Sun Beams, God Rays, Cloud Breaks, Jacob’s Ladder, Ropes of Maui… these are just some of the names that sun rays are known by. But their proper name is crepuscular rays. A fairly common optical phenomenon, crepuscular rays appear when sunlight is blocked by objects, causing a visible contrast between sunlight and shadow. Clouds, mountains, building and even trees can cause this to happen.
The name comes from their frequent occurrences during the crepuscular hours (those around dawn and dusk), when the contrasts between light and dark are the most obvious. Crepuscular comes from the Latin word “crepusculum”, meaning twilight.
Crepuscular rays visible in fog as the light passes through trees. Image credit: ©Matt Fricker
If the atmosphere contains particles that ‘capture’ or reflect the sun light (like very small water particles, smoke, dust and even airborne aerosols) this allows the sun light passing thought the atmosphere to become ‘visible’.
Crepuscular rays are usually red or yellow in appearance because particles in the air scatter short wavelength light (blue and green) much more strongly than longer wavelength (yellow and red) light. Also, because the light traveling through the atmosphere at this time of the day has to pass through as much as 40 times more air than it does when the sun is high in the sky, the filtering effect is more prominent. This process is known as Rayleigh scattering.
Image credit: ©Matt Fricker
As you can see in the images above, crepuscular rays seem to radiate outwards from behind the object. This is in fact an optical illusion due to our perspective as observers.
It is a similar effect to what you see when you stand next to railway tracks or power lines. As you look along the lines toward the horizon, they appear to get narrower.
Image credit: unknown
Crepuscular rays are (almost) parallel. Interestingly, it is actually the shadows that get slightly narrower. This is due to an effect called umbra, and it is best explained in the following diagram.
The illustration on the left represents the situation we have on earth, with the sun as the source of light being much larger. This causes the darkest part of any shadow to gradually taper the further it gets away from the source.
Image credit: unknown
Crepuscular rays can also form when the sun is high in the sky and light rays break through holes in the clouds.
Anti-crepuscular rays are similar to crepuscular rays, excepting that they are seen opposite the sun in the sky (the anti-solar point). Anti-crepuscular rays are most commonly seen at sunrise or sunset and are much less visible. This is due the amount of atmosphere that the sun light has to pass through.
Although anti-crepuscular rays appear to converge onto a point opposite the sun, the convergence is actually an illusion. The rays are in fact (almost) parallel, and the apparent convergence is due to the vanishing point at infinity.
To see anti-crepuscular rays, turn your back to the rising or setting sun and look towards the horizon.
Below are some great examples of anti-crepuscular rays captured by some of the PWL chasers.
Image credit: ©Cameron Fisher
Image credit: ©Jeff Miles – Near Esperance, Western Australia. See more of Jeff’s photography here.
Image credit: ©Grahame Kelaher – Grahame captured this image in Guam. See more of Grahame’s photography here.
We will add more images to this article as they become available.
**Parts of this article have been adapted and used with permission under Creative Commons Attribution ShareAlike License.