"Three things must happen for you to see a rainbow's colors. First, the sun must be shining. Second, the sun must be behind you, and third, there must be water drops in the air in front of you. Sunlight shines into the water drops, which act as tiny prisms that bend or "refract" the light and separate it into colors.
Actually, the rays of light bend twice. As they enter the drops, the rays of light bend, then reflect off the back of the drops. Then they bend again, this time while exiting the drops. That's when the light appears before our eyes.
Each drop reflects only one color of light, so there must be many water drops to make a full rainbow. You'll see the brightest rainbows when the water drops are large, usually right after a rain shower.
The rainbow is circular because when a raindrop bends light, the light exits the raindrop at an angle 40 to 42 degrees away from the angle it entered the raindrop.
The violets and blues bend at a 40-degree angle, and the oranges and reds bend at a 42-degree angle. As a result, the only beams of light you see are from raindrops that are 40 to 42 degrees away from the shadow of your head. This gives the rainbow its curved appearance."