$A$ girl was playing with a thin beam of light from her laser torch by directing it from different directions on a convex lens held vertically. She was surprised to see that in a particular direction the beam of light continues to move along the same direction after passing through the lens. State the reason for this observation.

What is the distance between the pole and the principal focus of a spherical mirror called?

The ray diagram given below illustrates the experimental setup for the determination of the focal length of a converging lens using a plane mirror.
$(i)$ State the magnification of the image formed.
$(ii)$ Write two characteristics of the image formed.
$(iii)$ What is the name given to the distance between the object and the optical centre of the lens in the given diagram?

The distance between the object kept at $2 \, m$ from a plane mirror and its image is $.....$ (in $, m$)

Why does a light ray incident on a rectangular glass slab immersed in any medium emerge parallel to itself? Explain.

What is the focal length of the objective of an astronomical telescope as compared to that of the eyepiece?