The Rainbow FAQ
Frequently Asked Questions about Rainbows
- Q. How is a rainbow formed?
- Q. Why do you see rainbows after it rains?
- Q. Can you see a rainbow anytime of the day?
- Q. Can rainbows appear in random places?
- Q. How do you make a rainbow?
- Q. Why is it darker outside a rainbow than inside?
- Q. How do a rainbow get it's colours?
- Q. What are the colours of a rainbow?
- Q. Why are there only 7 colours in a rainbow?
- Q. How do you remember the colours of a rainbow?
- Q. Why is red on the outside of a rainbow and blue inside?
- Q. What is the shape of a rainbow?
- Q. Why do rainbows always appear round?
- Q. Why are rainbows arcs or semi circles?
- Q. Can a rainbow be a complete circle?
- Q. Why is there sometimes two rainbows?
- Q. Why is the order of colours reverse in a secondary rainbow?
- Q. Can you see a rainbow at night?
- Q. Can moonlight produce a rainbow?
- Q. How far away is a rainbow?
- Q. Can you stand at the end of rainbow?
- Q. What is the anti solar point?
- Q. Is their a pot of gold at the end of rainbow?
A. You need a few things to see a rainbow. The main things are rain, you need that rain to be illuminated by bright sunshine, and the rain must be in the right position relative to you. A primary rainbow is always somewhere on an arc 42 degrees around the shadow of your head (called the anti solar point). So the brightly illuminated rain must be in this direction away from you to see a rainbow. For a secondary bow it is 51 degrees around the shadow of your head.
A rainbow is caused by the refraction and internal reflection of light inside rain drops, which results in the white sunlight being separated out into the colours of the rainbow. See the detailed pages which are linked to at the top of this page for a more in depth explanation.Q. Why do you see rainbows after it rains?
A. You need a few things to see a rainbow. The main things are rain, you need that rain to be illuminated by bright sunshine, and you need to be in a position that the rain lies on the circle 42 degrees around the shadow of your head. So if you get rain from a shower that moves in the direction away from the sun relative to you then you have a good chance of seeing a rainbow. You are more likely to see rainbows on days with scattered shower or storms than on dull overcast rain days.Q. Can you see a rainbow anytime of the day?
A. If you want to see a natural rainbow in rain, the answer is no. To see a primary bow in the sky above the horizon, the anti solar point, the shadow of your head, must be less than 42 degrees below the horizon (and 51 degrees for the secondary bow). This means you usually can only see rainbows in the morning and afternoons before the sun is too high in the sky. The lower the sun the higher in the sky the rainbow will be.Q. Can rainbows appear in random places?
A. Sort of. As explained in one of the above answers, the primary rainbow is some part of an arc 42 degrees around the shadow of you head. But if you call something like a fountain a random place, then on a sunny day you can walk around the fountain until the spray from it lies on this arc, and you will see a rainbow. You can do that for lots of random artificial sources of drops of water in the air, like garden sprinklers, irrigation sprays, and the spray from artificial water falls. To see a secondary rainbow you would use 51 degrees instead of 42.Q. How do you make a rainbow?
A. All you need is bright sunshine and lots of water drops. Go outside on a hot summer day with a hose with a nozzle on it which will produce a fine spray of water and spray water in the air in the direction you would expect to see a rainbow. The direction is 42 degrees away from the shadow of your head. 42 degrees is about twice the width of your strached out hand at arms length. So find the shadow of your head, put your thumb over it at arms length, move your streched out hand again one width, and that is the direction to spray the water. You should see a rainbow! The lower the sun the easier it is. If the sun is high you will have the ground as a background of the rainbow, and have less room between you and the ground to spray the water.Q. Why is it darker outside a rainbow than inside?
A. The angle of the primary rainbow is determined by the maximum angle light which has passed through raindrops can be scattered away from the anti solar point. For the primary rainbow light is scattered at all angles closer to the anti solar, which means inside the rainbow. Thus the brighter area is light scattered by raindrops in which the light has reflected once inside the raindrop. The secondary rainbow is caused by light which has reflected twice inside raindrops and is the minimum angle towards the anti solar point which this light can be scattered. Thus the light area outside the secondary rainbow is light scattered after reflecting twice inside raindrops. Light which has passed through raindrops will not be scattered into the area between the primary and seconded rainbows, and this darker region is known as Alexanders Dark Band. In this region the light is either background light or light which has reflected off the surface of raindrops.Q. How do a rainbow get it's colours?
A. A rainbow has colours because the index of refraction of light in water depends on the wavelength of the light, which means the same as colour, of the light, and because sunlight, which is white, is made up of a whole spectrum of colours.
If you look at the rainbow simulator to see the how the angle of the rainbow is determined by the minimum angle away from the anti solar point that light from the sun will be scattered, you will also see that this angle depends on the index of refraction. The longer wavelength coloured light, such as red, has a large rainbow angle, then the short wavelength colours, such as blue.
