The image formed by a lens is always erect and diminished. What is the nature of the lens?

An object is placed $15\, cm$ from a convex mirror of radius of curvature $60\, cm$. Find the position of the image and its magnification.

How can you identify the three types of mirrors without touching them?

The magnification of a real image formed by a concave mirror is always ...

An object is placed $50\, cm$ from a lens that produces a virtual image at a distance of $10\, cm$ in front of the lens. Calculate the focal length of the lens. (in $, cm$)

$(a)$ $A$ 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 $X$.