A boy is trying to start a fire by focusing sunlight on a piece of paper using an equi-convex lens of focal length $10 \mathrm{cm}$. The diameter of the sun is $1.39\times {10}^9 \mathrm{m}$ and its mean distance from the earth is $1.5\times {10}^{11} \mathrm{m}$ What is the diameter of the sun's image on the paper?

Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combination is:

A thin prism of angle $15°$ made of glass of refractive index ${\mu }_1=1.5$ is combined with another prism of glass of refractive index ${\mu }_2=1.75.$ The combination of the prism produces dispersion without deviation. The angle of the second prism should be