Define the term reflection.

At what position should an object be placed in front of a concave mirror to obtain a magnification of $(- 2)$?

The focal length of a concave mirror is $30 \, cm$. Find the position of the object in front of the mirror so that the image is three times the size of the object.

$(a)$ Define principal focus of a spherical mirror.
$(b)$ For what position of the object does a concave mirror form a real,inverted and diminished image of the object? Draw the ray diagram.
$(c)$ An object $4 \,cm$ high is placed at a distance of $6 \,cm$ in front of a concave mirror of focal length $12 \,cm$. Find the position of the image formed.

Draw ray diagrams showing the image formation by a convex mirror when an object is placed:
$(a)$ at infinity
$(b)$ at a finite distance from the mirror

The velocity of light in air is $3 \times 10^{8} \, m \, s^{-1}$. What is the velocity of light in water? (The absolute refractive index of water is $\frac{4}{3}$.)