One of the icons of classic western movies is the sunset. Even in black and white, rays of sunlight radiate into the sky like jets of water erupting from a fountain. They add a bit of grandeur to any sunrise or sunset.

They’re known as crepuscular rays, from the Latin word for twilight. Technically, the name applies to rays that appear during morning or evening twilight, while the Sun is below the horizon. In modern usage, though, it applies to rays of sunlight shining from behind clouds or other obstacles at any time of day.

The bright rays alternate with darker shadow bands, where clouds or mountains block some of the sunlight. The rays appear to radiate in all directions. But that’s an illusion. All of the rays are parallel. They appear to converge on the Sun because of perspective. It’s like looking down a set of railroad tracks. The tracks appear to converge as you look farther away, even though they’re the same distance apart. In the case of crepuscular rays, they all converge at the Sun.

We see the rays because they scatter off small particles in the atmosphere, such as grains of dust or pollen. The rays usually look yellow or orange, and for the same reason the twilight sky shows those colors: Air molecules scatter most or all of the blue light, leaving the redder wavelengths to shine through — adding some golden rays to any sunrise or sunset.

Script by Damond Benningfield