The illusion that lets you see the ghosts of clouds

You know that scene in movies where a hero looks in the mirror, or into water, and sees someone standing behind him, and then he turns around and sees nothing? Clouds actually pull that off in real life. There are times when you can see clouds in the water, and turn around to find nothing there. Why? Optics. Or ghosts.

Actually, it’s a relatively poorly-known trick of the light. For the most part, we think of the light from the sun as coming down in no particular order. It’s true, the sun emits light all willy-nilly. But because the light travels through the atmosphere, it becomes slightly polarized. Polarized light travels with its waves vibrating in the same direction – horizontally, vertically, etc. The light from the sun hits the particles in the atmosphere, and is scattered. But because the angle at which it hits the atmosphere contributes to its polarization after the scattering, the entire sky is polarized in different ways depending on where the sun is. When the sun is directly overhead, the light is polarized tangentially all around it.

If you held up a polarization filter to a part of the sky and rotated it, it would morph between light and dark as it alternately let in and screened out polarized light. It’s said that Viking used natural polarization filters to navigate, and that bees and birds use find their way around by seeing polarized light in the sky and marking the position of the sun.

At sunset, when the sun is low in the sky, most of the light is polarized vertically. The atmosphere isn’t the only thing that polarizes light. When light hits a reflective horizontal surface, like a lake or river, it is polarized horizontally. A perfectly vertically polarized wave of light can’t be turned horizontal just like that, so when the vertically polarized light from the setting sun hit the horizontal filter that is a lake, a person looking at the lake will not see any reflection.

The illusion that lets you see the ghosts of clouds

Clouds, however, mess with polarization. They scatter the light so many times, going so many directions, that it no longer is polarized in any particular way. Light coming from clouds will not be polarized. The above split image is a shot of clouds in the sky. The left-hand side is shot through a normal lens, the right-hand side is shot through a polarizing filter. Suddenly the sky gets darker, as the polarized light is filtered out, but the clouds pop out, looking bright. The same thing happens when the light hits the lake. The vertically polarized light is filtered out, and the sky goes dark. Much of the unpolarized light from the clouds reflects, and suddenly the clouds are visible.

Why can’t the person see the clouds when they turn? The light from the sunset overwhelms the light coming through the clouds. It would be like putting a light bulb in front of the sun. Because our eyes can’t see polarization, it all looks like one diffuse glow, until we turn around and most of the sunset is erased by the filter of the lake. When we look at the sky, we see no clouds. When we turn to the lake, we can see them clear as day.

Obviously, all the conditions have to be right. The sun has to be low in the sky and just bright enough. The clouds have to be light enough to blend in with the sky, but still give off enough light that they’ll be visible in reflection. And there has to be a relatively clear and calm body of water for a person to see both the polarized and non-polarized versions of the scene. Still, when it happens, it’s amazing. You can see two entirely different worlds, and still know that one is the reflection of the other.

Top Image: NOAA

Via Polarized Light in Nature, Polarization.com, JSTOR, UC, Pilgrim at Tinker Creek.