Science Question from a Toddler: The color of light


Why does a glow-in-the-dark Frisbee glow green? Why does a spark from a light socket look blue? Two different questions, but one intertwined answer.

Hopefully, readers Inga Foster—who asked about electricity—and Stewart Haddock—the man with the glow-in-the-dark query—don't mind being lumped together. As it turned out, they were really asking about the same thing. Both these phenomena stem from the weird ways light interacts with atoms.

Yes, we're talking about physics today. But don't worry. If I can understand it, you can understand it.

In one corner, we have the atom. You know this guy. He's the basic building block of everything, everywhere. Tinier than tiny. But also very powerful.

Each atom has a nucleus—a ball of particles that carry positive and neutral electrical charges—and is circled by electrons, particles with a negative electrical charge.

In the other corner: Electromagnetic waves. What these waves do depends on their frequency—how fast they vibrate. High-frequency waves bring light to our eyes, and determine what colors we see. There's a range of frequencies that can produce visible light, and we perceive the different frequencies as different colors.

It goes on a gradient, like a rainbow. We see higher pitched waves as blue, lower pitched ones as red and the other colors fall somewhere in between. The waves can also be so high frequency or low frequency that our eyes can't see them at all, and that's where you get into things like ultraviolet and infrared light.

Now, say you're a little atom, just hanging out, minding your own business, when you're hit by some form of energy. You can absorb some of that energy, but not all of it.

"When the atom absorbs energy, the electrons become very energized, but electrons don't like to be over-stimulated. They like to be home, just like everybody likes to be where it's comfortable," said Andrew Glassner, Ph.D.

Glassner is a former research scientist who designed computer graphics algorithms to produce true-to-the-real-world simulations of lightning and glow-in-the-dark objects in the 1990s and 2000. To get the models to work correctly, he had to study the physics behind the phenomena and incorporate that into the algorithms.

"If you take an electron and make it very, very excited, it will try to shed that excitement and go home again. The energy has to go somewhere, though, and the way electrons get rid of energy is by spitting it out," he said.

The atom spits out electromagnetic waves of a specific frequency depending on its charge and mass—which means that different atomic elements have different characteristic colors.

"That's actually how we know what chemicals make up the sun. We can look at the sun and see what frequencies are coming off. We can say, 'Those colors come from helium. So, by golly, the sun must have some helium in it!,'" Glassner said.

Them's the basics. But how does this play out for electric sparks and Frisbees?

"With electric sparks, the color you're seeing is mostly nitrogen from the atmosphere," said Bill Beaty, a research engineer for the chemistry department at the University of Washington who's consulted on textbooks and museum science education programs for kids. "If the air was neon rather than nitrogen we'd think electricity was orange."

What we see as blue light from an electric spark is simply the result of nitrogen atoms absorbing electrical energy, and spitting some back out in the form of electromagnetic waves—waves which, to us, happen to appear blue.

In fact, electricity doesn't always appear blue. The center of a spark, and lightning, both appear white. That's because when you hit an atom with higher levels of energy it will release waves of several different frequencies. Our eyes perceive each frequency as a different color—and white is just the color we see when we see lots of colors merged together.

The Frisbee work much the same way. Zinc sulfide is a cheap, naturally occurring chemical compound. About a century ago, people realized that if you took zinc sulfide and exposed it to light energy it would absorb some of that light, but also, slowly, spit light back out over several hours.

The atoms that make up zinc sulfide happen to spit out their waves at an frequency that, to us, appears ghostly green.

There's lots of other natural and man-made chemicals—called phosphors—that will do this, and in different colors. Glow-in-the-dark can really be any color you want these days. But zinc sulfide was the one that was put on watch hands, exit signs and (yes) Frisbees for much of the 20th century. So, really, the reason we think of glow-in-the-dark as green is more of a cultural thing, than a fact of science.

Image courtesy Flickr user methticalman, via CC


  1. One question that I’ve always had — what’s the orange color from flames? Carbon?

    I don’t think that it’s blackbody radiation due to the consistent color. Other typical flame colors: I’m guessing that the blue color is hydrogen. Greens are probably a salt or something.

    1. the orange is mostly sodium. That’s why sodium vapor lights used on roadways and some parking lots are so oddly colored and very orange.

    2. Yes, orange-yellow from wood is carbon. Green can vary, but copper salts is a common source for fireworks, while intense red is usually iron.

      1. Though I’ve always much preferred the red that results from lithium salts. They just have the nasty habit of being expensive and more toxic than iron salts.

        The lovely crimson you get with lithium is far more striking.

