If we are in a room that contains no sources of light at all and the room is also sealed off so no light can enter from outside, then we will experience total blackness, irrespective of the color of the walls or the objects in the room. But we can, in theory, also experience total blackness even if the room has a bright source of light, provided that the source of light is behind us and everything in the room is black such that it absorbs 100% of the light that falls on it, and no light gets reflected back into our eyes. This is because the color of objects that do not themselves generate light is determined by what light they reflect. Objects that appear to be red reflect light that is largely in the red range of the visible spectrum of electromagnetic waves, and similarly for other colors. Black objects are those that do not reflect any light at all in the visible spectrum.
Such perfect blackness in the sense of not reflecting any light at all is almost impossible to attain. While one can use pigments that directly absorb much of the light that strike them, perfectly absorbent pigments are not possible. But another way that blackness can be approached is by trapping the light that strikes a surface, so that while it is not directly absorbed, the light gets reflected over and over again within the object, each time getting a little more getting absorbed by its inner surfaces. Irregular surfaces are better at doing this.
The fact that what makes a body ‘black’ may have little to do with its intrinsic color is of course well known. Physicists have studied ‘black bodies’ since the 19th century. What they actually looked at was radiation coming out of a small hole in a hollow object maintained at a uniform temperature. Any light that happened to fall on the hole, once it entered the cavity, would ricochet around inside and thus get got trapped, with very little coming out again because of the unlikelihood of it happening to strike the hole from the inside at just the right angle to emerge. Hence any radiation coming out of the hole would be due almost entirely to the radiation emitted by the inner surface of the object and the spectrum of this radiation depends only on the temperature of the object and nothing else. Such properties that depend only on one or two parameters are of great value in understanding fundamental laws of science and indeed trying to explain the black-body radiation spectrum was one of the keys that led to the birth of quantum mechanics.
Dakota McCoy writes about birds of paradise that are native to areas in or near Papua, New Guinea that have feathers that appear to be super-black in that they absorb 99.95% of the light that hits them, about the same as the blackest of human-made materials called Vantablack. It does this in a manner similar to black-body cavities, by trapping the incident light that falls on it.
How does it do this? McCoy explains.
Normal feathers are flat, and look like fractals; when you zoom in using a microscope, each branch of the feather looks like a tiny, flat feather. Under a powerful scanning electron microscope, we were surprised to see that the super-black feathers look like miniature coral reefs, bottle brushes or trees with tightly packed leaves.
These tiny, specially shaped bits stick up to form a jagged, complex surface; together they act as microscopic light traps. When light rays strike these surface microstructures, they repeatedly scatter around the shapes and are absorbed, rather than being reflected back to an observer. It’s an iterative process: Each time a scattering event occurs, a portion of the light is absorbed until it’s almost completely absorbed.
…The Birds of Paradise’s super-black feathers are so good at absorbing light that even when we coated them in gold, a shiny metal, they still looked black. That’s because it’s not the inside of the feather making the color via pigment or ordered nanostructures; instead, just as with human-made black silicon, the super black comes from the physical surface structure. Evolution and human ingenuity arrived at the same solution.
Why did these birds evolve to be so black? We are not sure and McCoy discusses various evolutionary theories that have been proposed to explain it.
But we can, in theory, also experience total blackness even if the room has a bright source of light, provided that the source of light is behind us and everything in the room is black such that it absorbs 100% of the light that falls on it, and no light gets reflected back into our eyes.
I want to photograph in that room.
What would a person look like? They would be just a rim-lit silhouette if I also wore light-absorbent clothes.
(That’s a certain portrait photographer’s trick: put the subject between a giant softbox and the photographer, then the light on the subject is just the reflected bounce off the photographer’s white shirt)
In our lab we use stacks of razor blades as energy dumps for laser beams (Image of commercially available version here ). It still fascinates me how those blades turn from shiny and sharp to black, silky and smooth by simply stacking them.
@#1 Marcus:
Moving around in a 100% black room could be a challenge, basically the opposite of a white-out, but with the same orientation problems.
nastes
I would not be surprised if the darkness of the birds’ plumage contributed to avoiding predation in a shaded forest.
A term this post brings to mind is “schemochrome”, or structural color. These are common in biology. The blue of blue jay feathers is the result of feather structure, not pigment. Many beetles similarly have the color they do thanks to irregularities in the surface of the exoskeleton.
I’m also reminded of a passage from Douglas Adams’ The Restaurant at the End of the Universe:
“That,” he said, “that… is really bad for the eyes.”
It was a ship of classic, simple design, like a flattened salmon, twenty yards long, very clean, very sleek. There was just one remarkable thing about it.
“It’s so… black!” said Ford Prefect. “You can hardly make out its shape… light just seems to fall into it!”
The blackness of it was so extreme that it was almost impossible to tell how close you were standing to it.
“Your eyes just slide off it…” said Ford in wonder.
And
Zaphod Beeblebrox: It’s the weird colour scheme that freaks me. Every time you try to operate one of these weird black controls, which are labeled in black on a black background, a small black light lights up black to let you know you’ve done it. Hey, what is this, some kind of galactic hyper-hearse?
“I would not be surprised if the darkness of the birds’ plumage contributed to avoiding predation in a shaded forest.”
I forgot to add that I would be even less surprised if runaway sexual selection was the reason. That deep black probably makes the blue patches stand out even more during mating displays.
Having now read (or skimmed) the linked article, I see sexual selection is a favored hypothesis, as I suspected, since sexual dimorphism is so common among birds and is the reason for the extreme behavioral mating displays in the birds of paradise group.
Adaptation, considered as a form of “natural engineering”, will usually be marked by compromise. Ideal camouflage requires matching your surroundings. As an example on the other side of the spectrum, many types of squid have a bioluminescent paych under their ink sac to prevent it being silhouetted against the sunlight. Being darker then your surroundings does not make you camoflaged, it makes you stand out.
Extremes like this, especially when it’s just on the males, scream sexual selection. The females are likely the optimal color for camoflage. The males are the optimal color for courtship.