<p>Black is not a color; it is the absence of light.</p>

<p>I think you interpret the sentence as "the light is black", but it's just a way of saying "there is no light (at optical wavelengths, but there is at radio wavelengths)".</p>

<p>In my opinion this is not a good way of explaining the CMB. The sky is <em>not</em> black at any wavelengths, but has a background radiation at all wavelengths. In the optical, this background originates (primarily) from the integrated light from stars; in the microwave it originates (primarily) from the afterglow of Big Bang; in the infrared from dust, etc.</p>

<p>See the figure in <a href="https://physics.stackexchange.com/a/196373/70207">this answer</a> to a question about the number density of CMB photons, to see how large the background is in various wavelength regions.</p>

CC BY-SA 3.0
2016-11-26T05:55:47.110Z
2016-11-26T05:55:47.110Z
2017-04-13T12:40:15.377Z
pela
<p>How could visible light be in pitch-black? Can it be?</p>

CC BY-SA 3.0
2016-11-25T10:36:56.557Z
Read the sentence the correct way around. It's pitch black at visible wavelengths. As opposed to radio wavelengths, where it is not pitch black, but glowing.
AtmosphericPrisonEscape
CC BY-SA 3.0
2016-11-25T15:57:17.997Z
By sky, they mean deep space, not our nice blue atmosphere. Olbers' paradox: https://en.wikipedia.org/wiki/Olbers'_paradox
Wayfaring Stranger
CC BY-SA 3.0
2016-11-25T20:09:22.477Z
It seems you interpret the sentence as "the light is black", but it's just a way of saying "there _is_ no light (at optical wavelengths, but there is at radio wavelengths)".
I don't see why people would downvote your question, though.
pela
CC BY-SA 3.0
2016-11-26T01:55:40.590Z
@pela you should make your comment the answer.
LaserYeti
<big-bang-theory>
How could visible light be in pitch-black? Can it be?
Purple Rain Kim
# How could visible light be in pitch-black? Can it be?

(Asked by: username_0 on 2016-11-25)

How could visible light be in pitch-black? Can it be?

**Tags:** big-bang-theory

> **Comments:**
> **username_1**: Read the sentence the correct way around. It's pitch black at visible wavelengths. As opposed to radio wavelengths, where it is not pitch black, but glowing.  
> **username_2**: By sky, they mean deep space, not our nice blue atmosphere. Olbers' paradox: https://en.wikipedia.org/wiki/Olbers'_paradox  
> **username_3**: It seems you interpret the sentence as "the light is black", but it's just a way of saying "there _is_ no light (at optical wavelengths, but there is at radio wavelengths)". I don't see why people would downvote your question, though.  
> **username_4**: @username_3 you should make your comment the answer.  

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## 1 answers

### Answer by username_3 on 2016-11-26. Score: 3

Black is not a color; it is the absence of light.

I think you interpret the sentence as "the light is black", but it's just a way of saying "there is no light (at optical wavelengths, but there is at radio wavelengths)".

In my opinion this is not a good way of explaining the CMB. The sky is *not* black at any wavelengths, but has a background radiation at all wavelengths. In the optical, this background originates (primarily) from the integrated light from stars; in the microwave it originates (primarily) from the afterglow of Big Bang; in the infrared from dust, etc.

See the figure in [this answer](https://physics.stackexchange.com/a/196373/70207) to a question about the number density of CMB photons, to see how large the background is in various wavelength regions.


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