Question

In the secondary rainbow, what is the order of colours observed from earth?
A) Violet innermost, red outermost
B) Red innermost, violet outermost
C) Random
D) White and Dark alternatively

Answer 1

The secondary rainbow forms outside of a primary rainbow and is much fainter than that. It forms because the light undergoes two internal reflections when it enters the water drop.

Complete step-by-step solution:
A rainbow forms when light enters a raindrop at such an angle that it is reflected internally. Light is composed of different wavelengths and when it passes through a raindrop and undergoes internal reflection, all the wavelengths bend by different angles, which leads to the separation of all colours. When a single rainbow forms, the colours are ordered such that red is the outermost and violet is the innermost colour.

In the case of a secondary rainbow, the light entering the raindrop undergoes internal reflection twice. Because of this, light leaves the raindrop at a greater angle than before. The colours of the rainbow are also reversed because they are internally reflected again. This way, we see violet as the outermost colour and red as the innermost colour in the secondary rainbow. These rainbows are fainter than primary rainbows because a smaller number of light rays undergo two internal reflections in the raindrop.

Hence, the correct answer for this question is option B.

Note: Occasionally, we can also observe rings on the inside of the primary rainbow, called supernumerary rainbow. These rainbows are caused by interference of light rays emerging from the raindrop after one internal reflection.