# secondary rainbow

## How rainbows are created

How Rainbows are created. Optics lecture series – IV

“Primary and Secondary rainbows”, a lecture in Optics.

This lecture was delivered on February 02, 2017.

Sunlight is white in color.

Do you know Newton would have greatly disliked this statement, do you know why? “Color is not the property of light” but a property of an agent making observations assisted by light. We need a separate article for that. Don’t we?

That means it comprises of 7 primary colors. VIBGYOR is an acronym for these basic colors: Violet, Indigo, Blue, Green, Yellow, Orange and Red. Each color of light corresponds to a different wavelength. Violet has the shortest wavelength and Red has the highest wavelength.

Accordingly Violet has the highest intensity or consequently energy and Red has the lowest intensity or energy. In other words Red is the faintest color in the primary visible spectrum.

Different colors or wavelength of light have different refractive indices, this fact is known as dispersion, that is, different wavelengths of light would travel in different directions upon refraction at any optically denser or rarer media.

That means different wavelength or color component of light would travel at different speed and correspondingly different angles, upon incidence on a media whose refractive index differs from the medium from where incidence occurs.

The above phenomena is known as dispersion. Prism is an optical device which shows such dispersion of colors. We say that the angle of deviation of different colors of light is different at refracting interfaces of optical media. This is mathematically given by the Snell’s law of refraction which we write as follows:

Rainbows are caused by dispersion of light. Dispersion is an optical phenomena where wave velocity or phase velocity of light depends upon frequency or wavelength of light. Different colors of light emerge at different directions during refraction in a transparent medium such as water droplets during rainfall. When sunlight falls on small water droplets rainbow manifests as the result of dispersion of the incident rays.