Understanding the Science of Rainbows
What is a rainbow?
Everyone loves a rainbow! Even the grumpiest person cannot fail to feel lifted when they see this magical splash of colors in the sky.
Rainbows are a meteorological and optical phenomenon and have inspired, mystified and awed people across the world for thousands of years. Even though we now know its scientific explanation, the rainbow never fails to lift a heart and cause people to pause in wonder.
This page looks at the scientific reasons for why a rainbow occurs, how a rainbow is formed and the various characteristics a rainbow can have.
Table of Contents
When will you see a rainbow?
For a rainbow to occur there needs to be three things:
1. You need the sun to be shining behind you.
2. There needs to be a rain cloud or moisture in the air in front of you.
3. The sun must be shining through the rain cloud at an anti solar point of 42 degrees. An anti solar point is where the shadow of your head is, directly away from the sun.
Look at this photo below....
Can you see the people sitting on the bench? They have the sun shining behind them. You can see there's rain in the cloud in front of them. The sunlight is shining through millions and millions of tiny water drops, and as it does this the light refracts and disperses to show a beautiful spectrum of colors. Read on to find out how and why this happens!
You will see a rainbow at an anti-solar point of 42 degrees
The colors of the rainbow
Why do we always see the same colors in a rainbow in the same order?
Humans see light as white, but it is actually made up of many colors that we can't see because light moves so fast.
When the beam of sunlight passes through the water droplet it slows down and spreads out, separating each color that makes up white light into a spectrum of colors. This is called "dispersion". The colors of a rainbow are always seen in this order:
RED, ORANGE, YELLOW, GREEN, BLUE, INDIGO and VIOLET
There are other many other colors, but we can't see them through our human eyes - like infrared and ultraviolet!
Each color in light is measured in wavelengths - some travel on longer wavelengths than others. Red, for example, is on a longer wavelength than violet. Red bends and changes direction when it travels through the water droplet much less than violet does. The index of refraction is the measure of speed of the wavelengths of color. You will see the colors of the rainbow in this same particular order because of the index of refraction.
If you want to learn more about the mathematics of rainbows , read this excellent guide The Calculus of Rainbows
You can see how light is dispersed through a raindrop by experimenting with a glass prism....see this video
Light refracting through a prism
Why Rainbows are an arc - understanding light refraction
As the light travels through the water drop and disperses, it also does something called refraction.
Refraction is what happens to the light when it hits the water drop, it changes speed and bends. You can see this happening in the photo here of the glass prism on the right, refraction is happening as the white light (left) bends and disperses through the droplet.
So the reason why rainbows are an arc is because of the bending of and dispersing of light through millions of water droplets.
A more in depth article about refraction can be found here: "Snell's Law of the Refraction of Light."
The discovery of why rainbows happen
Rene Descartes (1596 - 1650) was a famous philosopher who came up with the modern theory for how rainbows occur in 1637. He was the first person to make the relation between round water drops and the interaction of refracting light. This is his sketch for how primary and secondary rainbows are formed.
A primary rainbow
Primary and secondary rainbows and the anti solar point
Primary rainbows shows red as the color on the outside, leading through to violet on the inside. You will see a primary rainbow at an anti solar point of 42 degrees. An anti solar point is the shadow of your head, directly away from the sun. In the photo above you can see the primary rainbow very clearly.
Secondary rainbows occur when the water droplets in the primary rainbow reflect light twice. Because it is the reflection of the primary, the colors are reversed. The colors are not so clear in the secondary and it is generally fuzzier and flatter than the primary. This rainbow occurs at 51 degrees from your anti solar point. Take a look at the image below, this shows both primary and secondary rainbows clearly.
Double rainbows showing primary and secondary bows
Characteristics of rainbows - Alexander's dark band
Alexander's Dark Band is the name for the dark sky between the primary and secondary rainbows you can see it really clearly in the photo above. It was named by Alexander of Aphrodisias, a Greek Philosopher who first noticed these optical phenomena in 200AD.
Alexander wrote commentaries on Aristotle's works and it was in writing a commentary to Aristotle's "Meteorology" book that he mentioned the dark sky between rainbows.