  2. Is wavelength the reason why the HID headlights on luxury cars appear blue or indigo when far away but white when close up? Like a visual Doppler effect?

    1. I think it’s more related to the light being more intense up close, and your individual color receptors equally overwhelmed so that judging color becomes difficult.

    2. Additionally, the color would only shift (albeit a very, very, very, minute amount) if the speed of the car changed drastically during the time you were perceiving it.

  3. 9re9: Light can undergo a doppler effect, but to create that effect, there must be a change in speed that is a signifigant proportion of light’s speed 186,000 miles/sec. A car doesn’t cut it. However, we see the doppler shift in every star we look at because of residual spreading of space following the Big Bang. :)

  4. I was wondering something similar after getting back from a road trip during which I saw a falling meteor. I’ve seen the “shooting star” type of meteor plenty of times, where it just looks like a white streak across the sky, but 3 or 4 times I’ve seen a much larger, brighter kind of meteor, and these have always been a distinctive green color. Is this related to the distance I’m seeing it from, or what the meteor is made from? Why are the greens ones always so much larger, brighter and lower in the sky?

  5. What we see as blue light from an electric spark is simply the result of nitrogen atoms absorbing electrical energy, and spitting some back out in the form of electromagnetic waves—waves which, to us, happen to appear blue.

    This is also why the sky is blue, by the way.

    I remember being taught in school all sorts of explanations that involved reflecting the ocean, or the ocean reflecting the sky (no one could ever work out which was which).

    No, the main reason that the sky is blue is because nitrogen absorbs and re-emits blue radiation — or, in other words, because nitrogen is blue.

    1. The sky is blue due to Rayleigh scattering. Inverse fourth power of the wavelength, blue has shortest visible wavelength, so blue is scattered the most.

  6. So, would high energy plasma in space, viewed from earth using a camera obscura tend to look blue? And would there be any reason to associate that observation with the flash leaking past shielding curtains around a plamsa cutter at work?

  7. “There’s lots of other natural and man-made chemicals—called phosphors—that will do this, and in different colors. Glow-in-the-dark can really be any color you want these days. But zinc sulfide was the one that was put on watch hands, exit signs and (yes) Frisbees for much of the 20th century. So, really, the reason we think of glow-in-the-dark as green is more of a cultural thing, than a fact of science.”

    This is not really true. Here’s why:

    First, it is important to note that the human eye is most sensitive to wavelengths around 555nm. Copper-doped Zinc Sulfide emits light very close to this. So, the color is not just a random choice, it allows us to see maximum brightness with minimum effort. As the light emission decays in darkness, we will continue to see green long after other colors appear to extinguish.

    Second, for a wide variety of materials, green is the brightest, longest-lasting glow-in-the-dark phosphor that can be made. This is even when you remove the human-eye sensitivity described above.

    Third, ‘any’ color is not possible. There are only a handful of colors which can be made with any industrial efficiency. Additional colors can be made by blending but it is not a simple task because glow phosphors decay at different rates. So you can have an orange light that gets dimmer and also shifts to green/yellow over time. That is not desirable.

    Fourth, at certain brightnesses, you aren’t seeing color anymore. Your cones are not producing an image and your mono-chromatic rods are delivering the information to your brain. Your brain ‘makes up’ color information based on context. Often this is the glow-in-the-dark green color in your mind’s eye.

    Why do night vision scopes show green and not red or blue? Because it is the most efficient color by orders of magnitude.

    The future of phosphorescence is bright (literally). Quantum dots and nano-technology will open many new doors and I expect it to show up in consumer products soon.

  8. spectroscopes are fun. i have a cheap $12 one i bought from a science education supply house that specializes in creationist text books (that’s a subject in itself). i was amazed to see that compact fluorescent bulbs radiate on only three or four lines of spectra, and that the “white light” they produce is just a simulation that fools the human eye. other eyes, such as those of birds and the the photoreceptors of plants, must find it very unnatural.

  9. #8 are you sure of that? I thought stars were traveling away from us at a constant rate, but their light was still red shifted. I know the comparisons between sound and light Doppler effects aren’t exact, but sound waves are shifted if their source is moving toward you at a constant velocity. Unless there’s some relativity effect I’m not taking into account I’d think light would do the same.

    1. #8 is correct. Red shift is the lengthening of the wavelength as objects are moving away from a fixed point. The amount of red shift for a star does not change unless the rate at which it is moving away changes (as in the acceleration or deceleration of the car). It’s just doppler shift on a cosmic scale.

