Why does it glow when the temperature reaches a certain level?

In life, we will meet some objects that glow when heated, and we also know that some luminous objects are very hot.

But why? Why do some objects glow when the temperature is too high?

Accurately speaking, anything above absolute zero will glow, but in most cases, these lights are invisible to the naked eye.

Objects above absolute zero will glow.

Why do you say that? Because objects with internal energy (i.e. objects with temperature) keep moving (at the atomic level) and bounce off each other, these collisions effectively convert thermal energy into radiant energy.

To better understand this, let's imagine two cars colliding. Did any debris fly out?

Now back to the atomic world, the same thing will happen. When they interact, some "fragments" will fly out. When some of these "fragments" exist in the form of photons, we can see that they are luminous.

This energy can also explain why fire can feel its warmth without contact, because heat "radiates" to our bodies (at least mainly).

Then why can't our eyes see all the heat?

There is a simple reason. We can't see radio waves or many other things. We can only see a small part of the possible "light" in the universe.

There is a rule that temperature is converted into particle motion, that is, the square root of particle (heat) velocity is proportional to temperature.

In a certain material (such as sintered red iron), the particles in the metal are brought together by a potential energy, and then thermally oscillate due to the change of temperature and remain below the energy level. At the same time, because they are oscillating, they must be accelerated. When charged particles accelerate, they radiate and glow.

The wavelength of this light depends on the acceleration, which (in our example) depends on the temperature. So the color of hot objects can actually tell us the temperature.

Similarly, if we know the temperature, we can also know what color it is (of course, it doesn't reflect sunlight). This equation is called Wayne's law.

So when the object is hot enough to make the wavelength within our visual range (380nm-780nm), we can see it.

The reason why we can only see the light with the wavelength between 380nm and 780nm is that our sun has been radiating and emitting light, bringing energy to the earth, and evolution has gained some favorable factors by taking advantage of this.

finally

Now that the problem has been extended to this level (evolution), let's consider the sun, which warms the earth through radiation and brings life to everything.

We can recognize the interaction of photons in some areas of the earth, so our eyes are sensitive to it.

When objects enter a certain temperature (or energy state) and are about the same distance from the sun, our eyes can see them.

Hot objects will glow under our naked eyes, isn't it a bit like warning us: don't touch them! The weather is very hot.