Formation of rainbow | Explain the formation of rainbow
Formation of rainbow | Explain the formation of rainbow :- One of the most interesting sources of color in nature is the rainbow. The basic cause of the formation of rainbow is a combination of refraction, internal reflection and dispersion of light by the rain drops of water present in the atmosphere. When the order of colors in the rainbow ranges from violet to red (40° to 42° i.e., red color on outer edge and violet color on inner edge) then we see primary rainbow. This rainbow is intense. It is formed by two refractions and one internal reflection. Sometimes a second outer rainbow also appears in the sky which is called the secondary rainbow. The order of colors in the secondary rainbow ranges from red to violet (50° to 53° i.e., violet color on outer edge and red color on inner edge). It is less intense than primary rainbow. It is formed by two refractions and two internal reflection.
Conditions under which a rainbow can be observed
(Formation of rainbow | Explain the formation of rainbow)
The conditions for observing a rainbow is that the Sun needs to be shining low in one part of the sky (say near western horizon) while it is raining in the opposite part of the sky (say eastern horizon). An observer can therefore see a rainbow only when his back is towards the Sun.
Let us now explain the formation of rainbow
Below figure shows the basic process of the rainbow formation, i.e., a combination of refraction, internal reflection and dispersion :-
Above figure might lead you to think that the top edge of a rainbow is violet but in fact, the top edge is red and violet is on the bottom. The rays emerging the drop in the above figure are spreading apart, so they can’t all reach your eye, as a result, we see red light from one drop and violet light from another drop below it as shown in figure below :-
In other words, the colors we see in a rainbow refract toward our eye from different raindrops, not from the same drop. Thus in the primary rainbow, the observer sees red color on the top and violet on the bottom.
Formation of Secondary Rainbow
(Formation of rainbow | Explain the formation of rainbow)
Secondary rainbow is four-step process, a combination of refraction, two internal reflections and dispersion. In secondary rainbow light rays undergoes two internal reflections inside a raindrop, instead of one as in the primary rainbow. The intensity of light is reduced at the second internal reflection and hence the secondary rainbow is fainter than the primary rainbow. Also, the order of the colors is reversed in secondary rainbow as shown in figure below :-
