What is a Rainbow?

The Reason a Rainbow Appears

Rainbows appear when it's raining or soon after it's stopped if the sun appears.  We see a rainbow because sunlight reflects and refracts off millions of water droplets in the air. The light is then reflected back to our eyes.

They say there's a pot of gold at the end of the rainbow, but does the end of a rainbow even exist?

A rainbow can be seen when there's water droplets in the air and the sun is shining from a low angle from behind where we're standing. 

As light passes from one medium into a denser substance it bends. The bending of light is known as refraction.

When light bends another optical phenomenon known as 'dispersion' takes place.  Dispersion is the splitting of light into its component colours.

Daylight, which is white light, is made up of red, orange, yellow, green, blue, indigo and violet. When these seven colours combine the human eye only sees white.

Dispersion takes place when white light passes through a prism.  Each raindrop acts as a tiny prism and that's why we see a rainbow.  

Light only refracts when it enters a more denser medium at an angle. The bending or change of direction is caused as the speed at which light is travelling changes.

The amount by which light bends depends on the speed at which it passes through an object. The more light speeds up or slows down, the more it bends.

A rainbow appears because the seven colours that make up white light refract at a slightly different angle as they enter the raindrop. They do the same once inside and refract again as they leave.

A double rainbow appears when light reflects twice within the raindrop.
We see a rainbow because of where we’re standing. We can see it because we're looking through the raindrops with the light from the sun coming from behind us.

It’s not possible to find the end of a rainbow because its shape depends on where we’re standing and the direction of the sun. However, if we view it from a different angle, the rainbow moves.

The rainbow is caused by the reflection of light off the raindrop which is round so it's the same shape.  We only see an arc or part of an arc because of where we’re standing. Were we to be in the air looking down, the rainbow is likely to be a full circle.

The fine mist from a garden hose will produce a circular 'rainbow' when the sun is shining on the water droplets from behind where you're standing. 

When light travels from air into water it slows down. So as the combined colours of white light pass from air into another medium (such as water), they slow down and pass through at different speeds.

As the colours exit the water droplets they continue to travel in their separated paths.  That enables us to see each colour individually.

Using a prism is an easy way to see the seven colours that make up white light. A prism is a transparent object with two identical ends and flat sides, like a triangle or a quartz crystal.If light travels through an object head-on, although its speed will change it won’t bend. If you hold a quartz crystal up towards natural daylight and look straight through it you won’t see a spectrum of colours.

Look through the crystal at an angle and you will.  The strength of the colours depends on how much the light is bending.

Rainbows in Quartz Crystals

The rainbows in some clear quartz are visible because of interference as light passes through the object.

In crystals with internal fractures or inclusions, rainbows can be far more prominent.  This is because these anomalies enhance the scattering and bouncing of light which enhances the appearance of iridescence.

Interference causes light to reflect off surfaces and different layers within the crystal.  Diffraction is the bending of light as it passes through and dispersion separates light into its component colours.

All three play a part in creating the rainbows that can be seen in a quartz crystal. 

Inclusions are often (but not always) made up of multiple fine layers.  Iridescence can occur almost anywhere light reflects off a surface with multiple layers.


The photos of the rainbows in this article are courtesy of Stan Celestian.   

The quartz crystal is from our collection.  The iridescence in the granite worktop was taken in my kitchen.

All images are clickable and redirect to the original photo.