# Is the light from the Sun the same as the light from a bulb?

I'm arguing with a friend of mine on whether the light emitted from the sun is of the same type of that emitted by a bulb. Her insistent ignorance is laughable, unless I'm wrong...

She's talking about how light from the bulb is "artificial" ...

I've tried explaining that that makes no sense, and the only difference between the light is the way they're produced, and the intensities across their spectra.

The bulb will emit minuscule amounts of varying wave-lengths, but with the intensity focused around visible light, right?

The Sun will produce higher intensities of different types of wave-lengths, right?

Her counter-argument is that light bulbs don't inflict harm (via harmful radiation, like the sun).

• Please note that the question only makes sense if you talk about incandescent light bulb, which are getting replaced by low consumption models nowadays. If you talk about fluorescent tube or LED, the answer is obviously that the light is very different from the sun's. Note also that "natural" or "artificial" does not make much sense when talking about physics ;) – DSuchet Mar 10 '17 at 0:31
• All of the answers below talk about how the spectrum of the sun is different from the spectra of incandescent lamps operating at different temperatures. (And, FYI, the spectrum of a fluorescent lamp or an LED lamp can be very different.) But is spectrum what your friend is asking about? or does your friend think that there is some other quality that distinguishes "artificial" light from "natural" light? (Hint, looking at any individual photon, there's no way to tell whether it came from the Sun or, from some other source.) – Solomon Slow Mar 10 '17 at 2:10
• She thinks that because the suns light is "natural," and "harmful" it is different to the type of light emitted by a bulb. I've reiterated this is simply due to different intensities of different frequencies, and that the way the photon is produced has no effect on it, excluding its energy ($\propto$ frequency $\propto \frac1{wavelength}$). – Tobi Mar 10 '17 at 2:14
• Ask her if she thinks that the "artificial" lights in a tanning booth are somehow not "harmful" - and then just take the third option just moving on and doing more important things than trying to win or lose this argument. – HorusKol Mar 10 '17 at 5:12
• Just because your friend is not particularly literate in the matter shouldn't matter. Directly answering to the title of your question: There are numerous differences between the light from a bulb and light from the sun. The spectrum of light will be different, bulbs will have a refresh rate and PWM (if, for instance, is a LED bulb) which the sun does not. The effect that has both in your eyes and your neurological system will depend on the individual. – devoured elysium Mar 13 '17 at 1:23

She's right that there's a difference, and you are right that it's all just electromagnetic waves!

The key to this is that there is no such thing as "white light" when you really get down to it. Each light emits a range of wavelengths of light. If they have a sufficiently even distribution of wavelengths, we tend to call that light "white," but we can only use that term informally.

Both the sun and the light bulb emit so-called "Blackbody radiation." This is the particular spectrum of light that's associated with the random thermal emissions of a hot object. Cool objects tend to emit more of their energy in the longer wavelengths like reds and IRs, while hotter objects emit more energy in the shorter wavelengths like blues and UV.

(Note, there are other possible emission spectra, but those are associated with different materials doing the emissions and, for the purposes of this discussion, they aren't too important. We can just claim the emissions are all blackbody)

If you notice, as you get hotter, a larger portion of the energy is emitted in the blue, violet, and ultraviolet. That's how you get a sunburn from the sun. It's harder to get a sunburn from an artificial light, not because it's artificial, but because those lights are almost always cooler than the sun. They don't have as much UV content. Instead, they have more red and yellow, which incidentally is why pictures taken indoors look very yellow. If you use a strobe, however, all those yellow hues go away because a strobe light is very warm, with lots of blues.

You can get a sunburn from artificial light, of course. Tanning beds are the obvious example, but there are other interesting ones. When you're a jeweler working in platinum, for instance, you need to wear UV protective gear (like glasses or even sunscreen). Platinum's melting point is so hot that it actually emits quite a lot of UV light and can give you a sunburn!

Other than these spectra, there is nothing different between light from an artificial source and light from the sun. Photons are photons.

