why is a rainbow shaped like an arc?

Answer 1

It is not.
You see it like that because, from your point of view, those are all the points with the correct angle for you to see the colors diffracted by a shower.
If you move, the arc will move with you.

Answer 2

Its all to do with where you view the rainbow from. To answer this question will involve the use of some simple geometry. The first point to note is that a rainbow is not a two-dimensional arc; it is in fact a three-dimensional cone with the apex at the observer’s eye.

The reason that it appears to be only a two-dimensional flat object is the same reason spherical bursts of fireworks in the sky appear in the form of a disc, which is because there is no evidence of distance. All the drops, which disperse light towards the observer to form the rainbow, lie in the shape of a cone with many different layers. The outside layers disperse the red component, the layer below that the orange component and the layer below that the yellow etc. In reality there will, possibly, be many layers, which will disperse the spectrum of colours. Including those outside the normal range of the human eye (450 – 750 nm).

In order to see this further, consider only the dispersion of the red component of the light*. It has been shown previously that the red component is seen when the angle between the incident rays and dispersed rays make an angle of 42°. Of course beams are dispersed at 42° from drops all over the sky in all directions up, down, left and right. However the only red light, which reaches the observers eye, comes from the water drops which are on the cone with side to axis angle of 42°. I.e. where the angle between the line of incidence, (which is in the antisolar direction) and the edge of the cone make an angle of 42°.

If the observer’s eye is at the apex of the cone, then to see the violet part of the bow they would need to look at 40° to the conical axis. Therefore, the cone that produces the violet component comes from a cone inside that of the one which produced the red part. A similar argument can be made for all the other colours and, because different colours are formed by different cones, then when they are viewed from the apex they appear in the form of a bow.

* Again the colours are being considered as the seven traditional ones.

Because only the drops on the cone with apex at the observer’s eye are responsible for the formation of the rainbow, which an observers sees, then an important implication is that each person looking at a rainbow is really looking at a different rainbow.

I.e. that everybody sees his or her own personal rainbow and two people can never see the same rainbow as their eyes cannot occupy the same space at the same time.

This last statement also leads to the interesting fact that both of the observer’s eyes actually see a different rainbow, as they are both at the apex of different cones.

Another consequence of the fact that there are no distance cues is that the rainbow an observer sees is always at right angles to the observer. When they move the drops that form the rainbow change, because there is a different cone of drops with an apex at their eye, the rainbow also appears to move with them. However it is in fact a different rainbow the observer sees. So it is impossible to see the side of a rainbow.

Throughout this reply the rainbow has been referred to as a bow, arc, or semi circle. However, I should have indicated, that in fact the rainbow is formed by the light reflected from a circle of drops on the cone at a common distance from the observer; Including the ones on the bottom side of the cone and therefore a complete rainbow is really a full circle.

So why do we usually see only a semi circle? The reason for this is simply that the ground usually prevents a full rainbow from forming, due to the fact that sunlight cannot be reflected off moisture in the ground. However, if you were in an aeroplane or on a high mountain when a rainbow formed, allowing for cloud cover, you should be able to see a complete or almost complete circle.

Answer 3

It's actually a complete circle. Or to be more exact, it's cone-shaped, with the small end near your eye and stretching away as far as the atmosphere goes - or the wet part of it anyway (imagine looking through the mouthpiece of a megaphone).

You don't see the bottom half because the earth is in the way. If you were high enough (in a very high plane, say), you would see a complete circle.

The water droplets have to be in the right place to reflect the colours into your eye, and that place is anywhere in that cone-shaped bit of atmosphere relative to your eye,

That's why you'll never find that pot of gold, because as you move towards it, the cone moves forwards with you.

Answer 4

It all has to do with the length of the colors. The shorter the lenght the closer to the inside of the rainbow. The longer the length the closer it will be to the top. The same principle applies when you go in the ocean. The deeper you go the more colors you loose. The first will be red. The last to be lost is blue because it has the longest lenth. Blue is only lost to darkness.

Answer 5

The way the colors are diffused by liquid droplets projects it in the shape of a giant double arc if the liquid is rain. If the liquid is simply water from the hose for instance, the arcs are smaller and often cut off.

Answer 6

It follows the shape of the earth.

Answer 7

Try this website. Hope it helps.

http://askabiologist.asu.edu/research/seecolor/rainbow.html

Answer 8

because the earth is shaped like a sphere

Answer 9

thats like asking why is a tree formed with bark and leaves.
hope you find your answer

Answer 10

it isn't it is a circle, but u only see half of it