# Does a black body radiate all colors of the rainbow?

## Main Question or Discussion Point

When a black body is heated the colors listed are IR, red, orange, yellow, white, blue-white, UV. Why doesn't a black body radiate all colors when heated: Red Orange Yellow Green Blue Indigo Violet? And why not shades of these colors?

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Integral
Staff Emeritus
Gold Member
The colors radiated depend on the temperature of the body. All colors can be radiated.

The colors radiated depend on the temperature of the body. All colors can be radiated.
Thanks. I know that the wavelength of emitted energy decreases as the black body gets hotter and it seemed to me that ALL colors should be radiated between IR and UV. How about a fire poker? I can see it glowing red, then orange, then yellow, then white, But I don't see it radiate in the green ???? Is it because it's not a perfect black body?

K^2
Thanks. I know that the wavelength of emitted energy decreases as the black body gets hotter and it seemed to me that ALL colors should be radiated between IR and UV. How about a fire poker? I can see it glowing red, then orange, then yellow, then white, But I don't see it radiate in the green ???? Is it because it's not a perfect black body?
It always radiates across the whole spectrum, but in different proportion. Your eye, however, is only sensitive to three overlapping regions which are roughly red, green, and blue. When a black body gets just hot enough to visibly glow, it radiates mostly in the IR, a bit in red, and so little in green and blue that your eye doesn't really pick up on that. You perceive the result as red. As it gets hotter, it starts to radiate more at lower wavelengths. You start seeing orange and yellow. However, what you see is yellow is already due to both red and green receptors in your eye getting excited, as yellow wavelengths are on overlap of the two. As the object gets hotter, the amount radiated in red doesn't reduce significantly, but you start getting more radiated in blue. The appearance turns from yellow to white, since white light excites receptors for red, green, and blue equally. When an object gets hotter yet, amount radiated in blue becomes higher than that in red and green. The object's apparent color shifts from white to blue. From there, it shifts into violet.

In short, the reason you don't see an object going through all of the colors of the spectrum is because radiation is never at just one frequency. It's a fairly broad spectrum. You can get "pure" colors at the edges of the spectrum for relatively cold or really hot objects, because they just clip the visible spectrum. But in the middle of visible spectrum, you get all of the wavelengths, so you are dealing with colors that are almost white.

The fact that its black dosent make a diffrence it turns red because thats the wavelength that most metals emit when the electrons go into a higher orbit and back. you can make a green fire by burning black ink even though its black. Also copper will burn green even though it won't turn green.

When a black body is heated the colors listed are IR, red, orange, yellow, white, blue-white, UV. Why doesn't a black body radiate all colors when heated: Red Orange Yellow Green Blue Indigo Violet? And why not shades of these colors?
It's like a rainbow. It has all wave lengths of light so you would think that it would see all colors, but instead the great majority of people only see seven. This is because vision is complicated and we actually see combinations of colors. It is a lot like chords in music. Almost all people hear chords, not individual notes. So we see a color, not the individual wavelengths it is made of.

Suppose that in sound you piled thousands of unlike chords on top of one another. You would get a sort of not-unpleasant noise, a sort of combination of a rumble and a hiss, but it wouldn't sound like a chord. White light is kind of like that, thousands of colors all mixed together. So yes, all of the colors are there, but they are all mixed together so we don't see any of them.

If you wanted to filter the sound to get one chord, you could. It you wanted to filter white light to get one color, you could.

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So I guess if you are asking about the perception of the colour of black body radiation the answer is no. The colour cannot be green, pink or blue. This should give more information http://en.wikipedia.org/wiki/Color_temperature

mfb
Mentor
The fact that its black dosent make a diffrence it turns red because thats the wavelength that most metals emit when the electrons go into a higher orbit and back. you can make a green fire by burning black ink even though its black. Also copper will burn green even though it won't turn green.
Well, those are not black bodies - not even a good approximation.
They emit more greenish light than black bodies would do (and less at other wavelengths).

Thanks for the great replies! I've now got some more reading to do . . . .