What Angles Produce Primary And Secondary Rainbows

The angles of a rainbow describe the angle of deviation between the incoming sunlight and the light that exits the raindrop toward the observer's eye. This angle is measured from the antisolar point (the center of the rainbow).

Primary Rainbow (Angle: ~40°–42°)

A primary rainbow is the brightest and most common rainbow. It is produced when light undergoes one internal reflection inside a raindrop.

• Light enters the drop, refracts (bends), reflects off the back of the drop, and then refracts again as it exits.
• The different colors of light (wavelengths) are refracted at slightly different angles.
• The light is concentrated at a specific angle of deviation. For violet light, this angle is about 40°. For red light, it is about 42°.
• This is why we see the primary rainbow as an arc with red on the outer edge (at 42°) and violet on the inner edge (at 40°).

Secondary Rainbow (Angle: ~51°–54°)

A secondary, or double, rainbow is fainter and appears outside the primary rainbow. It is produced when light undergoes two internal reflections inside the raindrop.

• The light bounces twice off the back of the drop before exiting. Each reflection causes some light to be lost, which is why the secondary bow is dimmer.
• This double reflection also "reverses" the order of the colors.
• The light exits at a larger angle. For red light, the angle is about 51°. For violet light, it is about 54°.
• This results in a rainbow with red on the inner edge (at 51°) and violet on the outer edge (at 54°).

The dark, unlit band of sky between the primary and secondary rainbows is known as Alexander's band. In this region, no light from either one or two reflections is directed toward the observer.