Question

A prism ABC (with BC as base) is placed in different orientations. A narrow beam of white light is incident on the prism as shown in figure. In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky? [NCERT Exemplar]
(a) Only (i)
(b) Only (ii)
(c) Only (iii)
(d) Only (iv)

Answer 1

The diagrams in the question is given in the pic that's attatched.

Answer: (b) Only (ii)

Explanation: When the prism is placed with angle A in the upward direction, the third colour from the top is yellow. But we have been given that the third colour from the top is blue (colour of the sky).

Thus base BC must be on the top. That is, the prism must be inverted.

Hope it's helpful. Mark it as the brainliest if it was.

Answer 2

After dispersion, the third color from the top corresponds to the color of the sky in (b) Only (ii).

• When light travels through a prism (with BC as a base), after dispersion it is observed that the colors are in the order ROYGBIV.
• But, when light travels through the same prism inverted, it is observed that the colors are in the order VIBGYOR.
• In this case, blue is in the third position from the top. Hence, when a narrow beam of light is incident on the inverted prism, the third color from the top corresponds to the color of the sky.