ELI5: Why do hotter things begin to glow and emit light as they increase in temperature? And why don’t cold things get darker?
ELI5: Why do hotter things begin to glow and emit light as they increase in temperature? And why don’t cold things get darker?
Comments
Black-body radiation.
Cold things are emitting light, it’s just that that light is in the infra-red portion of the spectrum and as an object gets colder it does emit less, becoming “darker” it’s just that the mk.1 human eyeball cannot see that type of light to begin with.
Two different things at work here: reflection and emission.
Most matter reflects light to a certain degree, which is how we can see it normally.
Additionally, all matter emits light due to its temperature. Usually that happens in the infrared range, heat basically. As you heat up that emission starts to move to higher energy levels, which means it’s shifting from infrared towards blue and thus visible light. That’s the reason it goes from dark red to red and orange as it gets hotter.
It’s called “black body radiation.” Basically, every object radiates its heat away in the form of light, and the hotter the thing is, the more it radiates. “Cold” things (including you!) radiate in the infrared spectrum which is invisible to the naked eye, so while they still reflect light just fine, they can’t get visibly any darker. A white lamp is still a white, even if it’s turned off.
If you heat up a piece of metal in a furnace, the light it radiates becomes stronger, until eventually it reaches the visible part of the spectrum and goes from infrared to a visible deep red. Further heating it up makes it eventually glow white-hot as it climbs towards the ultraviolet.
> And why don’t cold things get darker?
They do.
Things glow hot because hot bodies emanate electromagnetic waves. As the item heats, the frequency of the waves rises. At some point the frequency moves out of the infrared commonly referred to as radiative heat, into the visible light. Eventually if they get hot enough they’ll start spitting out even higher frequencies such as gamma rays, so the peak doesn’t stay in the visible light spectrum, it moves right off into the very high frequency stuff.
So things that are cold are emitting their electromangetic waves in the very low frequencies, but also the amount of energy is also very low, so if you could “see” in those wavelengths, they’d be very dark.
As for why “cold items don’t get dark” that’s because the light to see them with is light that bounced off from a light source. They’re just no emitting visible light, because they’re too cold for that, but they are emitting infrared light, which you cannot see. The light that makes them visible to you is merely bouncing off them.