I've heard that a blackbody radiator at 6500 K, for instance, emits pure white light, but does that mean it's made up of all wavelengths in the visible spectrum in equal proportions? Or is the combination of wavelengths unequal?
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The spectrum for a 6500 K blackbody is shown below. Notice is doesn't have equal strengths of all wavelengths. But the differences are relatively small.
White light is considered to have the same strengths but these maybe close enough to appear white. It probably depends more on the viewer for difference this small.
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$\begingroup$ Definitely among the whitest black body spectra. T less we get into more yellow, a bit more and we are into blue. 6500 K is it in the range of the so called colour Temperature White. $\endgroup$ Oct 28, 2017 at 21:43
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"Pure white light" is not a technical term, so it is unclear what exactly it means. But certainly, a blackbody does not emit a flat power spectrum across the visible range (or across any range, for that matter).
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$\begingroup$ Might be technical but not physical as to be used in physics. + 1 because it touch the point in the question $\endgroup$ Oct 28, 2017 at 21:34
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Don't confuse white light with a white surface.
A white surface reflects all wavelengths equally.
White light is a much harder concept to define. It is highly subjective.
Here's an experiment you can do: Light up one room in your house with "warm white" lamps, and light up the room next door with "cool white"/"daylight spectrum"/"full spectrum" lamps.
Wait for nightfall, and then spend some time in each room. Decide for yourself whether the light in the room is "white". "
Now, go out in yard, and look at the side-by-side windows of the two rooms from outside. Does either of them look white?
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$\begingroup$ Additionally, compare the light from ordinary incandescent bulbs to daylight: people (after adjusting) often see both of these as the same, but they're really not: you used to need special 'tungsten' film if you wanted to make colour pictures indoors. $\endgroup$– user107153Oct 28, 2017 at 19:15
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I don't think equal can be defined. Is the spectrum w.r.t wavelength, frequency, or number of photons? If any one of these is "equal", the other 2 are not.
That said, check out the "color triangle":
https://en.wikipedia.org/wiki/Color_triangle
https://en.wikipedia.org/wiki/CIE_1931_color_space
which is defined for "the average human eye". (And who says humans define white?).
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The three types of cones in the human eye are commonly labeled by the color to which they are most sensitive, Blue, Green and Red. A higher degree of stimulation of one of these groups of cones causes the individual to perceive the color of that spectrum more than the others, giving the object being viewed its particular color. Since white light is peculiar to the individual perceiving it,meaning the many variables that effect the perception of the color of light. If an individual is seeing a purely white light then the three groups of color stimulated cones in the eyeball must be receiving equal amounts of spectral stimulation to be seen as pure white light any degree of a higher elevation of stimulus to any one of the three groups would cause a shift of perception toward that individual color. Purely white light would describe light being seen through a human eye as having an equal dispersion across the visible spectrum.