Question

Why does a rainbow form an arc? Why does not it go straight?

Answer 1

Rainbow:

An arc of colors observed throughout the sky is generated by the refraction and dispersion of the sun's illumination in the atmosphere by rain or other liquid droplets.

The reason why a rainbow forms an arc and it does not go straight:

1. When sunlight strikes a raindrop, the red light waves are bent at a $42°$ angle from the sun's initial direction.
2. Violet light waves that are shorter are only bent at a $40°$ angle.
3. Because of this symmetry, the raindrop seems to be symmetrical when viewed along a line of sight that forms a $40°$ to $42°$ angle with the sun's direction.

As a result, we see the rainbow as an arc rather than a straight line.