You can project all sorts of colors from a video projector, but can you project the color black? Wouldn’t that be like shooting a shadow out of a flashlight? I mean think about it. When you give a presentation, you can have a slide with a black background. How does that work? Well, guess what? There’s some fun physics here, so let’s get into it.
How Do We See the Color Black?
I like to ask students some questions about how they see things. The first is the most fun: Have you ever been in total darkness? I mean no light source at all, not even from a crack under the door. You might have experienced this is on a cave tour, when the guide turned off the lights for a few seconds to freak you out. (Not a good idea; sometimes a light doesn’t come back on.)
The next question: What do you see in a dark room? If it’s not completely dark, your eyes will adjust and you eventually see glimmers of shapes. However, in a fully dark room you see nothing. A total void. It’s kind of unnerving.
Just for fun, I made a physics demo to help people understand the color black.
Courtesy of Rhett Allain
This is a box with a tube going into it. There’s also a door that you can open on the top. The students look into the tube and try to determine the color of the inside of the box. Everyone has the same answer—it’s black on the inside. I then open the top door so that light can get in, and they see that the walls are white and there’s a picture of me in there:
Courtesy of Rhett Allain
The point is that if there’s no light going into your eye, your brain interprets that as the color black.
All the Other Colors
Remember that light is an electromagnetic (EM) wave. EM radiation can have any wavelength, from radio waves hundreds of meters long to gamma rays spanning just a few picometers (trillionths of a meter). And there’s a tiny sliver of this range, from 400 to 700 nanometers, that we can see. We call this visible light—or just “light” (though technically it’s all light).
Our human brains interpret different wavelengths of the visible spectrum as different colors. You can see these colors by passing white light through a diffraction grating so that different wavelengths bend different amounts. Here’s what that looks like:
Courtesy of Rhett Allain
There you have a nice spectrum of colors, with red being longer wavelengths and violet being shorter. This shows that white light is a combination of all the colors of the rainbow. But what about hues like brown, magenta, or cyan that aren’t in a rainbow? Well, those aren’t a single wavelength.
So how do we see them? It’s actually pretty amazing. Our eyes are sensitive to only three specific wavelengths: red, green, and blue. But by mixing these three in varying intensities, you can create the perception of any color.
Don’t believe me? Well, if you’re looking at this post on a computer or phone with a color screen, that’s exactly how all the colors are created. There are a bunch of super tiny lights that are red, green, or blue (RGB). Each of these tiny lights can vary in intensity to produce whatever color you want. If you put a small drop of water on your phone screen, it will act as a magnifying glass and you can actually see the RGB lights inside.
Courtesy of Rhett Allain
But what about the colors from a video projector? It essentially does the same thing. However, instead of a bunch of tiny RGB lights, it projects RGB colors to different locations on the screen.
Projecting the Color Black
Now we’re ready for the fun stuff. How does a video projector shoot out black light? Black light would be the absence of any light, as we saw before. How do you project nothing? First, let’s consider a screen like a TV. If you have three tiny lights (red, green, blue), you can make a tiny black dot by just turning off the lights for all three colors in that spot. Boom, it’s black!
So, is it possible that you just turn off the projector to make black? That can’t be right, can it? If that was the case, you couldn’t tell the difference between a projected black color and the screen without the projector even on. Yes, that is exactly how it works.
Check it out: In the following image, I’m projecting a slide that is half black and half white. The slide takes up most of the screen.
Courtesy of Rhett Allain
You don’t see any black, do you? That’s because there’s nothing there, and the ambient light in the room just shows the silver screen. This is why we watch movies in darkened theaters.
So when Darth Vader makes his appearance in the original Star Wars, striding through Princess Leia’s spaceship? That’s right, it’s just a Vader-shaped hole in the projected image. The lit-up areas around him cause our eyes to see the contrast as a deep black. There are lights reflecting off his thermoplastic armor too, adding to the sense of something being there.
It kinda gives new meaning to joining the dark side, right?
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