  10. #14: what #8 means is that, if the color shifted during observation (i.e. from blue to white), then there must have been a change in velocity to cause that change in Doppler shift. If a star is moving away from you at a constant velocity, it will be red-shifted by the Doppler effect (i.e. redder than it theoretically should be for being mostly hydrogen/helium). If it changes color during the observation, it must be changing speed. If a car was heading towards you at a constant velocity while sounding its horn, you wouldn’t hear a change in sound until it hits you. The shift in siren/jet-engine sound that we think of is because something fast is passing you – i.e. changing velocity relative to you. First it is heading towards you and has a higher pitch, then it passes you and heads away from you with a lower pitch.

    1. I understand how that works with the red shift. But to my observation, a train’s sound rises in pitch as it approaches (as opposed to simply being higher in pitch by the same amount on the entire approach). It peaks when it passes and declines thereafter. I assume it’s probably going at a constant speed relative to the track; are you saying that the slight angle between me and its direct approach (since obviously I wasn’t hit by it) is enough to make it appear to rise in pitch?

      Or is it that sound doesn’t work quite the same way as light? The speed of sound in air is not a constant, unlike the speed of light in vacuum. Sound waves are a pressure phenomenon, after all, and they attenuate as they become more distant from their source: would the decrease in amplitude be accompanied by a decrease in frequency (/increase in wavelength)?

      If you blew two identical stationary train whistles, one 100 yards from me and one a mile from me, would their pitch sound different? If not, what explains the apparent rise in pitch as they approach the observer at a (more or less) constant velocity?

      1. Yes, Xopher, the angle between you and the train is why the apparent change in frequency occurs.

        Then the train is a long way away, the pitch stays essentially constant, because the angle it is approaching you is basically constant (it might as well be heading directly at you). As it gets closer, the angle changes, faster and faster, until the train is beside you. At this point, the train’s velocity in the direction between you and the train is 0, so you hear the natural frequency. As it passes you, it’s angle quickly changes to the point where it is traveling away from you at the same speed it was originally approaching you and so the sound is lower.

        For more mathematical details, check out the Wikipedia page.

      2. The apparent rise in pitch as a train passes is due to the angle of approach. You would experience the same phenomenon with Red shift and blue shift if your eye were sensitive enough to detect it, or if it were moving fast enough.

        The frequency of the identical unmoving train whistles will still be the same, or else the Tubas at the back of the orchestra would always be slightly out of tune with the violinists at the front!

        Unless the violinist was running away from you really really fast, then they’d be a cellist.

  11. #10: The emission spectrum of nitrogen is not the reason the sky is blue. The sky is blue due to Rayleigh scattering, which is dependent on particle size. The sky is blue because nitrogen and oxygen (which compose the majority of the atmosphere) are roughly the same size and intensely scatter blue light (or more accurately, they scatter the wavelength of light that we see as blue). This scattering effectively drowns out the rest of the color spectrum. This is also why sunsets are red. The light has to travel through more of the atmosphere as it nears the horizon. The blue light becomes so scattered that the wavelengths which are not scattered (i.e. red) will dominate what color you see. (Also the reason lunar eclipses appear red)

    1. Because I’ve been sick and spent most of the day watching Dr. Who episodes. Some of it must have worn off on this post.

  12. I always get nervous when someone says “If I can understand it, then you can understand it” to a crowd.

    Because there are some STUPID people out there.
    And such a statement is basically an admission that the speaker is the lowest rung on the intellectual ladder (relative to the subject at hand).
    Lower than the dumbest of the dumb.

    … so is probably not the best person to be doing the explaining.

  13. Blue sky during the day is due to Rayleigh scattering, which is a wavelenght/frequency dependent scattering of light. Shorter wavelengths (blue is shorter than red) are scattered more, whereby the longer wavelengths pass through the atmosphere with much much less scattering. Scatter is the reflection of light in all directions; the atmosphere seems to glow blue when viewed from space as well as from the ground.

    As far as light from lightning and phosphorescence goes, its all due to the acceleration of electrons. Just remember this,electrons have an electric field (electrostatic field), any time an electron accelerates (changes direction, or velocity) its electric field also changes and a subsequent magnetic field is also produced. This magnetic field propagates with the an electric field and you have electro-magnetic radiation, which is photons (when detected). Think about lightning, you have free electrons (current) jumping from one N2 (nitrogen molecule) to another, from a high potential to a low potential (home). Well since the electron is jumping it is accelerating. This movement creates an electromagnetic wave, which is photons. The distance that the electron jumps, determined by the element that hosts the electrons, ultimately determines the color of the photons emitted (E=hv), where E is the energy (how much the electron jumps), h is Planks constant, and v is the frequency.

