Light scattering causes the halo that appears on street lights. On rainy days, the air is full of countless small water droplets, each of which is like a prism. The street light emits different wavelengths of light, and the refraction angles of light of different wavelengths (colors) in small water droplets are different. The light spreads out as it passes through the small water droplets, and different colors of light are separated. Light refracts in these droplets and forms a halo around them.
More reading: Illuminance Uniformity of Street Light: How to Measure And Improve
