Light and Heat

The subject of the relationship between light and heat came up a few weeks ago, and I was lucky enough to be let in on a summary of the discussion:

“Light equals heat”

“Nope.”

“Well then, explain the Sun.”

I can’t possibly improve any further on this sophisticated exchange, but it did eventually bring to mind the fact that light is sometimes described by its “temperature”. Certainly you’ve heard the phrase “red hot” or “white hot” and you might even associate it with iron work or forging steel. Where it gets a little nerdy is when you assign a temperature to “red” or “white”.

Firstly, the Fahrenheit scale isn’t used and neither is the Celsius scale. No, for this we use another scale called Kelvin. This is the temperature scale that physicists and astronomers like because 0 Kelvin is also known as “absolute zero” or “penguin butt cold”. Nothing in the universe can get any colder than 0 Kelvin; that’s it.

For a little context, “room temperature” is generally taken to be:

77 Fahrenheit (F) = 25 Celsius (C) = 298.15 Kelvin (K)

This should also neatly explain why no one uses the Kelvin scale to discuss any kind of normal temperature. (Can you imagine: “Boy, it’s a hot one; feels like 303 Kelvin!”) Mercifully, a change in temperature of 1 Kelvin is the same as a change of 1 Celsius, so it’s not completely silly. But Kelvins are useful to discuss things that are very, very cold and also very, very hot: like stuff that’s so hot that it glows.

So, if you’ll notice on the LED light bulbs that you see in the stores, they typically have some number on it like 3500K, which is its color temperature. The temperature corresponds to a nerdy physics concept (a “black-body radiator”) that you can think of as follows: an ideal material of this type emits light that goes from red to orange to yellow to white to bluish-white as it gets hotter and hotter. Incandescent bulbs behave mostly like this. (FYI: we recently discovered that dimmable LED bulbs do not look more yellow/orange as you dim them. Regrettably, they stay exactly the same color.) And it is incandescent bulbs and how they respond to heat that make this relatively simple concept absurdly difficult to keep straight.

If you see light from a low-powered incandescent bulb that has a nice warm glow—yellowish orange, let’s say—it probably has a color temperature around 2500K. If you see a high-powered incandescent bulb, it looks cool—very white with only a slight yellowish tinge—it might have a color temperature around 6000K, which is a much higher temperature.

Yes, this feels like some kind of joke: a red-hot fireplace poker emits “warm” light on the low end of the temperature scale and a white-hot flare emits “cool” light at the high end of the temperature scale. I wish I could blame this on some buffoon from the 1700’s who didn’t understand heat but it kinda comes down to our association of “warm” with orange and “cool” with bluish-white. And besides, it would feel weird to talk about color temperatures that went from “warm” to “really, really hot”.

In any case, light may not quite equal heat: there’s a conversion you have to do to the temperature first and I’m pretty sure you have to divide the color of the light by heat. Go figure.