    Another interesting this is that a photon is a particle, but light propagates as a probability wave. When this light wave interacts with something it turns into a particle, which has momentum, but no mass. Light can actually move objects since it has momentum, but it does not have mass. It’s quite strange.

  14. The angle of the train’s approach would make the sound drop in pitch as it got very close, since it would be approaching the observer more and more slowly until it drew even with the observer and “stopped” before “accelerating” away until that angle grew shallow again and relative velocity became constant.
    Maybe the observed rise in pitch is from a greater proportion of high pitched mechanical noise getting to the observer when the train is closer.

  15. Yes, we’re talking about physics today. But don’t worry. If I can understand it, you can understand it.

    Non so se questo è vero. Io non parlo inglese.

  16. The atom spits out electromagnetic waves of a specific frequency depending on its charge and mass…

    Quibble. Not mass, but atomic number. I’m reasonable sure the atomic absorption and emission spectra of an element don’t vary with isotopic state, so mass doesn’t matter. Rather it is the number of protons in the nucleus — i.e. the atomic number — that controls energies of the electronic transitions.

    And yeah, I know the vibrational spectra of molecules are influenced by isotope effects, but the post clearly refers to atoms.

  17. A candle flame (white), burning wood (yellow/orange), and a oxygen starved acetylene flame (brilliant white) are continuous spectra, which long puzzled me. Continuous spectra are characteristic of incandescent solids. It turns out that is he source in these flames too– incandescent soot (carbon) particles that subsequently burn as they exit the flame envelope to the oxygen rich surrounding air.

  18. You librul intellecktuils have it all wrong as usual. The sky is blue because God’s only child was a boy. If it had been named Jessica Christ the sky would hav proboly been pink.

    (couldn’t resist)

  19. The question is often asked why more females aren’t involved in science.

    Maggie writes: In one corner, we have the atom. You know this guy. He’s the basic building block of everything, everywhere. Tinier than tiny. But also very powerful.

    Having never sexed an electron, I can’t say for sure, but I doubt they have little gonads. Pls to edit: “It’s the basic building block…” Do it for the children!

  20. Lorann: Having never sexed an electron…

    I can see it now: Sexing the Electron, Jeanette Winterson’s latest novel, about 20th century physicists in love.

  21. “There is no better, there is no more open door by which you can enter into the study of natural philosophy than by considering the physical phenomena of a candle” – Michael Faraday (not the guy from Lost)

  22. 9re9 #3 may actually be referring to the effect described by atmospheric perspective, which is seen when distant objects (on the planet’s surface) appear more blue than when they are closer. Atmospheric perspective is caused by Rayleigh scattering. I’m not certain it’s the same physics phenomenon as what makes Xenon lights appear blue when further away and white up close. The headlight issue might just be a human perception thing.

  23. Interesting posts wrt gender in science. First the issue of sexing the electron. The he/she/it issue always seems a little nitpicky to me, but fair enough. I happen to be of the gender that tends to dominate science, so maybe I’m just not seeing it.
    The more interesting issue is the matter of prefacing the article with, “if I can understand it, you can understand it.” That clanged with me for the same reasons stated by djdole in #27. But it also struck me as a very welcoming gesture by the writer, in an attempt to make the article more aproachable. I hadn’t made notice of the writer’s name, but from that line, it seemed pretty clear that the writer was female.
    I’m dealing in generalities which give way to stereotypes here, and I realize that many individuals out there do not fit these proposed molds. But when you ask yourself the question, why do not more women go into science?, generalities are what you have to deal with.
    A male writer might have started off by writing (or at least thinking), “you can understand it, because I am a smart person, and an excellent communicator.” That would have been better received by me, djdole (a guy too?), and other male readership. But it might have been a little alienating to female readers. (It is a tad obnoxious, no?).
    I can’t quote the studies, but it has been shown that female writers tend qualifiy their statements with phrases like, “I think,” or, “In my opinion.” (See, Writing Down the Bones, a great book on writing.) For reasons I can’t begin to get into, females tend to be less comfortable with direct, open conflict and disagreement. Some would characterize this as evasiveness, others would call it just being polite.
    The process of scientific discovery is born out of direct disagreement. “With all due respect, your excellency, the Earth might NOT be the center of the universe,” or “I think these two balls will fall at the exact same speed.” But it certainly could do with a little tempering in the form of good manners.
    The gender difference won’t balance out until more women decide to go into science in the first place, and making it more accessible in with the subtle use of language seems a good place to start. At least, that’s my opinion.


  24. @#40:
    My dad gave me a copy of Faraday’s lectures on the candle when I was just starting to read. That book started me on science – don’t give it to child without realizing what you’re embarking on…

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