Define the term power of a lens.

Draw a labelled ray diagram to locate the image of an object formed by a convex lens of focal length $20 \, cm$ when the object is placed $30 \, cm$ away from the lens.

What is the ratio of the velocity of light in a transparent medium with a refractive index of $1.5$ to the velocity of light in a vacuum?

$(a)$ Define absolute refractive index.
$(b)$ The path of a light ray from three different media $A, B$ and $C$ for a given angle of incidence is shown below. Study the diagrams and answer the following questions.
$(i)$ Which of the three media $A, B$ or $C$ has maximum optical density?
$(ii)$ Through which of the three media,will the speed of light be maximum?
$(iii)$ Will the light travelling from $A$ to $B$ bend towards or away from the normal?
$(iv)$ Will the refractive index of $B$ relative to $C$ be more than unity or less than unity?

The radius of curvature of a spherical mirror is half of the focal length.

$A$ light ray enters from medium $A$ to medium $B$ as shown in the figure. The refractive index of medium $B$ relative to $A$ will be