In a combination of lenses in contact, the power of first lens is$+2·5D$ and the focal length of second lens is$-25cm$. Calculate the power or focal length of the lens combination.

You know the phenomenon of the scattering of light by atmospheric particles. Write a few lines about the blue colour of the sky and reddish colour of the sky in the morning as well as in the evening.

In a prism ${\mathrm{r}}_1+{\mathrm{r}}_2 =\mathrm{A}$ and $\mathrm{\delta }=\mathrm{i}+\mathrm{e}-\mathrm{A}$, when the prism is at the position of minimum deviation ${\mathrm{D}}_{\mathrm{m}}$. Show that the refractive index of the material of the prism is:

$\mathrm{\mu }=\frac{\sin \left({\displaystyle \frac{\mathrm{A}+{\mathrm{D}}_{\mathrm{m}}}{2}}\right)}{\sin {\displaystyle \frac{\mathrm{A}}{2}}}$

 Also derive the expression for ${\mathrm{D}}_{\mathrm{m}}$ for small angled prism.

Which optical device can produce optical dispersion and which property is responsible for the dispersion of light? Draw a diagram to show that it is possible to obtain white light again after dispersion.