Why is a rainbow shaped like an arc?

[Melissa_dwl]

rainbow is caused when water drops bend light rays from the sun. but why is it like an arc?

[geofbob]

There is very good website explaining rainbows, including why it is an arc, at http://www.unidata.ucar.edu/staff/blynds/rnbw.html
. However, one fact that many people don't realise is that a rainbow is an optical phenomenon - ie there is no actual arc in the sky, just the appearance of one in the viewer's eye. In other words, the rainbow you see is not exactly the same as the rainbow I see.

[eram]

BHVMA writes: Different colors (different frequencies) bend at different angles when they enter and leave rain droplets. Droplets are spherical, so they act like lenses. You always see a rainbow opposite the sun because light entering the droplets bends slightly, reflects like a mrrror off the back surface inside the droplet, then is bent even more as it leaves the droplet, which is why sunlight becomes refracted into its different colors. That we see an arc shape is a function of our viewpoint - light refracts in all directions out of droplets, but we only see the light exiting on the path that enters our eyes. Due to the physics of light the angle of refraction are very precise, so we see rainbow colors only at the same exact angle all around the single axis of our line of sight. This phenomonon is particularly remarkable when a rainbow is seen in a full circle, as from a mountaintop, airplane, or even a tall building - any condition in which rain droplets are below you.