What Is the Nature of Black in the Visible Spectrum VIBGYOR?

[shihab-kol]

In the visible spectrum VIBGYOR, there is no black colour.So, what do we percieve as 'black' ?

Another of my queries is that when dispersion takes place there is a change in wavelength but not so in case of frequency. But they are related inversely. So, why does this happen?

 

[anorlunda]

In the visible spectrum VIBGYOR, there is no black colour.So, what do we percieve as 'black' ?

Black is the lack of light. White is the superposition of all visible colors.

Just think of a black-and-white picture. The dark parts are black and the bright ones are white, medium intensities are grey.

 

[shihab-kol]

Black is the lack of light.

What do you mean by that? How is light absent? Is it something related to absorption?

 

[sophiecentaur]

Bright / dark / black relate to intensity and not to wavelength or frequency (or chrominance).

 

[anorlunda]

Go into a sealed room and turn off the lights. What color do you see?

Is it something related to absorption?

Yes, that's one way. A surface that reflects no light to your eye is perceived as black.

 

[shihab-kol]

Go into a sealed room and turn off the lights. What color do you see?
Yes, that's one way. A surface that reflects no light to your eye is perceived as black.

So, something absorbing all light is perceived to be black?

 

[sophiecentaur]

So, something absorbing all light is perceived to be black?

Yes

 

[shihab-kol]

Bright / dark / black relate to intensity and not to wavelength or frequency (or chrominance).

The frequency part of the question does not relate to black.

Yes

Thanks for the clarification .
And about the second part ?

 

[nasu]

In your second part I would guess you misuse the term "dispersion". Dispersion means that the speed of the wave through a medium depends on frequency. You also can write velocity of wave as a function of wavelength. But dispersion does not mean a change in either frequency or wavelength. The speed in the medium is different than in the air and so the wavelength will change while propagating through that medium. The frequency does not depend on the propagation speed, is a characteristic of the source of the wave. But this happens even in a medium without dispersion.

 

[shihab-kol]

In your second part I would guess you misuse the term "dispersion". Dispersion means that the speed of the wave through a medium depends on frequency. You also can write velocity of wave as a function of wavelength. But dispersion does not mean a change in either frequency or wavelength. The speed in the medium is different than in the air and so the wavelength will change while propagating through that medium. The frequency does not depend on the propagation speed, is a characteristic of the source of the wave. But this happens even in a medium without dispersion.

Yes.
I tried to mean refraction.
Thanks

 

[sophiecentaur]

Frequency and intensity are independent. Intensity does have an effect on perceived color, though. You can see a yellow area and a brown area in the same scene. They can both have the same chrominance co ordinates. Our color perception falls off as light level drops and our vision becomes monochrome at very low levels. It's a bit of a grey area. ( pun intended)

 

[pixel]

Another of my queries is that when dispersion takes place there is a change in wavelength but not so in case of frequency. But they are related inversely. So, why does this happen?

It's true frequency and wavelength are inversely related, but in refraction the wavelength and the speed of the wave change, the frequency doesn't change.

 

[Drakkith]

So, something absorbing all light is perceived to be black?

Well, perfect absorbers do not exist. Typically an object with a "black" color absorbs a much larger percentage of incoming light across most of the visible spectrum compared to surrounding objects, though it never absorbs all of it. I am currently surrounded by about a dozen different objects that I would call "black" and each one actually looks slightly different than the others.

 

[russ_watters]

So, something absorbing all light is perceived to be black?

Or not having them created to begin with. Consider a white movies screen.

 

[shihab-kol]

Why does the frequency not change?

 

[jbriggs444]

Why does the frequency not change?

Why does the frequency not change when a wave enters a medium like a prism and thereby speeds up or slows down?

Because every time a wave crest arrives at the boundary the wave crest passes through and proceeds past the boundary. The frequency at which crests arrive is equal to the frequency at which crests proceed.

 

[sophiecentaur]

Black is not a particularly quantitative thing. It is not 'less than 10 photons arriving per nanosecond'.
The doorway into a bar can appear black at noon but, with the same lighting inside, it can be the brightest object in the street on a moonless night.