Primary bows light up the sky inside of the arc, the secondary rainbow is a reflection of the primary so it brightens up the sky on the outside of the arc. This makes the sky appear darker between the bands.
Can you see Alexander's dark band here?
A Supernumerary Rainbow is a selection of smaller multiple green, pink and purple colors in the inside of the main rainbow. The extra bands are created by the interference of light waves on the water drops. The colors are lighter than those in the primary rainbow, and can change color too.
The photo above shows the Supernumerary Rainbow in full, and the photo below is a close up of the extra bands.
You can sometimes see a supernumerary on the outside the secondary rainbow, but these bands are very faint.
A very good and detailed explanation of how these bands are formed can be found at www.atopics.co.uk
How many bands can you see in this supernumerary?
Twin rainbows are joined together at the base, unlike double rainbows which are separate.
There is currently no agreed explanation for twin rainbows, there could be a variety of reasons why they occur. Some scientists believe that they may be caused by a mixture of ice crystals and water droplets. Or twin bows could because of non spherical water droplets, this makes the light refract slightly differently, resulting in a twin rainbow.
Another explanation is that the second bow is a reflection of the first, but this hasn't been concretely proved because there have been rainbows that have a reflection and a twin too!
Rainbows in dew and mist - and a rainbow experiment for kids!
This photo below shows a rainbow of crepuscular rays in sprinkler spray. A crepuscular ray is light that appears from a single point in the sky, and look like columns of light. The columns of light here are from the trees above and as they shine onto the water droplets from the sprinkler they make a rainbow.
A fun experiment is to make a rainbow with hose water spray! On a sunny day go outside and set up a hose to spray a fine mist, stand with your back to the sun and turn the water on. Remember that the anti solar point is the shadow of your head and that the angle for a rainbow is 42 degrees from this point. Move the hose slowly from the shadow of your head to what you perceive 42 degrees will be - you will then make a rainbow! The brightest rainbows are later in the day, so try this as late afternoon for the best results.
Rainbow in water spray
Red rainbows can be seen at sunset, the low sun lengthens the red and yellow wavelengths in light and scatters the shorter greens and blues.
Moonbow, Kula, Hawaii
A moon bow is a very rare occurrence and you are lucky to see one! It is caused by the moon behind you, rather than the sun. It looks like something from a dream doesn't it?
A Fog bow is formed the same way as a rainbow, but the water droplets are smaller in fog so you cannot see the colors. This ghostly arc is most commonly seen on mountains and in cold sea mists. This photo was taken on a hillside above San Francisco.
These amazing halos are made up of tiny ice crystals known as diamond dust. These ice crystals are found in cirrus clouds, which are very high up wispy clouds at about 3 -6 miles up in the sky. Each minuscule particle of diamond dust refracts the light at 22 degrees. Just like the raindrops act like prisms in dispersing the light, so do the ice crystals and a rainbow colored halo appears around either the sun or the moon. Halos are also known as glorioles or icebows and folklore says that when you see one, rain is on its way.
An ice halo made from diamond dust
The circumhorizon arc
This stunning image below is a Circumhorizon Arc, nicknamed a "fire rainbow". But it is not a rainbow! The circumhorizon arc is created by a high sun, at about 58 degrees and is created by the "diamond dust" ice particles found in high level cirrus clouds. This optical phenomena is seen in mid latitude countries, so whilst it is very rare to see this in countries nearer to the Northern Hemisphere. Do you live in Los Angeles, Houston or Melbourne? If so then you can see this beautiful sight up to 6 times over the summer, if you live in Northern Europe then to see this would be very unusual indeed!
A circumhorizon arc, or fire rainbow
A circumzenithal arc is also created by the tiny particles of diamond dust ice crystals and the sun. The ice crystals must be flat and six sided, the sun has to be angled at 32 degrees - with these conditions you can see why this is a rare occurrence! It is most commonly seen in colder climates and will often stay visible in the sky for up to 30 minutes.
A smiling sky - the circumzenithal arc
Different parts of the world see different bows more often than others, have you seen any unusual rainbows? Vote below!
What Type of Rainbow Have You Seen?
© 2010 LadyFlashman