$A$ ray of light passing through the principal focus of a concave mirror,after reflection,travels in which direction?

Through which point of a convex lens does a ray of light pass without undergoing refraction?

$A$ $10 \, cm$ tall object is placed perpendicular to the principal axis of a convex lens of focal length $30 \, cm$. The distance of the object from the lens is $20 \, cm$. Find the $(i)$ position,$(ii)$ nature,and $(iii)$ size of the image formed.

How can you distinguish between a plane mirror,a convex mirror,and a concave mirror by merely looking at the image formed in each case?

Where is the object placed in a compound microscope?

$(a)$ $A$ concave mirror produces a three times enlarged real image of an object placed at $10\, cm$ in front of it. Calculate the focal length of the mirror.
$(b)$ Show the formation of the image with the help of a ray diagram when an object is placed $6\, cm$ away from the pole of a convex mirror.