Rainbow

A rainbow is a multicolored arc made by light striking water droplets. 

The most familiar type rainbow is produced when sunlight strikes raindrops in front of a viewer at a precise angle (42 degrees). Rainbows can also be viewed around fog, sea spray, or waterfalls. 

Rainbows are the result of the refraction and reflection of light. Both refraction and reflection are phenomenathat involve a change in a wave's direction. A refracted wave may appear "bent", while a reflected wave might seem to "bounce back" from a surface or other wavefront.

Light entering a water droplet is refracted. It is then reflected by the back of the droplet. As this reflected light leaves the droplet, it is refracted again, at multiple angles.

The radius of a rainbow is determined by the water droplets' refractive index. A refractive index is the measure of how much a ray of light refracts (bends) as it passes from one medium to another—from air to water, for example. 

There are some variations of rainbows, such as double rainbow, reflection rainbow, reflected rainbow, etc.

Double Rainbow

Sometimes, a viewer may see a "double rainbow." In this phenomenon, a faint, secondary rainbow appears above the primary one. 

Double rainbows are caused by light being reflected twice inside the raindrop. As a result of this second reflection, the spectrum of the secondary rainbow is reversed: red is on the inner section of the arch, while violet is on the outside.

Reflection Rainbow

A reflection rainbow appears above a body of water. A primary rainbow is reflected by the water, and the reflected light produces a reflection rainbow. Reflection rainbows do not mirror the primary rainbow—they often appear to stretch above it.

Reflected Rainbow

A reflected rainbow appears directly on the surface of a body of water. A reflected rainbow is created by rays of light reflected by the water surface, after the rays have have passed through water droplets. Reflected rainbows to not appear to form a circle with a primary rainbow, although their endpoints seem to meet in an almond-shaped formation.

Fogbow

A fogbow is formed in much the same way as a primary rainbow. Light in a fogbow is refracted and reflected by fog (water droplets suspended in air). A fogbow seen in the clouds is called a cloudbow.

Moonbow

A moonbow, also called a lunar rainbow, is a rainbow produced by light reflected by the Moon. The Moon itself does not emit light, of course. Moonlight is reflected sunlight, as well as some starlight and "Earthlight." Because moonlight is so much fainter than sunlight, moonbows are dimmer than rainbows.

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