Difference between vibration and wave.And why the rainbow is curve?

The  rainbow   is curved because of the required observation angle. The dispersion of light in a  rain drop   requires a specific angle between the observer and the sun in order to be seen. Hence, the curved (arc) rainbow is seen. Not all rainbows are arcs however, other shapes can occur in different condition.

Rainbows are always circular. Rainbows are formed by small water droplets (spherical in shape) in the air splitting the suns light into colors. Each color has a consistent angle to the incoming light and so makes a circle (like a compass). When we see rainbows we see them as arcs when in truth they are formed as circles. There is a point called the antisolar point that the rainbow seems to form around. We do not see the full circle of the rainbow because the horizon gets in the way. As the sun goes down we are able to see more of the rainbow, and the higher the sun is in the sky the smaller the arc seems. Sometimes people at high elevations, like in planes, see rainbows as full circles because they do not have the horizon to block their view.

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