• The term "white light" can absolutely be defined and used formally. And it does not require an even distribution of wavelengths. E.g. a spectral distribution with zero everywhere except at 3 spikes at at suitable wavelengths respectively in the red, green and blue regions (with suitable intensities), will be just as white as a flattish distribution. Note that "white" is a color science (which is defined partially in terms of human eye/brain biology) term and not a physics term. – Stefan Monov Mar 10 '17 at 18:43
• An unambiguous definition of white light is: Convert the given spectral distribution into the CIEXYZ colorspace, convert the CIEXYZ value into the CIELAB colorspace and check if both the a and the b resulting coordinates are zero. They are iff the light is white. – Stefan Monov Mar 10 '17 at 18:43
• @StefanMonov That is true. I should probably have been more careful and used more words to capture those details. – Cort Ammon Mar 10 '17 at 20:28
• You are spot on. One suggestion to even improve it: Talk (briefly) about the physical mechanisms which generate Black Body Radiation on the molecular or atomic level, and stress that it is exactly the same mechanism creating the sun's and the filament's light. In particular, although there are violent thermonuclear processes in the core of the sun, producing all kinds of radiation you don't want in your light bulb, they are shielded by 500,000 km of matter. All we see when we look at the surface is the resulting heat making it glow, exactly like the coal in your grill, or that filament. – Peter A. Schneider Mar 12 '17 at 8:09
• That was quite a revelation when I realized it as a boy: The sun just glows because it is hot, like anything else would at that temperature. No magic, no fusion, no special effects. We immediately know the temperature of the surface when we look at the light, without ever going there. And concerning the overall state of affairs in the sun, it's surprisingly cool. – Peter A. Schneider Mar 12 '17 at 8:11

What we experience as light are photons impinging on our retina. First we must understand the distribution of photons, and see if they are the same.

You are right to say that the spectrum of light from the sun is different that that from a light bulb. The solar spectrum compared to an incandescent bulb (the ones with a filament) looks like this:

We see both the sun and the bulb pretty much emit photons all over the UV/Visible/InfraRed spectrum. But, do so at different intensities. That means there are a different number of photons of different colours impinging into your eye. So the total light, or the spectrum of photons, is not the same. But, what about the photons themselves?

So are these photons the same - the short answer - no.

The long answer is that photons have other features you can detect, for instance, the polarisation (the direction in which the electric and magnetic fields oscillate). Further, the photon has a "length" of the wave packet but this cannot be measured directly.

For the example you give, the photons from the sun and the bulb would have a broad range of polarisations and "lengths".

But, let's consider if we could make them all photons the same, i.e. can we make them indistinguishable? As photons are bosons, you can put them into the same state and if you did this - you'd get a laser. The photons here have the same wavelength and polarisation but would have a slight spread of energies. We can take this idea further and consider a bose-einstein condensate of photons, only then would all the photons be doing the same thing.

So, to summarise, when you have lots of photons, you can make assumptions where the light come from because of the spectrum, but if you were given one photon which could plausibly be produced from the sun or the blub, it would be impossible to tell if it came from a filament or the sun.

Edit: correction for incandescent light bulb

• An "incandescent bulb" is usually not a "long white tube". That would be "fluorescent lamps/tubes". – user1583209 Mar 9 '17 at 23:58
• I don't think I understand your point about photons populating several modes. Following this argument, two sources with exactly the same parameters (for instance, two suns) would not emit the same light. – DSuchet Mar 10 '17 at 0:27
• So they are not the same but they are indistinguishable? – Bergi Mar 10 '17 at 16:03
• Do you have a source for your image? And what is the gray box? – Paŭlo Ebermann Mar 10 '17 at 22:20
• The grey box is the approximate range of wavelengths within which electromagnetic radiation is visible light. – Gareth McCaughan Mar 11 '17 at 0:04

Here are already several good answers, but one thing hasn't been addressed, which might be what your friend refers to. The Sun and an incandescent bulb both emit (close-to) Planck spectra (as shown in Tomi's and Cort Ammon's answers). In contrast, fluoerescent bulbs, or tubes, emit spectra that have multiple spectral lines. Depending on which gas is used in the tube, or what material the tube is coated with, various spectra can be achieved.

