The laws of reflection are true only for plane mirrors and not for curved mirrors.

$A$ thin lens has a focal length of $-12\, cm$. Is it a convex lens or a concave lens?

Which of the following is the correct general form of Snell's law?

$(i)$ Define a real image of an object.
$(ii)$ Name the mirror that:
$(a)$ can give a real as well as a virtual image of an object.
$(b)$ will always give a virtual image of the same size as the object.
$(c)$ will always give a virtual and diminished image of an object.
$(d)$ is used by a doctor in examining teeth.
$(iii)$ With the help of a ray diagram,explain the use of a concave mirror as a solar concentrator.

The refractive index of water with respect to air is $\frac{4}{3}$ and the refractive index of glass with respect to air is $\frac{3}{2}$. What is the refractive index of glass with respect to water?

What is the $SI$ unit of the power of a lens?