What Makes A Rainbow Appear?

As many of you will probably have seen, a rainbow generally appears when it has been raining and the sun comes out. The reason we see a multitude of beautiful colours in the sky is the result of the sun rays shining through the raindrops at different angles and splits into different colours. A rainbow is actually an optical illusion. When the sun shines onto the droplets of water, it acts like a prism and takes on the form of a multi-coloured arc with red on the outside and violet on the inside. A double rainbow is when a second, fainter arc with the colours in the opposite order.
Even though a rainbow goes over a spectrum of colours, traditionally the full sequence of colours is most commonly cited as red, orange, yellow, green, blue, indigo and violet. Many people believe that because rainbows are magical and have some special reward at the end, such as a croc of gold. However, although they might appear magical to us, as far as I am aware, they don't have a special surprise at the end of them – although I can't say I've ever reached the end of a rainbow.

A rainbow is the refraction of light.  Refraction is the bending of light ever so slightly which results in the colors of the rainbow.

Adding to the previous answer: The light is refracted by some form of water in the air (such as a raindrop after it rains)

What is a rainbow?  

A rainbow is merely a large band of parallel stripes, blended at the rims, which displays the full spectrum of colors that make up the sun's white light. This brilliant display appears to the naked eye when the sun's light breaks up as it passes through, the prism-like raindrops during a rain-shower.

This immense, curved spectrum of light appears only when both the elements of sunshine and rainfall present. As the sunlight enters the falling raindrops, it breaks up into its true colors of red, orange, yellow, blue, and violet. These colors are always arranged according to their wavelengths, with red being at one end of the spectrum, and violet at the other. Once inside the droplet, the particles of colored light bounce from side to side, reflect off of the far side of the droplet, exit the droplet, and reassemble, according to their wavelengths, to form a rainbow.

Simply because you happen to be in the right place at the right time, when both elements necessary to form a rainbow are present, does not mean that you will actually see one. For the human eye to see these multi-colored bands, ranging from red to violet, his body must be strategically positioned between the sun and the rain, with his back to the sun.

If the sun, the eye, and the center of the rainbow's arc are not in a straight line, the show is over, before it began. This explains why we only see rainbows in the early morning or late afternoon…it is physically impossible for us to align our eyes with the sun at other times of day, as it is high above our heads! Logically, a morning rainbow appears when the sun shines in the east, and the rain falls in the west, and an afternoon rainbow appears when the sun shines in the west, and the rain falls in the east. If lucky, and a bit superstitious, and you do find a pot of gold somewhere over the rainbow, please let us know. After all, we gave you the directions!

The sun after the rain