The color perceived by humans depend on the ratio between the intensity at three wavelength intervals in the blue, green, and red range, respectively. This is because we only have three different color-sensitive photoreceptor cells, called "cones" (in contrast, dogs only have two types of cones and thus lack one "color dimension", while butterflies have five, and mantis shrimps have 16!).

This means that different spectra can be percieved by humans as the same color. An example of a typical fluorescent lamp is shown below. On top of the spectrum, I've drawn the three spectral ranges that humans are sensitive to. The lamp spectrum is seen to have some larger peaks in the blue, green, and red range, and humans would interpret this roughly as "white". But the same color could be made — "artificially" as your friend might call it — with some other lines, e.g. with the line labeled "5" replaced by two smaller lines to either side and with slightly different peak ratios. Or, with a Planck spectrum of roughly 6000 K.

Spectrum of a typical fluorescent lamp (black), and sensitivity curves of the three different human cones (blue, green, and red). Source: Wikipedia + my own hand-drawing.

• I think non-continuous spectra are very important to mention in this discussion (because everything there was to say of incandescent light sources was already said in the question, actually). One remark: Since the visible light from fluorescent sources is emitted from the fluorescent material (which absorbs the originally emitted UV), I'd assume that it is the fluorescent material determining the spectrum of visible light emitted, not the gas proper. (A given fluorescent light will probably not work at all with different gas. The gas is important for the color of non-fluorescent tubes.) – Peter A. Schneider Mar 12 '17 at 7:58
• @PeterA.Schneider: I'm not sure I understand: What is the difference between "the fluorescent material" and "the gas proper". – pela Mar 12 '17 at 8:19
• The fluorescent material is a thin coating on the inside of the glass tube; that coating is what actually emits the visible light. Its electrons are excited by the UV coming from the gas. They emit photons in the visible spectrum when they fall back to the ground state. See en.wikipedia.org/wiki/Fluorescent_lamp#Principles_of_operation. – Peter A. Schneider Mar 12 '17 at 9:16
• @PeterA.Schneider: Wow, I didn't know that. Thanks! I edited the text. – pela Mar 12 '17 at 9:20

The sun puts out about 8% of its energy in UV (which does the damage), about 44% in visible, and the rest in IR. A standard incandescent puts out effectively no UV, 10% visible and the rest in IR. Halogen lamps can be operated at higher temperatures with a reasonable lifetime, and produce some UV, with perhaps 15% visible.

The difference is expressed in terms of the temperature of the emitting surface. The sun is effectively at about 5500 degrees K, while a normal incandescent will run about 2700 K, and a halogen about 3000 K.

On the one hand, in principle it's "simple" to produce a lamp with light essentially equivalent to sunlight - just run it at 5500 K. Problem is, no known substance will take that heat without melting.

On the other hand, there are a large number of M class stars with a surface temperature of about 3000 K (not ours, of course - ours is a G class). Tourists to a planet in orbit around an M class would not need sunblock, for the same reason you don't need sunblock under incandescent lighting,

So, no, incandescent lighting is not unnatural or artificial in the sense your friend thinks. The same cannot be said for most white LED bulbs, which have a spike in the blue portion of the spectrum, and this does not occur in nature.

Either light from a bulb, or light from the Sun, will illuminate my path well enough that I can avoid stepping on the cat. In that sense, they're the same.

The light from the Sun has a color blip, right where early atomic physics suggested the element with two protons in its nucleus would radiate. That element, called Helium (from Helios, Greek word for the Sun) really does exist.discovery of Helium

There isn't any of it (nor evidence of it) in light from a typical light bulb. So, in that sense, light from the Sun and from a light bulb are NOT the same.

Your friend sounds like an interesting person to have an argument with but I suspect that the lovely graphs from the other replies are not really going to help you. One comment was about UV tanning lamps. This is the way to go with your friend, bring other examples into the discussion so you can illuminate (!) exactly where her artificial/natural ideas come from. I am thinking light from a candle, light from a fire/flame, lightening, electrical sparks, moonlight, reflections from mirrors, stars in the night sky, etc. You should then be able to find out where her (fuzzy or fixed) boundaries are and tweak the discussion accordingly.

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