rainbow is a familiar sight when the sun is shining and
rain is falling. It can also be seen in the spray from
sprinklers and car-washes and in the spray above waterfalls.
Sunshine and showers are the ideal condition for rainbows
to be seen as they are formed by sun shining through
Where should you look for a rainbow, and when?
To see a rainbow you should look opposite from the
sun, against a showers or thunderstorms. In the UK,
rainbows tend to be most common in the late afternoon
and early evening period when the sun is in the west.
Remember: the sun rises in the east and sets in the
When does this type of fog form?
have been talked about for many years. The ancient
Greeks wrote about rainbows as a path made by
Iris (the messenger of the Gods) between heaven
and earth. Chinese mythology speaks of a slit
in the sky sealed by the Goddess Nüwa using
stones of five different colours. The Bible
in the story of Noah talks about the rainbow
of a sign from God that life would never again
be destroyed by floods. But perhaps the most
famous is that the Leprechauns keep their pot
of gold at the end of the rainbow.
Rainbows were first explained by the infamous
scientists Sir Isaac Newton and Rene Descartes
in the 1600’s. Descartes explained that rainbows
were caused by the reflection of light from
raindrops, but couldn’t explain why. However,
Sir Isaac Newton (shown in the picture) explained
with the use of a glass prism experiment in
1666 how raindrops separate light into the colours
of the rainbow we see.
What colours do we normally see?
The colours we normally see in a rainbow are
red, orange, yellow, green, blue, indigo and
violet, but really a rainbow has an unlimited
number of colours! There are a few different
mnemonics that help you remember the seven colours
of the rainbow, but one of the favourites is
‘Richard Of York Gave Battle In Vain’ – that’s
Red, Orange, Yellow, Green, Blue, Indigo, Violet.
is the science behind rainbows?
Sunlight is refracted in raindrops and is split
into the different colours that make up the sunlight.
The refracted light is then reflected off the back
of the raindrop at an angle of around 42 degrees,
which defines the angle in the sky that we see
the rainbow. The blue light is a shorter wavelength
and so is refracted at a bigger angle than the
longer wavelength red light, which means that in
the bow you see the red at the top and the blue
near the bottom. The spreading out of light at
different wavelengths is called dispersion. Because
we see only one colour from each raindrop, a great
many drops must be present for us to see a rainbow.
Why can we never reach the end
of the rainbow and find the pot of gold?
Unfortunately you will never reach the end of the
rainbow for two reasons. The first is that because
it’s an optical effect then it moves as you move
and so you can never reach the bottom. Secondly,
and perhaps more importantly, a rainbow is really
a circle, it’s just that we see half of it.
- Sometimes you can see a secondary rainbow. The secondary rainbow occurs when the light undergoes a double reflection in the raindrop. Because this is a second reflection the colours occur upside down compared to the primary rainbow, and they are dimmer. We call the area in between the two bows Alexander’s band after the ancient Greek Alexander of Aphrodisias who wrote about it. It is possible on very rare occasions to see a third bow, but as by this stage the light is very dim and it appears in the direction of the Sun it is extremely difficult to spot.