Why can't light be in black color?

[Yashas CS]

When we pass light through a colloidal solution we can see the path of light which os called tyndall effect and we can say that in its path it vibrates the particles in its way providing considerable energy to the particles that is why we can see the light. The area where the light does not pass we can say that it is dark or it is same as when you pass light through a box with a hole on one of the sides.

 

[Jon Richfield]

I regard most of the foregoing as missing the original point and I reckon that that physics teacher would agree with me, bearing in mind his broad hint about the sun.
OK, some of us speak of "black light" as UV, but even as a schoolkid I always thought that a stupid and misleading term, and that was before I even had heard of black body radiation. I am sure that the teacher was NOT speaking of anything of the type.
What is colour? Most of us seem to agree on that, not counting a bit of a red herring about whether light is visible or not. Ignoring another (subjective) red herring about our eyes and brains, it (objectively) amounts to the relative intensities of relevant frequencies of light, together with the absolute intensity of the ambient light relative to the detection apparatus.
In particular, consider "white light".
In everyday experience, and in the absence of certain classes of gross subjective distortion (such as recently having lived in green light for a few weeks on end) this amounts to black body radiation within a given spectral range (for humans, between roughly 400 and 800 nm), and intensity ranges within a very few orders of magnitude of those typical of clear-air desert sunshine.
Now, let us take our source, diffuse the light and progressively reduce its gross intensity without altering any other objective attributes.
Still white light, right?
Well yes, but keep dimming it and you will find yourself in blackness; dark if you like. It still is radiation from the same black body at the same temperature, only there is less of it. It still is there, as you can confirm with starlight cameras etc.) And long before the light has vanished you would say it is black (or grey, which is simply an intermediate degree).
Talking rubbish am I? Try harder!
Set up a white light as described hereintofore, to shine on a suitable screen. Say a nice wide circle of roughly even intensity.
Now interpose a few filters of various intensities, but all of neutral grey (otherwise known as "dark white" in colour) . They will cast shadows of which colour?
Errrr... grey, except where it is dead black. Right?
But then, the only objective difference between black, grey, and white is the intensity.
Black light is white light of a sufficiently low range of intensities, as originally emitted by a black body of a sufficiently high temperature. such as the ... sun?
Much as any other colour effect can be seen as a range of intensities of a given frequency mix.
Yes?
Quibbling, am I?
Then try to make sense of this well-known, (brilliant, though not white) illusion, that includes white, grey, and (in some versions) black light:
https://en.wikipedia.org/wiki/Checker_shadow_illusion

 

[zinq]

Instead of debating physical color vs. perceptual color, why not agree that both things exist and are separate concepts.

Physical color can be thought of as a point in a (portion of) an infinite-dimensional Hilbert space of arbitrary combinations of frequencies having finite total energy.

Perceptual colors are points in a (portion of) a 3-dimensional Euclidean space that was discovered using color matching experiments: CIE 1931 XYZ space, and its later refinements.

The process of perceiving light is the phenomenon whereby physical light enters the eye and after stimulating cells in the retina is interpreted as a color in perceptual space. This determines a mapping from physical color space to perceptual color space.

So, the study of color perception is the study of this mapping. (Of course we are glossing over niceties like the fact that people have two eyes and that everyone perceives color a bit different from everyone else.)

 

[davenn]

So, did you notice the sunspots on the Sun? They appear black, yet emit bright light. Even black and white are sometimes relative.

not really.

"Sunspots are temporary phenomenaon the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They correspond to concentrations of magnetic field flux that inhibit convection and result in reduced surface temperature compared to the surrounding photosphere..."
https://en.wikipedia.org/wiki/Sunspot

yes really ! They are around 4000K, they only appear dark compared/ in contrast to the 5700K photosphere around them

In a novel (Nebula award winner) the following question is asked: "If the speed of light is 3⋅108m/s, what is the speed of dark?"

same.

no, it doesn't exist, therefore it doesn't have a speed
Again black isn't a "colour" it isn't in the spectrum of visible light, it doesn't have a wavelength, it IS the ABSENCE of lightDave

 

[Jon Richfield]

yes really ! They are around 4000K, they only appear dark compared/ in contrast to the 5700K photosphere around them
no, it doesn't exist, therefore it doesn't have a speed
Again black isn't a "colour" it isn't in the spectrum of visible light, it doesn't have a wavelength, it IS the ABSENCE of lightDave

Get real Dave! Read a book called The Shadow Club by Roberto Casato. It should make you more careful with such assertions.
A hint: before claiming that an entity exists or not, you should analyse its nature.
Would you say that a hole in the ground does not exist? Or a hole in a semiconductor?

 

[rootone]

Would you say that a hole in the ground does not exist?

