Our eyes can see many colors of visible light. Sunlight is a mixture of many colors together. Light from the sun looks white to our eyes. All of the colors are in white light, they are just all mixed up. To see all the colors separately, you can use a prism. A prism is a piece of glass or plastic in the shape of a triangle. The colors of the rainbow in order are: red, orange, yellow, green, blue, indigo, and violet. Many people remember this by the first letters that spell Roy G. Biv.
A prism works because the different colors of light travel at different speeds inside the glass. Because the colors of light travel at different speeds, they get bent by different amounts and come out all spread out instead of mixed up. Violet travels the slowest so it is on the bottom (least bent) and red travels the fastest so is on the top (most bent). The other colors are somewhere in between. When the air is full of water, like after a rainstorm, the water droplets act like a prism and can make a rainbow. Rainbows are circular in shape because the prisms (raindrops) that created them are spherical.