Rainbows.

[carrust]

Do all rainbows that appear in the sky have the same radius? If they to, what is the criteria that sets the size. If not, why not.

[vascop]

The light comes to your eye from the sunlight refracted and reflected from raindrops. The sun is behind you and the rain in front. So the rainbow light is coming from those raindrops closest to you right out to those furthest away from you. As the angle between the rainbow and your eye has to be about 42 degrees (this is calculated from physics - refraction etc), this would seem to indicate that as you walked towards the nearest rain, the radius of the rainbow would decrease and that the top point of the rainbow would be lower, and vice versa.
However this can all be affected by how high the sun is above the horizon. The higher the sun is in the sky the lower will be the top of the rainbow and the further away from the rain you would have to be to see the rainbow at all.
If the sun is vertically overhead then you would not see a rainbow unless you were high above the ground e.g. in an aeroplane.
The lower the sun is above the horizon the closer to the rain you can get and still see a rainbow - the higher the sun is above the horizon the further away from the rain you need to be to still see a rainbow.

[heathfield]

Rainbow's radius = distance to rain / tan 40�

[vascop]

I think you mean:
distance to rain x tan 40 which means that the radius is roughly equal to the distance from the rain since tan 40 is approximately 0.83.

[vascop]

I meant also to add that the radius is only roughly equal to the height of the rainbow when the sun is very low. When the sun is high the height of the rainbow is less than the radius.

[carrust]

Thanks for your answers.

Errr, I think I got that...my blood runs cold when tangent or sine is mentioned...looks like you 2 are science or maths teachers:-(

[chakka35]

There's some fascinating stuff about rainbows in Richard Dawkins' book Unweaving the Rainbow.

[Rev. Green]

The centre of a rainbow is where the shadow of your head would be. All raindrops which appear the same colour (e.g. green) are on a cone centred on the centre of the rainbow, with its point where your eyes are. So the "green" raindrops which are close to you have correspondingly smaller radii than "green" raindrops which are further away.