Light travels in a straight line in a uniform medium, but it bends when the medium changes. For example, if you look at a pencil partially submerged in water, it appears to be bent at the surface of the water. This is due to the bending of the light coming from the submerged part of the pencil when it passes from the water to the air. Rainbows occur when the light of the Sun passes through millions of tiny water droplets. In this case, light is bent both when entering the droplets and when going out of them. The bending further differs from one colour to another, which leads to the colour gradient. In fact, the light of the Sun is bent by a precise angle for each colour, and this is why it forms an arc if the Sun was right behind you, the rainbow would form a circle defined by this angle.
Picture Credit : Google
