Q.Which of the following is not involved in formation of a rainbow? (A) refraction (B) angular dispersion (C) angular deviation (D) total internal reflection

Primary rainbow: 1 internal reflection, red outside, ~42°, brighter. Secondary rainbow: 2 internal reflections, red inside (reversed), ~51°, fainter.

Primary rainbow. It is produced when sunlight entering a raindrop is refracted, undergoes one total internal reflection at the back of the drop, and is refracted again as it leaves. The emerging light is dispersed into colours. In the primary rainbow the red colour appears on the outer (upper) side and violet on the inner side; it makes an angle of about $42^\circ$ (red) with the incident sunlight direction and is comparatively bright.

Secondary rainbow. It is produced when light undergoes two internal reflections inside the drop (with two refractions). Because of the extra reflection, the order of colours is reversed — violet on the outer side and red on the inner side — the rainbow subtends a larger angle of about $51^\circ$, and because more light is lost at the second reflection it is fainter than the primary rainbow.

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