This article, "Rainshine", was written in an attempt to convey the essence of what a rainbow actually is in simple words that most people would understand. It is hoped that readers will now really comprehend what they are looking at when they see a rainbow rather than just learn a few words about it.

Dan Hinmef

When sun shines on rain, all the rain glistens but we only see a narrow strip because our view is restricted - like looking through a letterbox.

Turn a coffee mug on its side. Light is reflected off all its lit surface yet you cannot see it all at once. You see only a narrow strip at one place on the curve.

If you move your head to one side you will see a different strip of that light at a different part of the curve (move your mouse left and right across the top of the picture on the left to see this). This is because the light is reflected at a different angle from each part of the curve and you have to be at that angle to see it.

Remember, all the surface is shining all the time but you can't see it all. Move your mouse over the bottom of the mug in the picture to see what that might look like if you could.

There is no Rainbow in the Sky

In the same way, there is no rain bow up in the sky. It is true that the light of the rainbow is there but it is the entire wall of raindrops that is shining. If you think about it, this must be so. All raindrops in the sun must be reflecting light not just the ones in a narrow strip.

Why is our view restricted to just a narrow arc of that wall of glistening raindrops? The rainstorm is not curved like a coffee mug so why can't we see the other raindrops shining as well as the ones in the narrow strip of the rainbow?

Most light passes straight through a raindrop because water is transparent but as the light goes through the drop some reaches the back at such an angle that it reflects off it. Because of the way that light bends as it enters and leaves water and because of the shape of the drop, this light is reflected back and out of the front of the drop mostly at an angle of about 40 degrees to the direction it entered. This means you only see those raindrops which are shining from that angle. The shine from the other raindrops misses you and, as with the coffee cup, you must move to see it.

Some think this means you will always see the top of a rainbow up in the sky at 40 degrees from the ground but that is only true at sunrise and sunset. At those times the sun's rays are horizontal - parallel to the ground - and do reflect back down off the rain to the ground at about 40 degrees. But when the sun is higher in the sky, the top of a rainbow will be lower because it is the angle between the sun, the raindrop, and you that is 40 degrees.

If the sun itself rises to 40 degrees up in the sky then to a viewer on the ground, the very top of a rainbow would seem to disappear below ground level. If it were not for the ground being in the way you would see the rainbow as a complete circle. Simliarly, a rainbow cannot be high enough in the sky for someone at ground level to see its full circle because then the sun would have to be so low behind the viewer that it would be below the horizon - it would have set. By going up high in aircraft you can see the full circle of the rainbow. Even on a high hill or mountain you could see more of the circle than is seen at ground level. Alternatively, you might see the full circle in a fountain or waterfall spray on a sunny day if you have the right vantage point.

When Searching for Rainbows
Look away from the Sun!

The best times to see a rainbow are on a warm day in the early morning after sunrise or late afternoon before sunset. If it's too cold and the raindrops start to form ice crystals in the air then a rainbow will not show; the drops need to be liquid to refract and reflect the light correctly. If you want to go rainbow hunting then try a day with mixed rain and bright periods. Look away from the sun. To see a rainbow you must be standing with the sun behind you.

Everybody has their own personal rainbow

Yes, everybody has their own rainbow because nobody can be at the same exact spot. Remember, there is no 'bow' up in the sky; all the rain is shining but you can see only a narrow strip of it. The rainbow you see must be your rainbow because it does not exist as an external object. It only exists as your restricted, 'letterbox' view of the whole rainshine.

But why is this band of light in an arc?

Firstly, you need to know that although sunlight is reflected from each raindrop at about 40 degrees, it is not all 'downwards' at 40 degrees as shown in the above diagrams. Light is reflected in every direction at 40 degrees: up, down, left, right, and everywhere in between, like the spokes of an umbrella on its side where every spoke is at 40 degrees to the handle.

If you could see one raindrop in the rainbow, all that you would see is a single point of light which is not the whole drop shining but the end of a single 'spoke'; all the other spokes from that raindrop point away from you.

In the sky inside a rainbow's arc, you cannot see any raindrops shining brightly at all. There is rain there - and the sun is shining on it but you can't see it. This is because you are looking directly 'into' the 'umbrellas' of each drop from the front. the 'spokes' of light reflected from these raindrops all point outwards and go around you completely. They point over you, to right and left, or into the ground ahead of you, but not at you.

But a raindrop actually in the rainbow itself, for example in the top right corner of the arc, will be pointing a 'spoke' of light towards you from the bottom left corner of the rain drop (8 o'clock on a clock face). At the top of the rainbow, a downwards pointing 'spoke' of light will point to you from the bottom (6 o'clock) of each drop, and so on.

Now, look at a wall and imagine it is a wall of raindrops, each drop with a tiny umbrella of light-spokes coming out from it. Take a square of cardboard and draw a line from one corner to another to form a diagonal. Stand it upright and place one upright edge against the wall with the marked diagonal at the top next to the wall. Tilt the card left and right. The top corner will trace out an arc.

The TOP EDGE of the card is the line of the light from the sun.

The diagonal is the light reflected back from a single 'light-spoke' of each raindrop along the curve.

The viewer is at the bottom of the diagonal.

In the diagram you will see the corner of the angle between the top line from the sun and the diagonal back to the viewer is marked in red. With a square card this angle will be 45 degrees but the principle is the same no matter what the angle. If you are not used to degrees, this gives you some idea. 40 degrees is just a slightly narrower angle. Imagine the card is a slightly flatter rectangle instead of a square and that angle will be about 40 degrees.

The sky seems full of rainbows

Wherever the sun reflects back from the rain a viewer will see a rainbow. It is as if the rain is covered with billions of rainbows overlapping one another and if you walk along you see a different one at every step. In fact there are none. Each one is simply your restricted view of the entire wall of rainshine. The diagram on the right shows just two of these perceived rainbows but the one on the right would not be visible from the position at the left and vice versa. In all cases, the angle between the light from the sun (shown in yellow) and the light reflected back from the rain (shown in lilac) to the viewing point, is always about 40 degrees.

Why do all the rays of light come to a focus at one spot? They don't! They come to a focus at millions of spots. The diagram doesn't show these other rays of light because there are so many that the diagram would be completely filled with lines! Nobody can see any of them except the ones that just happen to focus in their direction.

Why does a rainbow have all the different colours?

The white light from the sun is a mixture of all the colours. When this sunlight is reflected back from a raindrop it is not all at exactly 40 degrees. It varies with the colour. It actually ranges from 40 to 42 degrees. Red is at about 42 degrees whereas blue is at about 40. This diagram only shows one ray of light but remember the drop will be reflecting a complete 40 degree cone of colours up, down, in all directions.

The drops on the outer edge of a rainbow are at just the right angle to reflect their red light towards you. The blue light which they also reflect is bent wider or higher so it misses you, for instance, the blue reflected from the raindrops at the very top of the rainbow is bent upwards slightly higher than the red and so goes over your head - only the red reaches you.

The drops towards the inside of the rainbow's arc are at just the right angle to reflect their blue light towards you but their red light misses you.

Red light is reflected from every raindrop - even those that look blue to you. In the same way, every colour is reflected from every raindrop no matter what colour it looks!

In Conclusion

Hopefully this article makes clear the essential nature of the rainbow but there is more. Try the links below which have additional fascinating information about the rainbow as well as many photographic images.

Dan Hinmef