Define angular dispersion.

Draw a ray diagram of a refracting astronomical telescope when final image is formed at infinity. Also write the expression for its angular magnification(magnifying power).

Radius of curvature of an equi-convex lens of glass$\left(n=1.5\right)$ is $30 \mathrm{cm}$. Find its focal length. An object of height $5.0 \mathrm{cm}$ is placed at a distance of $60 \mathrm{cm}$ from the optical centre of the lens. Find the position and the height of the image formed.

On the basis of Bohr's theory, derive an expression for the radius of the $n^{th}$ orbit of hydrogen atom.