What is the power of a convex lens with a focal length of $20 \text{ cm}$?

$(a)$ For the same angle of incidence $45^{\circ}$,the angle of refraction in two transparent media $I$ and $II$ is $20^{\circ}$ and $30^{\circ}$ respectively. Out of $I$ and $II$,which medium is optically denser and why?
$(b)$ Light enters from air to diamond,which has a refractive index of $2.42$. Calculate the speed of light in diamond,if the speed of light in air is $3.00 \times 10^{8} \text{ m s}^{-1}$.

$A$ ray is incident along $ACB$ on a spherical mirror,where $C$ is the centre of curvature. In which direction will the ray be reflected?

Does the magnification equation change depending on whether the lens is a concave lens or a convex lens?

What happens when a light ray incident normally on the surface of a glass slab enters from air into the glass?

The refractive indices of turpentine oil,kerosene,and alcohol are $1.47, 1.44$,and $1.36$ respectively. Based on this information,complete the following ray diagrams to show the path of a ray of light through each medium. (Give a reason for your answer.)