How do rainbows form?

A rainbow forms when sunlight enters water droplets in the air, bends, reflects off the back of each droplet, and bends again as it exits. Because different colours bend by slightly different amounts, the white light spreads into a spectrum. The result only appears when the Sun is behind the observer and droplets are in front of them. NOAA describes a rainbow as an optical phenomenon rather than a thing in a particular place, since it depends on the viewer's position. [1] [2]

Light slows down when it passes from air into denser water, which makes it change direction. This bending is called refraction. After entering the droplet, the light reflects off the inside back surface and then refracts again on the way out. Sunlight is a mixture of wavelengths, and violet, the shortest visible wavelength, bends the most while red, the longest, bends the least. The colours therefore leave the droplet separated. [1] [3]

The Bureau of Meteorology explains the geometry. Looking away from the Sun toward rain, the raindrops where the angle between the Sun, drop and observer is 42 degrees send red light to your eye. Drops a little lower, at 41 degrees, send green, and those at 40 degrees send violet. Each colour therefore reaches you from a different band of drops, producing the stacked arcs. [4]

To see a rainbow, the Sun must be behind you, and the Sun must be no higher than 42 degrees in the sky. The lower the Sun, the more of the arc you see. The Bureau of Meteorology notes that partly cloudy days, with gaps between rain clouds, and mornings or late afternoons give good chances. Rainbows can also appear in waterfall mist or in spray from a sprinkler or hose. [5] [4]

Rainbows usually look like semicircles because the ground cuts off the lower part of the circle. From a high viewpoint such as an aircraft or hot air balloon, with droplets below you, a full circle can be seen. The Royal Meteorological Society adds that the scattering angle differs for each person, so every rainbow is unique to its observer. [3] [2] [5]

The familiar seven colours, red, orange, yellow, green, blue, indigo and violet, were popularised by Isaac Newton. In reality a rainbow contains a continuous range of hues, and modern definitions often merge indigo into blue and violet. The colours also blend and blur, so it is not a true spectrum. [2] [5]

A fainter secondary rainbow appears when light reflects twice inside droplets. It exits at a different angle, about 50 degrees for red instead of 42, so it sits above the primary bow with the colour order reversed. The longer light path makes it dimmer. Fogbows form from much smaller droplets, which diffract the light and smear the colours, so they look pale and broad. [1] [3] [2]