Not all incident light striking a raindrop crosses the boundary into the watery interior of a droplet. Some of the incident light is reflected off the surface and a small proportion of that travels towards the observer.
- Incident light reflected off the surface facing an observer undergoes neither refraction nor dispersion.
- Because the outside surface of a raindrop forms a shiny convex mirror, reflected light diverges in every possible direction depending on its initial point of impact.
- Just as raindrops form the coloured arc of a primary rainbow, they can also reflect white light towards an observer.
- White light reflected towards an observer off the outside of raindrops helps to account for why the sky on the inside of a rainbow (between its centre and coloured arcs) appears brighter and lighter than the area of sky outside.