Why does the rainbow form semicircle?

Answer 1

There's an excellent explanation here http://www.eo.ucar.edu/rainbows/

Answer 2

Actually it doesn't. It forms a circle... Two infact (there is always a slightly fainter rainbow accompanying the main one, which is not always visible).

The reason we see it as a semi-circle is only because we (usually) see it from the ground, so so, the bottom half seems to disappear below the horizon.

If you are viewing the the rainbow from a high vantage point. Say a Tower or sky-scraper, you will see the whole cirlce. I've seen it and it blew my socks off. Rainbows are a thing of beauty.

2 little facts for you:

1. No two people are ever looking at exactly the same rainbow, because if they are standing in two different positions (even if they are next to each other) they are looking at the effect from a slightly different angle. Therefore, the water droplets which are causing the effect for man.1 are working differently for man. 2.

2. When you are looking at a rainbow, face it head on. The sun is always directly behind you. Without fail... If this were not the case you wouldn't be looking at a rainbow... The sun's light passes you and effects the water droplets directly in front of you... you will always be in between the sun and the rainbow.

In reality rainbow's don't really exist. They are merely a trick of the light, which is why you'll never find that pot of gold.

Answer 3

A rainbow is formed when light (naturally from the Sun) hits a droplet of water, the different wavelengths of the colours cause them to be bent by different amounts as they enter the droplet and so spread out in a fan, the light then is reflected from the opposite side of the droplet and comes out the side it went in being bent more in the process. That is why rainbows are seen when looking away from the Sun.
Because the Sun is so far away, all droplets will see the light coming from the same direction no matter how far apart they are, the light is bent (or refracted ) by a specific angle for each colour by each droplet so that colour will end up in a different place from each droplet but for every droplet that is at the correct angle from the Sun to the droplet and back to us we will see the same colour. It is like looking down a cone or funnel from the narrow end, all points on the cone are at the same angle, no matter how near or far. So the droplets causing the rainbow can be at any distance and the rainbow will look the same, neither bigger or smaller (that is why you can never reach a rainbow ). If the Sun is higher than you (almost always unless you are very high up and the sun is setting) the cone will be tilted down and the bottom will be below the horizon, the higher the sun the less of the cone is visible.
So difficult to describe in words, so easy and beautiful to see. Try making one yourself. When it is dark, with your back to a bright light, spray water in the air away from you ,to either side and above. Get someone else to spray the water farther away (and wider), move the light above you and below your eye level.

Answer 4

I don't think it does - there was a R4 progamme on this years ago (Punters) which investigated the rainbow - and they are circular given the right circumstances (over water I seem to recall) - the reason we see them as semicircles is because the land gets in the way. Also - for extra trivia value, we each see a different rainbow - the rainbow itself is the product of light refraction through raindrops / water vapour (?) and is interpreted by your eyes on the precise angle you are at.

Answer 5

I don't think the horizon comes into it at all. I saw a demonstration on TV in a fire service station where they put a fine spray into the air. From ground level only an arc could be seen but as the TV camera was hoisted up, more and more of the circular rainbow could be seen.

Not so sure about the light bouncing back from the direction it came from...If you take a triangular prism and shine a narrow beam of light through it you quite plainly see the refracted light coming out of the other side of the prism.
As it travels thru the prism its quite easy to that its bent twice due to the refraction but very little internal reflection.
Nice image here

Answer 6

It doesn't, it forms a circle. One of my Dad's fave stories is that he has skydived right through the middle of a rainbow! Basically, when it is, or has been raining, the air is full of water droplets. These act as prisms, which means that when a beam of sunlight hits it, it gets all bent in the middle, and split up into seven colours.
This is because what we see as 'white' light is actually a combination of lots of different types of light - the seven visible colours, and some invisible colours like ultraviolet and infrared.
Usually, light beams travel in straight lines, so all the different colours reach our eye at once, but when light hits a prism, the different colours all get slowed down by a different amount, and so each changes direction and they spread out _think of a toy car hitting carpet at an angle - it turns a corner).
Because red gets bent furthest, the light makes a circle, with red on the outside and violet on the inside.
And it's very pretty!!

Answer 7

The 'doughnut bisected by horizon' is what I've always understood to occur.

Just thought I'd note that 'the seven colours of the rainbow' are a cultural construct. Some cultures see only four - I've also come across eight.

Anyone come across a culture that sees more than eight?

Helen

Answer 8

It actually forms a doughnut shape we only see half of it the other half is over the horizon which we could never see as it depends on our position and moving towards it would cause it to disapear.
This is becuase the (white) light that refracts through the raindrops and the light is centered a point where the light is heading but then sent into a circle around that point.

Answer 9

Actualy it is not in a semicircle.And it is an illusion.
Cause of rainbow is the dispersion of light through
raindrops. So it is visible as a semicircle

Answer 10

The sun refracting through water droplets gives the prism effect. Because of the wavelengths of light, the order is always the same.

A rainbow is actually a sphere, we see the edge of the sphere, cut in half by the horizon.

Answer 11

Rainbow can be straight,but, the Earth is round and the surrounding Sky goes along with the shape of the Earth, therefore, the Rainbow is Curved.