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Asked by anon-244767 on 27 Apr 2020. This question was also asked by anon-252013.
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Tom Louth answered on 27 Apr 2020:
As metals (actually all items that haven’t caught fire) get hot they emit light. Normally this is infra-red, but as they get hotter the wavelength gets shorter and so they glow red. The colour is actually made of lots of wavelengths, so as they get hotter it eventually goes white. This is why really bright things look white rather than blue or green.
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Chris Dawson answered on 27 Apr 2020:
https://en.wikipedia.org/wiki/Black-body_radiation
In the diagrams in the article here you can see:
As an object gets hotter (the temperature in K goes up) the curve gets higher and the peak moves to the left. This means that hotter objects produce more photons (more light) overall and the “colour” they make most of moves up the EM spectrum towards blue. However hotter objects still produce the reds and oranges like cooler objects, the colours just all get added up to make white.
In the second diagram you can see a colour chart where it shows the colour the different objects appear, following the black line on the diagram as the temperature increases. You can see here that it never goes to green but gets closer and closer to a bluey-white.
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Susan Cartwright answered on 27 Apr 2020: last edited 27 Apr 2020 4:20 pm
As Chris says, heated metals, being solid objects, emit a range of different wavelengths, approximately as described by the Planck formula (see the link in Chris’s answer). There is a range of temperatures where the maximum emission is in the region of the spectrum we see as green, but because this is a distribution and not a single wavelength, when you factor in the response of our eyes metals at this temperature actually appear white, not green (you may have heard the expression “white hot”). It is possible for an extremely hot object to appear blue-white rather than pure white, but not a proper sapphire blue.
Although hot solids cannot appear green, or really blue, it is possible for hot gases to do this. This is because hot gases emit only at a few specific wavelengths, not over the whole range of wavelengths like a hot solid. A very hot flame appears blue, for example. You can get a green flame by throwing finely powdered barium, boron or copper salts into it: there are some pictures at https://en.wikipedia.org/wiki/Flame_test
In astrophysics, there are objects that are made of hot, low-density gas, and can therefore glow in unusual colours. Planetary nebulae, which despite their name have nothing to do with planets, but are actually what’s left over when a star like the Sun dies, are so often green that it is possible to select them for study just by picking green things out of an image. In this case the green colour is produced by an unusual state of ionised oxygen.
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Ry Cutter answered on 28 Apr 2020:
To add to the already great answers, like with cooler objects that emit in the infrared (above the wavelengths we can see), we can also heat things up so they emit beyond this point into the ultra-violet, or UV (below the wavelengths we can see). This is especially important when we look at stars as they’re so hot they emit in many wavelengths that we can’t see usually. We need to design telescopes to look at them in different ways, specialised for different wavelengths.
What’s harder still, the Earth’s atmosphere is really good at absorbing UV light; so, we need to put telescopes like Hubble in space so they get a chance to catch that UV radiation before it’s blocked out by our atmosphere!
Great Question,
Ry
P.S. (Now you know blue actually means hotter, do you think the colour scheme for taps is a bit misleading!)
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anon answered on 28 Apr 2020:
Here’s an analogy.
Imagine that the individual atoms in metals start wiggling when heated. The hotter, the faster. And as they wiggle, they radiate light. Faster wiggles = more energy = bluer light. Now, because all atoms wiggle differently – disorganised, think a crowd versus a marchband – this gives a black body radiation profile. But our eyes see an average of the whole thing, so when the peak emission is in the green part of the spectra, we perceive the whole thing as white. -
Holly Campbell answered on 29 Apr 2020:
When you heat up a metal, it emits thermal radiation, also known as incandescence. I’ll explain how it all works.
Metals can be thought of as made up of an ordered lattice of positive ions, surrounded by a ‘sea’ of free moving electrons around them. At any given temperature the ions in the lattice all vibrate together. The movement of the positively charged ions causes light to be emitted. Generally when you look at most things, the vibrations of the ions or molecules causes low energy infrared light emission. As you increase the temperature, the vibrations increase in frequency and thus visible light is emitted. Thermal radiation follows a pattern with increased temperature: as the metal heats up, it will begin to glow red, and then as it gets hotter it will begin to glow yellow because of the increase in the frequency of the vibrations, and eventually white at very high temperatures.
One way of heating up a metal is to pass an electrical current through it. Metals are good conductors, but except in the case of cooled superconductors, they all have some degree of resistance to the passage of current. The current flowing through the metal loses energy in the form of heat, and this is known as Joule heating. Power loss in a conductor = (current)squared multiplied by the resistance. Therefore high current flow through a metal wire causes it to heat up drastically, resulting in it glowing red, yellow or even white! That is the mechanism of how an incandescent light bulb works! -
David Sobral answered on 29 Apr 2020:
We also glow, but at our normal temperature you need to look in the infrared to see us. However, at hotter temperatures, we can start seeing objects as red, then more yellow, then blue. This is the reason why stars and ultimately galaxies have different colours for example: the bluest stars are the hottest, and the red are the coldest. Now it can sound confusing for red stars to be the “coldest” stars because when you heat up metal and it gets “very hot” it turns red, but a metal being “very hot” is still the temperature of a very very cold star 🙂
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Paul Saffin answered on 29 Apr 2020:
There are a couple of aspects to this, first we need an understanding of light, and secondly an understanding of how we see light.
Following the pioneering work of Max Planck, Albert Einstein realised that light was made up of packets of energy, which we now call photons, and that the energy of these photons is proportional to their frequency (how rapidly they oscillate). So, high frequency photons have more energy than low frequency ones. Hotter things produce more energetic photons.
Now we can go back to much older physics, where it was realised that the colours of the rainbow from red light to violet light are made of light with different frequencies, with red being a lower frequency than violet light.
So, yellow photons have more energy than red photons, which is why a yellow-hot poker is hotter than a red-hot poker.
Comments
anon-244767 commented on :
Thank you, everyone! Your responses were extremely interesting and very useful. I now understand in-depth. It is a really interesting topic, and I’d love to learn more about it!