens if its lower part is blackened?

An incident ray falls normally on a glass slab. What is the angle of refraction (in $°$)?

$A$ student focused the image of a candle flame on a white screen by placing the flame at various distances from a convex lens. He noted his observations:

Observation	Distance of flame from the lens $(cm)$	Distance of screen from the lens $(cm)$
$(i)$	$60$	$20$
$(ii)$	$40$	$24$
$(iii)$	$30$	$30$
$(iv)$	$24$	$40$
$(v)$	$15$	$70$

$(a)$ From the above table, find the focal length of the lens without using the lens formula.
$(b)$ Which set of observations is incorrect and why?
$(c)$ In which case will the size of the object and the image be the same? Give a reason for your answer.

An object $1 \, cm$ tall is placed at a distance of $15 \, cm$ from a concave mirror of focal length $10 \, cm$. Find the position,size,and nature of the image formed.

If the magnification of a body of size $1\, m$ is $2$,what is the size of the image (in $m$)?