 

[XZ923]

Well, perfect absorbers do not exist. Typically an object with a "black" color absorbs a much larger percentage of incoming light across most of the visible spectrum compared to surrounding objects, though it never absorbs all of it. I am currently surrounded by about a dozen different objects that I would call "black" and each one actually looks slightly different than the others.

I'm just curious, isn't a black hole a perfect absorber beyond the event horizon? Further, would it be accurate to state that the only known existence of "perfectly black" that we know of at this point is inside a black hole's event horizon as perceived by someone outside it? Or is this negated by the existence of Hawking radiation which should be visible in the IR range?

I'm sorry if these are fairly stupid questions; I'm fascinated by the concept of black holes but don't know much about them.

 

[jbriggs444]

I'm just curious, isn't a black hole a perfect absorber beyond the event horizon?

Wiki can answer this. If one considers a black hole as if it were a "black body"...

"A black hole of one solar mass (M☉) has a temperature of only 60 nanokelvins (60 billionths of a kelvin); in fact, such a black hole would absorb far more cosmic microwave background radiation than it emits. A black hole of 4.5×10²² kg (about the mass of the Moon, or about 13 µm across) would be in equilibrium at 2.7 K, absorbing as much radiation as it emits. Yet smaller primordial black holes would emit more than they absorb and thereby lose mass.^[11]"​

 

[pixel]

Why does the frequency not change?

Another example: a light string attached to a heavier one. If you start a wave train in the lighter string, as each crest meets the heavier string it will become a crest there, so the frequencies are the same in each string. But for equal tension in each string, the wave in the heavier string is slower and the wavelength there is smaller than in the lighter string.

 

[Drakkith]

I'm just curious, isn't a black hole a perfect absorber beyond the event horizon?

I'm talking more about physical objects you could encounter in real life. Not crazy spacetime geodesics.

 

[sophiecentaur]

Why does the frequency not change?

How could it? What would drive the parts of one medium at a different frequency from the parts in the previous medium? You need Phase Continuity at an interface and that demands no frequency change.

 

[shihab-kol]

But shouldn't the wave lose energy along the way? And frequency is a measure of energy.In a wave the energy is transferred from one place to another but along the some dissipation should occur.
So, why?

 

[jbriggs444]

But shouldn't the wave lose energy along the way? And frequency is a measure of energy.In a wave the energy is transferred from one place to another but along the some dissipation should occur.
So, why?

Why should a wave lose energy along the way?

In the case of electomagnetic waves quantized as photons, the energy in a photon is determined by its frequency and vice versa. But photons do not "dissipate" (disregarding cosmological red shift).

In the case of more mundane waves (e.g. sound waves, surface waves on a lake or transverse waves on a taut string) there is no association between frequency and wave energy. The energy is associated with amplitude, not frequency.

 

[shihab-kol]

Why should a wave lose energy along the way?

In the case of electomagnetic waves quantized as photons, the energy in a photon is determined by its frequency and vice versa. But photons do not "dissipate" (disregarding cosmological red shift).

Ah, I am being taught the classical nature of waves where they do dissipate energy (like sound), so the quantization of energy in a photon did not occur to me.
Thanks.

 

[shihab-kol]

Well, another question.
I draw lines of black ink consecutively on a paper.When viewed from a distance they appear to be one solid thick black line.But when viewed from a near position one can make out the separate lines.Why is this so?

 

[A.T.]

I draw lines of black ink consecutively on a paper.When viewed from a distance they appear to be one solid thick black line.But when viewed from a near position one can make out the separate lines.Why is this so?

https://en.wikipedia.org/wiki/Angular_resolution

 

[sophiecentaur]

Well, another question.
I draw lines of black ink consecutively on a paper.When viewed from a distance they appear to be one solid thick black line.But when viewed from a near position one can make out the separate lines.Why is this so?

Diffraction.

 

[shihab-kol]

Diffraction.

How?

 

[shihab-kol]

I read in a brief history of time that the Earth is a geodesic and assumed it to be some sort of a sphere. But Drakkith says that black holes are also geodesics .
What exactly is a geodesic?