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Suppose I stand in a room without windows or lights, but there is a radio playing. Then I won't see anything, but there is light! It just isn't visible. Is the room black?

Now consider an object usually termed a "black body? In what sense is this black? How does it differ from the first?

Finally, I look to the sky at night. I see light from stars and much blackness. Does this mean space is black?

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  • $\begingroup$ Define what you mean by "black" and by "color" and then you can answer these questions according to your definitions. Of course, someone else with different definitions may give different answers... $\endgroup$ – Nathan Reed Jul 22 '15 at 4:16
  • $\begingroup$ More on the color black. $\endgroup$ – Qmechanic Jul 22 '15 at 6:25
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A black body is an ideal physical body that absorbs all incident radiation, regardless of frequency or angle of incidence. Thus, there is no light reflecting from it; for example, Black Holes.

The definition of "Black" is different in Physics from Everyday Life. In Physics, color is a term that anglophones use to describe the various frequencies of electromagnetic radiation, to be more precise, the visible part of electromagnetic spectrum. Therefore, "Black" is not considered as a color in Physics, because the color "Black" is not associated with any frequency of electromagnetic radiation. It causes by, in contrast, the absent of any radiation entering your eyes.

"Blackness in Space" is complicated. Vacuum doesn't mean nothing is there. There is radiation in every part of the universe. You can test this by using your radio and tune it to some random frequency (not stations' frequencies). You will hear the static sound. A small percentage of that sound is coming from outer space (from the Big Bang, to be exact). The technical name of that radiation is called the Cosmic Microwave Background Radiation or CMB. You can't see it because it is in the Microwave region of the electromagnetic spectrum. That is why you can see nothing in deep space, or as most people interpret it, black.

Therefore, if you ask "Does this mean space is black?", it is hard to answer because space, in Physics, has no color due to no visible radiation except for regions of stars. You can't say "space is black" neither because black is not a color. enter image description here

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Okay, so here's the deal about so-called "black bodies": we can in some ways define a thing's color by saying, "if I shine a light on it, what are the frequencies of the extra light that I get back?" In this precise sense, the Sun is black. The Sun can't help but shine the exact amount of light that it shines; if you shine a light on the surface of the Sun, that light is entirely absorbed, not reflected. In fact, it turns out in general that good light-emitters are also good light-absorbers. So that's why we say that the blackbody spectrum is a black-body spectrum: it's about what light gets reflected and what light gets absorbed.

So you have to choose what you mean. Do you just want the peak frequency of the light that you're seeing when you point a spectrometer at the thing? Then a truly pitch-black room at absolute zero is black, a pitch-black room with a radio could in an extended sense be "radio"-colored, the Sun is green, and space is "microwave"-colored.

Or do you mean "what color do I see?" -- in which case the Sun is white, the pitch-black room is black, space is black, the radio room is black.

Or do you mean this idea of "I'm going to shine a light on it and see how the spectrum I'm seeing changes"...? Then the Sun is black, the pitch-black room is whatever color its walls normally are when it's illuminated, same with the radio room, and space is black.

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I think your question is a matter of semantics.

Our eyes are sensitive to radiation between approx. 400-700nm wavelength those frequency limits correspond to blue and red respectively. But the perception of color is actually quite a bit more nuanced than that - many things we see don't emit/reflect light at a single frequency, but our eyes/brain still gives it some kind of color value. We also often talk about gold and silver as colors, which implies something about their reflective/surface qualities in addition to the frequencies of radiation they emit.

Generally, your eyes aren't sensitive to radiation outside that spectrum - that's when you perceive black. Whether you call that a color or lack thereof is a matter definition.

As for black bodies: confusingly, they don't have to be black. The sun is a reasonable black body. They're called black bodies because they don't reflect any light; they absorb all incident light and reemit it according to Planck's Law. It's worth noting that at room temperature, a black body does appear black, because the light is lower frequency than you can see.

As for your last question: space is filled with radiation, you just can't see most of it.

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