Two thin lenses of $+10 \mathrm{D}$ and $-10 \mathrm{D}$  powers are placed in contact with each other. Find the power, focal length and nature of this combination.

Draw ray diagram to illustrate the formation of a real image after refraction of light at an air-glass spherical surface.

Explain the meaning of the first and the second focus of a convex lens, drawing separate ray-diagrams.

Define the first focus of a concave lens, giving a suitable ray-diagram.

A lens is made of two different materials of refractive indices $n_1$ and $n_2$, as shown. A point object is placed on the principal axis. How many images of the object will be formed by the lens (neglecting spherical and chromatic aberration)?