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 2.21) In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky ?
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Explanation

Generally, in case of a prism (II), the formation of spectrum is shown below

In the above figure, from top the third colour is yellow. But we can see that from bottom the third colour is blue which is the colour of sky. So, we can obtain the correct situation by inverting the prism. Thus the required orientations can be bound in case II.