thermal radiation - Is darkness really light?

According to this wikipedia article

"Consequently, most objects that absorb visible light reemit it as heat. So, although an object may appear dark, it is likely bright at a frequency that humans cannot perceive."

Can someone explain this to me in lay man's terms? Does this mean that darkness is really light?

Answer

I think it is best to start with some basic facts:

1. Light is made of photons


2. Photons have a wavelength (and a corresponding frequency and energy)


3. Our eyes detect photons within a certain range of wavelengths


4. Materials can reflect, absorb, or emit photons of different wavelengths

"Consequently, most objects that absorb visible light reemit it as heat. So, although an object may appear dark, it is likely bright at a frequency that humans cannot perceive."

This quote is describing what happens when a photon of visible light strikes a material and is absorbed. An electron that absorbs the photon is excited - which means it moves to a higher energy state. Then, it eventually "falls" back down to a lower energy state, and in the process, emits another photon. This photon typically has a wavelength that is longer than the originally absorbed photon, which puts it outside of the range we can detect with our eyes and into the range that we can feel as heat - the infrared region.

Does this mean that darkness is really light?

To answer your question, we have to start with a solid definition of darkness. I like "darkness is the absence of light."

Using that definition, then the answer has to be "no, because dark objects do not emit visible light."

The confusion comes in when you start thinking about the full range of the spectrum. Typically, we think of "light" as "visible light" alone. But all light is electromagnetic energy, which is made of photons. And photons can have a broad range of wavelengths, some visible, and some not.

The wikipedia quote is referring to this. They are bending (or maybe expanding) the definition of "darkness" to mean "does not emit photons." Since the object could be emitting photons in the infrared region (or ultraviolet, or radio, or anything other than visible) you could call it "bright" in that spectrum.

Incidentally, radar works on this principle. Many objects that do not reflect photons in the visible region do reflect photons in the radio region very well.