Why do materials get white hot?


What causes the color change when things start glowing because they are so hot?

In: 15

disclaimer: I’m not a thermal dynamics expert, so take this as all eli5 should be taken; as a general description, not to be confused for a physically accurate model.

As temperature is increased, energy levels are also increased. One of the byproducts of increased energy are photons (light) that are emitted at faster rates. This causes the wavelength to shorten. As the wavelengths are shortened the result is they produce colors that are interpreted by our visual center differently.

This process is called incandescence.

Light is made of photons. Photons carry energy, and their energy is related to their frequency.

Color is also related to frequency.

So, blue light has a higher frequency and therefore more energy in each of its photons than red light.

Hotter objects have more energy in each atom bouncing around, so the photons produced by these atoms bouncing off of one another have (on average) higher energy.

So, a hotter bunch of atoms produces higher energy photons, and higher energy photons are bluer.

At room temperature, most of the glow happens at frequencies so low that humans can’t even see them. As it heats up, the glow begins to include redder frequencies, and at really high temperatures it includes all visible frequencies.

All visible frequencies is what we call “white”.

Black body radiation. Everything that has heat emits electromagnetic waves (light). Most things on earth don’t emit light in the visible spectrum but they do emit in the infrared spectrum (that’s why infrared cameras work for night vision). When you get something hotter it emits higher energy light entering into the visible light spectrum following the same pattern that we would see in a rainbow started with red then to yellow and so on as they all get mixed in we see white.

When you zoom in enough, heat is just how fast atoms randomly move around. When you have something hot, the laws of thermodynamics dictate that it needs to exchange heat with its surroundings.

This happens through a few different mechanisms, and one of them is called [black body radiation](https://en.wikipedia.org/wiki/Black-body_radiation). The basic idea here is that all objects, regardless of their color or material (hence “black body”) will emit light depending on temperature.

At room temperature, objects radiate in the infrared. IR cameras can see this. However, there’s a special temperature called the [Draper point](https://en.wikipedia.org/wiki/Draper_point) which marks when an object begins to emit light in the visible spectrum.

Red is the lowest-energy color, so that’s what the coldest hot objects radiate.^* As their temperature goes up, the light becomes bluer, then moves into UV, and then moves into _really_ exotic colors like X- and Gamma rays.

The most visible example of this isn’t even hot metal or glass, it’s actually sunlight! Most of the light we get from the sun is its blackbody radiation.

^* This isn’t actually true. Humans can see radiant heat at lower temperatures – it’s possible for your eyes’ low-light cells (rods) to perceive dim glows as a greyish hue as long as you’re in a dark room. The light could be too dim to be seen by your color cells.

Ok. Every object that’s warmer than absolute zero (-273.15 degrees Celsius) emits photons (light).

Cold objects emit very low energy photons, infrared light (you and me are that cold, that’s why we’re visible on an infrared camera.). Then as an object becomes hotter it starts to emit some of those photons in the red range (525 degrees celsius), then orange, then yellow etc…

Now white is when you perceive every colour of light fairly evenly. So when a material is so hot that it sends out as many blue photons (one of the most energetic wavelengths of visible light) as it sends out red photons it tends to balance out and you see it as white hot (1300 Celsius).

But it can become hotter. Our sun is much hotter than white hot steel, having a surface temperature of 5500 degrees celsius. It’s so hot that it sends out UV light (beyond the visible spectrum). It’s still cold enough that the light is full spectrum (emitting red, yellow, green and blue light so that it appears to be white), but there are stars that are so hot (7000 degrees celsius) that they appear blue-white as the majority of the visible light they send out is blue (most of their light is actually UV light though).

The answer to your question led physicists to quantum mechanics.


I hope this comment is allowed here.