A glass lens is immersed in water. Will its power increase or decrease?

A converging lens of focal length $40 \mathrm{cm}$ is kept in contact with a diverging lens of focal length $30 \mathrm{cm}$. Find the focal length of the combination.

Two lenses producing linear magnifications $m_1$ and $m_2$ are joined together to form a lens combination. What is the magnification $m$ of the combination?

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.