What if shadows had colors?

Stand in the sunshine. Look down. There is your shadow! It is dark.

What if your shadow could be a color, like green or yellow?

Your shadow follows you everywhere, and it's usually dark gray or black. A shadow is simply a patch where your body blocks the light, so less light lands there.

But is a shadow always dark? What if there were more than one light, and the lights were different colors? This lab is a thought experiment that turns out to be something you can actually try with a grown-up and some colored flashlights.

A shadow is the outline of an object on the part of a surface that light from a source does not reach. With a single source it receives none of that source's light and looks dark. With several sources, each casts its own shadow, and each shadow is still lit by the others.

This thought experiment asks what color those shadows would be if the sources were red, green and blue, and what that reveals about how our eyes combine colored light.

One lamp makes one shadow. It is dark because no light gets there.

Two lamps make two shadows. Each lamp makes its own!

Shine one flashlight at a white screen and hold a pen in front of it: you get one shadow, and it is black. A Science Buddies activity, published by Scientific American, shows why with a ruler: light travels in straight lines, so the shadow lands exactly where the pen blocks the flashlight's beam.

Add a second light and you get a second shadow. Each shadow belongs to the light it blocks, and the other light can still shine into it.

The Science Buddies activity has students line up a ruler from the flashlight through the pen to the screen: the pen always sits on that line, because light travels in straight lines, and the shadow forms exactly where the pen blocks all the light. With one colored flashlight the screen takes that color and the single shadow is black.

The Physics Classroom adds that with three spotlights, three shadows are cast, one for each light. Each is missing one source's light but still receives the others'.

Now the lamps are red and green. Where both shine, the wall looks yellow.

One shadow is green and one is red! Each shadow gets light from the other lamp.

Now shine a red and a green flashlight on the screen. Where both lights overlap, the screen looks yellow. Hold up the pen and there are two shadows. Where the pen blocks the red light, only green light reaches, so that shadow is green. Where it blocks the green light, the shadow is red.

Add a blue light and the screen looks white. Block the blue and the shadow is lit by red and green: yellow. Block one of the others and you get cyan or magenta. Where all three shadows overlap, no light gets in, and that spot is dark.

Science Buddies explains the two-light case: each shadow has the color of the other light source, because the pen blocks one beam's straight-line path but not the other's. Red and green combine into yellow, red and blue into magenta, blue and green into cyan.

The Physics Classroom works through three sources: the shadow from blocking blue is filled with red and green light and appears yellow; regions where two shadows overlap receive only one light; the region behind the object where all three overlap is not lit at all. So in a world lit by colored sources, shadows really would have colors.

Our eyes have special helpers for seeing color.

Some see red best, some green and some blue. When they all get light, we see white. When some miss out, we see a color!

The backs of your eyes have color-sensing cells called cones. There are three types, each most sensitive to red, green or blue light. When red, green and blue light all reach a spot, all three are stimulated equally, and you see white.

In a shadow, one color is missing. If only red and green light reach the shadow, your cones send a mix your brain reads as yellow. Nothing is painted on the wall: the color is made by which lights land there and how your eyes respond.

Science Buddies describes three cone types, each most sensitive to red, green or blue; the brain reads the different signals as color, and perceives white when all cone types are stimulated equally. OpenStax adds that our eyes respond to mixtures of wavelengths, and that an object can look a different color under non-white light: a blue object under pure red light appears black because it absorbs the red.

A shadow lit by two primaries stimulates two cone types strongly and one weakly, which the brain interprets as a secondary color. Color vision is subtler than this simple picture; OpenStax notes it is not completely understood.

Many TV and computer screens make colors from red, green and blue light.

You can make colored shadows at home with a grown-up and colored flashlights.

You can see this for real. The Science Buddies activity uses red, green and blue flashlights, a white paper screen, a pen and a ruler. Its extras suggest trying different background colors and overlapping the circles of light only partly.

The same mixing is all around you. Science Buddies notes that many television and computer monitors make their colors using only red, green and blue. NASA explains that our eyes can see light with wavelengths from about 380 to 700 nanometers, from violet to red.

Science Buddies calls this additive color mixing: superimposing beams of light to mix them into new colors. Partly overlapping three circles of light shows primaries, secondaries and further blends at once, and many television and computer monitors produce their colors from red, green and blue in the same way.

NASA notes that the human eye typically detects wavelengths from about 380 to 700 nm, with cone cells tuned to that band. Stage lighting can mix colored sources too, but we have not checked how designers use colored shadows, so that stays an idea.