You can say a hole in the ground is absence of any solid matter in the same sense that 'black' is an absence of any electromagnetism.
Both do exist, but both are defined as being an absence of something.

 

[alw34]

Get real Dave!

Jon, now play nice.
Dave is from 'down under'.
Everything there is 'upside down'.
But they do outrank North America in infra red light all year round.

 

[Drakkith]

Get real Dave! Read a book called The Shadow Club by Roberto Casato. It should make you more careful with such assertions.
A hint: before claiming that an entity exists or not, you should analyse its nature.
Would you say that a hole in the ground does not exist? Or a hole in a semiconductor?

I would agree with Dave. Defining something to be the absence of something else doesn't mean it exists as a real physical object with properties such as velocity, charge, etc. However, for convenience and simplicity we usually talk about such things as if they actually exist. It's easier to say, "Hey Jim, watch out for that hole" instead of, "Hey Jim, watch out for that spot where we removed all the dirt". It's also easier to make models and do calculations if we model holes as being real objects, but again, that doesn't mean they are 'real' in the sense I described above. A hole in a semiconductor material can be modeled as if it is a positive charge moving the opposite direction to the flow of electrons, but in reality there's simply no electron at that location.

Trying to assign a speed to darkness makes no sense. For example, a shadow can move across the ground at any speed you want. Even superluminal.

 

[alw34]

I vote this the craziest discussion yet.

Someone award the OP a prize!

 

[Drakkith]

I vote this the craziest discussion yet.

Someone award the OP a prize!

Oh, if only you could see some of the threads us mods have had to remove...

 

[zinq]

You can define anything you want (that makes sense); such definitions are not "wrong".

In mathematics, a hole in a space is a very precise concept, that most certainly exists.

There is nothing wrong with defining darkness as the absence of light.

But do no confuse it with light, which carries energy and wavelength, can be a wave or particle, etc. Dark could be defined as the zero element in the (portion of) the infinite-dimensional Hilbert space that comprises all possible light rays. But it is different from the nonzero elements, in that it doesn't "carry" any information, since it doesn't "go" anywhere.

IF we wanted to make sense of "the speed of dark", that might actually make sense: Suppose we are continuously shining a light at a screen at a distance D away. Suddenly we turn of the light source; assume for simplicity that the light emission stops instantly. Then if as usual, c is the speed of light, the screen should stop reflecting light at a time equal to D/c later. So in this sense the speed of dark is the speed of light.

 

[Drakkith]

IF we wanted to make sense of "the speed of dark", that might actually make sense: Suppose we are continuously shining a light at a screen at a distance D away. Suddenly we turn of the light source; assume for simplicity that the light emission stops instantly. Then if as usual, c is the speed of light, the screen should stop reflecting light at a time equal to D/c later. So in this sense the speed of dark is the speed of light.

Now take that light source and sweep it across the screen. What is the maximum speed of the shadow/light spot across the screen?

 

[zinq]

Sure, no maximum, but that's not dark "moving". Because, in no sense is the shadow at one location the "same" as the shadow at another location. Nothing moved, so no speed is relevant.

 

[Grinkle]

What is the maximum speed of the shadow/light spot across the screen?

Musn't it be c, since we could never measure with any experiment that it were faster?

 

[Grinkle]

If dictionary.com is an acceptable reference, then below citation should resolve the question of whether black is a color. ;-)

noun
20. the color at one extreme end of the scale of grays, opposite to white, absorbing all light incident upon it.

 

[Drakkith]

Musn't it be c, since we could never measure with any experiment that it were faster?

Sure we can. Place two light sensors on the screen some distance apart from each other and then sweep the light across them. You'll find that the difference in the time that one sensor goes on/off and the other sensor goes on/off can be anything from 0, corresponding to just pointing the light source directly at the sensors and turning it on or off, all the way up to any value, corresponding to how fast you sweep the light source across the screen.

Sure, no maximum, but that's not dark "moving". Because, in no sense is the shadow at one location the "same" as the shadow at another location. Nothing moved, so no speed is relevant.

The only stuff that moved was the light on its trip from the light source to the screen. But that's not any different from your example. It still makes little sense to talk about the speed of darkness.

 

[davenn]

Get real Dave! Read a book called The Shadow Club by Roberto Casato. It should make you more careful with such assertions.
A hint: before claiming that an entity exists or not, you should analyse its nature.
Would you say that a hole in the ground does not exist? Or a hole in a semiconductor?

back off Jon .. stop being obnoxious !

 

[Drakkith]

I think this thread has gone on and off topic long enough. Since the original question has been beaten to death as well, I'm locking this thread. Anyone interested in the 'speed of dark' or something else in this thread can start a new one.