This circular rainbow was generated by light from the morning sun coming through a peephole in a door. Why do you think this happened? Why do you think that the red was on the outside and blue on the inside?
Different wavelengths of light have different indexes of diffraction in different materials. That means that different colors bend differently. This is the same reason that the sky is blue during the day when the sun is directly overhead but red when the sun is rising or setting. This is a concept known as Raleigh scattering.