III. Weather Phenomena

4. Rainbow and secondary rainbow

Rainbow and Secondary Rainbow

Have you seen the beautiful seven-colored spectrum of light after a rain? When that happens do you often find yourself getting excited and saying to yourself, "Oh, that's a rainbow"? If you look really close, you can sometimes find two parallel spectrums of light. The brighter and more colorful one is known as a rainbow or a primary rainbow. The second spectrum of light we can occasionally see is called a secondary rainbow.

A rainbow occurs when light hits the numerous water drops in the atmosphere at a certain angle. It is an atmospheric phenomenon that is formed through optical processes such as refraction, dispersion, internal reflection and secondary refraction. If the light source is the sun then the rainbow will be colorful and bright; if it is formed by moonlight then the hue will be darker. As shown in Fig.1, when a beam of sunlight enters the water drop at point A, the degree of refraction of the different colors is variable. Violet light has the shortest wavelength therefore its degree of refraction is also the greatest; red light has longer wavelength therefore its degree of refraction is the smallest. The degrees of refraction of the other colors lie within this range. Therefore, a solar halo with violet light in the innermost and red in the outermost edge is formed, and that's why the rainbow we see is always violet on the inside and red on the outside. The visual angle (the angle from the ground surface to the vertex of the rainbow) is about 42° (Fig.2)

The refraction of light in the formation of rainbow

The path of light when a rainbow is formed

Fig. 1: The refraction of light in the formation of rainbow.

Fig. 2: The path of light when a rainbow is formed.

A secondary rainbow is different from a primary rainbow only in that it is formed by secondary reflection of light within raindrops. Therefore, when we see the light reflected by the raindrops the spectrum of the light is the exact opposite from that of a primary rainbow, which means the secondary rainbow is red on the inside and violet on the outside. It has a visual angle of about 50° and is concentric with the primary rainbow. Sometimes there will be third rainbow or even more in the atmosphere, but is dim and extremely rare.

The colors and the width of a rainbow are affected by the size of the raindrops. The bigger the raindrops the narrower and more colorful the rainbow is and vice versa. But if the raindrop is too small for dispersion or clear reflection of sunlight, there will be no rainbow. Furthermore, if the angle of the sun is too large (for example, around noon) or too small (during sunrise or sunset) a rainbow will not easily be formed. Since the rainbow is seen in our eyes as the reflection of sunlight through raindrops, it always appears opposite the sun. Therefore a rainbow in the morning is seen in the west and an evening rainbow is seen in the east.