When white light enters a glass prism from air, the angle of deviation is least for:

A ray of light passes through a prism as shown in the figure given below: The angle $\delta$ is known as 

At a particular minimum value of angle of deviation, the refracted ray becomes:

Two identical prisms $ABC$ and $A\text{'}B\text{'}C\text{'}$ are given. White light is passed through $ABC$ as shown below: Which of the following positions of $\mathrm{A}\text{'}\mathrm{B}\text{'}\mathrm{C}’$ will again yield white light:

A prism $\mathrm{ABC}$ (with $\mathrm{BC}$ as base) is placed in different orientations. A narrow beam of white light is incident on the prism as shown in the figure ahead. In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky :