The magnification in a plane mirror is always ........

$(a)$ $A$ ray of light falls normally on a face of a glass slab. What are the values of angle of incidence and angle of refraction of this ray?
$(b)$ Light enters from air to a medium $X$. Its speed in medium $X$ becomes $1.5 \times 10^{8} \ m \ s^{-1}$. Speed of light in air is $3 \times 10^{8} \ m \ s^{-1}$. Find the refractive index of medium $X$.

When light rays parallel to the principal axis fall on a convex lens,they refract and converge at a point on the principal axis. What is this point called?

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

How many images are formed by two parallel mirrors?

$A$ student wants to project the image of a candle flame on a screen $60\, cm$ in front of a mirror by keeping the flame at a distance of $15\, cm$ from its pole.
$(a)$ Write the type of mirror he should use.
$(b)$ Find the linear magnification of the image produced.
$(c)$ What is the distance between the object and its image?
$(d)$ Draw a ray diagram to show the image formation in this case.