Rainbow

RAINBOW is a natural optical illusion!

The rainbow is a natural phenomena caused when sunlight enters the raindrops.

Mnemonic for rainbow

The most famous are…
ROY G BIV (red, orange, yellow, green, blue, indigo, violet)
VIB G YOR (violet, indigo, blue, green, yellow, orange, red)

What is a rainbow?

A rainbow is a beautiful multi-colored arch formed in the sky.

  • Red
  • Orange
  • Yellow
  • Green
  • Blue
  • Indigo
  • Violet

How is a rainbow formed?

  • Rainbow needs numerous raindrops to form.
  • The rainbow appears opposite to the sun.
  • The sunlight is actually made of seven colors. When the sunlight enters water droplets, the water droplets acts as prism causing the sunlight to split into a spectrum of seven colors.
  • When sunlight enters raindrops it gets refracted, that is sunlight bends as it passes through the raindrops. Bending of sunlight causes dispersion, here the sunlight is split into a spectrum of seven colors according to its wavelength. Then the split colors strike the inner surface of the water drop to form total internal reflection. At last, while leaving the rain drops the spectrum of seven colors refracts again to form the primary rainbow.
Sketch by René Descartes (1596 – 1650) on how primary and secondary rainbows are formed.

PRIMARY RAINBOW: The sunlight A enters the spherical raindrop at B, the light is refracted and dispersion takes place (sunlight is split into seven colors). The seven colors, then strike C where total internal reflection takes place. At D the seven colors are refracted while leaving the spherical raindrop to form the primary rainbow.

SECONDARY RAINBOW: The sunlight F enters the spherical raindrop. At G the light is refracted and dispersion takes place (sunlight is split into seven colors). The seven colors strike H to from total internal reflection and strike I. Again, the seven colors strike at I form total internal reflection and strike K. Now, at K the colors are refracted and they come out of the spherical raindrop to form the secondary rainbow.

How is a double rainbow formed?

The Double Rainbow
The Double Rainbow
  • In primary rainbow, two refraction and one total internal reflection takes place. In secondary rainbow, two refraction and two total internal reflections takes place.
  • The primary rainbow will be darker in color, whereas the secondary rainbow will be lighter in color.
  • In double rainbow, the primary rainbow becomes the inner one and the secondary rainbow appears outsides the primary rainbow.
  • The primary rainbow will have (ROY G BIN) while secondary rainbow the pattern is reversed (VIB G YOR).
  • In between the two rainbows the sky appears dark this is called Alexander’s band. It is named after Alexander of Aphrodisias who first described the dark sky between the bands of two rainbows about 2000 years ago.

Where is the end of the rainbow?

No, actually there is no end of the rainbow as it is a circle. To the observer from the ground, the rainbow appears to be an arc shaped as it is cut by the horizon. To an observer from airplane the rainbow appears circle.

Why is a rainbow circle?

The sunlight that enters the rain drops bends at an angle of 42º to form a rainbow. Hence, the rainbow is a circle.

Does everyone see the same rainbow?

No, two persons cannot see the same rainbow as it depends on the angle. Obviously, two persons will see the rainbow from different angles.

Can you touch a rainbow?

No, the rainbow is not a thing to be touched as it the reflection of light in water droplets.

Can rainbow appear at night?

Yes, it is called moonbow or lunar rainbow. The moon should be bright enough and it should rain. But, it is very rare.

Other than sky where is the rainbow formed?

Rainbow formed in waterfalls
Rainbow formed in waterfalls

It can be formed in waterfalls.


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