Al Stahler: Making colors
It was back in kindergarten that we first mastered the medium of fingerpaint … and learned that, by combining just three colors – red, yellow, blue – we could create any color of the rainbow: Mixing red and yellow made orange; mixing blue and yellow, green.
Sunlight – white light – is a mix of all the colors of the rainbow. We see rainbows because raindrops play with sunlight, sending different colors off in slightly different directions.
Fingerpaint also creates colors by playing with light, but it plays a different game. When light hits paint, the paint sucks some of the colors out of the light – it subtracts some colors – and reflects back whatever colors are left. When light hits a painted pumpkin – or a real pumpkin, for that matter – the pumpkin subtracts from the light all the colors that would keep us from seeing orange, and reflects back only the colors that trigger our eyes to see orange.
An extreme example of color-subtraction is black. Black paint sucks up all the colors of the rainbow, reflecting not a single color. White paint, on the other hand, sucks up no colors … white reflects all colors. Which is why, when a black car and a white car sit side-by-side in the sun, the black car grows warmer than the white one … nice in winter, not-so-nice in summer.
Having established that color happens when pumpkins or paint suck up some colors and reflect others – that color is caused by subtracting out some colors from light – I’ll now admit that that is only half the story. There is another way to make color … to make the orange and yellow of a candle flame, the blue of the flame on a cook stove.
This second way of making colors starts out, not with white light, but with no light at all … with nothing from which to subtract a color. Rather, this new way of making color involves adding colors together, to make the color we see.
This addition process also needs just three colors to make all the colors of the rainbow. But when we make colors by addition, red plus yellow does not make orange. Rather (brace yourself!) red …plus green … make yellow.
Don’t take my word for this – as with fingerpaint, you can do the experiment yourself … though not with paint, but with your phone.
The picture on the screen of your phone is painted with LEDs – light-emitting diodes – think of them as teeny, tiny light bulbs … red, green and blue light bulbs, the three colors we need to mix to see all the colors of the rainbow. (The lights that grace many of our appliances are also LEDs).
Small problem: The LEDs in the screen are so tiny, we need a strong magnifier to see them. But that problem is easy to solve … you might have already done this, by accident.
First, get a photo on the screen that has lots of yellow – a sunflower works well.
Now, go to the sink, and wet your hands … but don’t dry them with a towel. Rather, shake the water off your hands. When you cannot shake off any more water, flick your fingers toward your phone, flicking tiny water droplets onto the screen. Sitting on the screen, those droplets act as magnifying lenses – fairly strong ones..
Looking through a droplet, look down into a yellow region of the screen. You should see the tiny LEDs – tiny, rectangular dots of color, sitting cheek-by-jowl – that paint the pictures on the screen. Notice that, where the screen glows yellow, there are no yellow dots (in fact, there are no yellow LEDs, anywhere on your screen). What you see are red dots, and green dots. When our eyes see the right combination of red and green, our brain shouts, “Yellow!”
Elsewhere on the screen, you’ll also see the blue LEDs lit-up.
The colors of the universe result from both addition and subtraction. The image here is of a stellar nursery – a star-birth region in a small galaxy that orbits our own Milky Way. The red is the glow of gases – mostly hydrogen – adding together. The blue is the light of hot, baby stars, reflecting off surrounding dust. Hot stars emit lots of blue light (in a column next summer, I’ll point readers to a star so hot, its bluish tinge is visible to the naked-eye). The dust reflects that blue light … after subtracting out some color.
Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and can be reached at email@example.com.
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