The index of refraction of light in water is a measure of the speed of light in water. All colours, or wavelengths, do not travel at the same speed.Q. What are the colours of a rainbow?
A. The colours of a rainbow are a continuous spectrum, not a set of discrete colours. This is where the saying "all the colours of the rainbow" comes from - there are many. However we typically only have specific words for a smaller number of colours which are part of the rainbow of colours. We normally say a rainbow has the colours red, orange, yellow, green, blue, indigo, violet. Rainbows also go into the infrared and ultraviolet, but we can not see this with our eyes.Q. Why are there only 7 colours in a rainbow?
A. As I mention in the question before this there are many colours in a rainbow, almost infinite if you want to look very very closely. However we only have specific common words for 7 of them, and thus it is commonly known that a rainbow has the 7 colours red, orange, yellow, green, blue, indigo, violet.Q. How do you remember the colours of a rainbow?
A. We commonly say that a rainbow has the colours red, orange, yellow, green, blue, indigo, violet. The common mnemonic I was taught to remember this is Richard Of York Gave Battle In Vain.
A. The colours are caused by the different wavelengths (same as saying colours) of light having different indexes of refraction (see question above). The maximum angle away from the anti solar point which light from the sun is refracted by the raindrops is greater for red than blue. Thus red is on the outside and blue is on the inside of the rainbow.Q. What is the shape of a rainbow?
A. Rainbows are an arc at a constant angle from the anti solar point (shadow of your head). For a primary bow this angle is 42 degrees, and for a secondary bow 51 degrees. So the shape is an arc along the circumference of a circle.Q. Why do rainbows always appear round?
A. Rainbows always appear round because they are formed at a constant angle around the anti solar point (the shadow of your head). The shape of something described by an arc a constant distance away from a point is a circle, which is called round.Q. Why are rainbows arcs or semi circles?
A. Rainbows are usually only partial - part of - the cirlce around the anti solar point (shadow of your head) on which rainbows are seen. They can not be square or straight, because of the way rainbows are formed. Read about how a rainbow is formed to understand this better.Q. Can a rainbow be a complete circle?
A. Yes. It is difficult to produce the conditions for this to occur. It isn't really possible to have a complete circle from a naturally occurring rainbow as seen from someone standing on the ground looking at a rain shower. However in an aircraft you could possible be able to look down on water drops in the atmosphere and see a rainbow. In reality you are more likely to be looking at cloud or fog where the water drops are very small. When the water drops get very small the wave like nature of light starts to dominate which will wash out the colours in a rainbow until the point the drops are too small and a rainbow will not been seen at all. So a complete circular rainbow seen from an aircraft is going to look more white than coloured, with the shadow of the aircraft seen in the center.Q. Why is there sometimes two rainbows?
A. A secondary rainbow is produced by light which has reflected twice inside raindrops. The secondary rainbow occurs on an arc 51 degrees around the shadow of your head (the anti solar point).Q. Why is the order of colours reverse in a secondary rainbow?
A. A secondary rainbow is produced by light which has reflected twice inside raindrops. The angle is the minimum angle (note not maximum as in the primary) away from the anti solar point in which this light can be scattered. It happens that the minimum angle for red is less than for blue, the opposite, of the primary and the order is reverse. Another way of looking at it is that the second reflection inside the water drop reverse the order of the light like looking at writing in a mirror makes it look backwards.Q. Can you see a rainbow at night?
A. Yes, the moon is bright enough to produce a rainbow, and while rare, people do see it. We all just have to get out more at night. Since the moon is not near as bright as the sun, the rainbow produced by the moon at night is much weaker than a rainbow produced by the sun during the day. Our eyes see dim things as black and white, not in colour. So a night "moonbow" will look gray not colourful.Q. Can moonlight produce a rainbow?
A. Yes it can. See Q. Can you see a rainbow at night?Q. How far away is a rainbow?
A. A rainbow is caused by sunshine on rain. The rainbow is as far away as the rain is. A rainbow can be seen in other sources of water drops, such as fountains, or even dew drops on grass or spider webs. In these cases it is easier to know how far away a rainbow is.Q. Can you stand at the end of rainbow?
A. A rainbow is not a physical thing by itself. When you see a rainbow you are looking at sunlight that has been reflected and refracted by raindrops. A rainbow caused by rain can get very close. As I say in the question above, the rainbow is as far away as the raindrops. The rain get fairly close but usually the cloud that is producing the rain will block the sun, or the rain itself will, and the rainbow will go away. So you can't really stand at the end of a rainbow.Q. What is the anti solar point?
A. The anti solar point is in the direction exactly away from the sun. When the sun is above the horizon the anti solar point is the shadow of your head.Q. Is their a pot of gold at the end of rainbow?
A. Of course the idea is that you can never reach the end of a rainbow, and since you will never get there to check, it can't hurt to think there is a pot of gold. Even if there isn't a pot of gold, a rainbow is such a beautiful thing that it should be considered a pot of gold in it's own way and we should be grateful for every chance we get to see one.