The process that makes the sun appear red at sunset is called Rayleigh scattering. This is the scattering of light by particles that are smaller than the wavelength of light. When sunlight passes through the Earth's atmosphere, the blue and violet light is scattered more than the red light. This is because the blue and violet light waves are shorter than the red light waves, and they are therefore more likely to be scattered by the molecules and particles in the atmosphere. As a result, the red light is what reaches our eyes, and the sun appears red at sunset.