Light travel straight but rainbow is not straight.why?
Answers
Rainbows are not just curved, but in fact are circular. The reason for this shape is that rainbows are not fixed objects, but are the result of an optical phenomenon that occurs when sunlight is reflected by water droplets at a specific angle as shown in this cartoon. As the light from the sun passes through the water droplets suspended in the air, because the refractive index of water depends on wavelength, each droplet will effectively act as a small prism, splitting the light into its spectral components as shown here.
The key to explaining the shape of rainbows that the strongest reflection occurs at an angle of 42o between the direction of the sunlight, the water droplets, and your position as shown in the diagram above. You can have such an angle not just at one position, but basically around a cone where this angle stays fixed at 42o as you can see in this picture. Normally we think of rainbows as bows, but that is only because the ground gets in the way. If there was no obstruction, a rainbow would look like a full circle.