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

[pixel]

I expect some people will disagree with this blanket statement but the aberrations that are caused in glass lenses are still, effectively due to the failure of the various paths to add up perfectly (in phase). To my mind that is just a sub-set of diffraction.

I guess I'm one of those people, although maybe we're getting more into the realm of philosophy here.

In diffraction, we consider the different path lengths that light takes to reach a given point. With aberrations, the light is directed to different points, e.g., with spherical aberration the rays that goes through the outer parts of the lens meet closer to the lens than the rays that go through the center of the lens. I would not consider the inability of all these rays to meet at the same point as diffraction. And usually diffraction is what provides the ultimate limitation on imaging, after all the aberrations are minimized, hence the term "diffraction limited" optics.

 

[sophiecentaur]

I guess I'm one of those people,

Fair enough. But I cannot see your distinction between how a lens produces an image and the way that, for example, a Zone plate will also produces an image. No one would claim that a zone plate doesn't work by diffraction - it produces points on a plane where the light from a particular point on an object adds constructively (in phase) but nowhere else from that source point. Precisely how a lens operates. You have chosen to look at the operation of a lens in terms of Rays but it can also be treated in terms of what it does to a spherical wave front, emanating from one point on an object. The lens delays each part of the wave front so that the the path length is equal at only one point in the image plane. The only difference between the two is that the zone plate works by selecting parts of the emanating wave front to produce constructive interference at certain parts of the image plane and the lens adjusts the phases by suitable delays through different thicknesses of glass.
I appreciate that the terms 'diffraction' is commonly used in a restricted sense but it's a fairly arbitrary classification - in the same way that the patterns from two slits tends to be referred to as an interference pattern - largely because it can be predicted fairly well by treating the slits as point sources. But it's still a diffraction pattern which can be calculated approximately using a simple summation of terms instead of using the full integration.

 

[shihab-kol]

I know it is a lot of questions but since the heading contains both light and colours here goes.
Blue + red = magenta
But just combining rays would not give me magenta.At least that's what I think.
So,what are the criteria?
I think an object which gives out magenta will absorb every color except blue and red.
But is this all?
And yeah,geodesics

 

[shihab-kol]

Oh, come on !
You all going to leave me hanging??!?

 

[russ_watters]

I know it is a lot of questions but since the heading contains both light and colours here goes.
Blue + red = magenta
But just combining rays would not give me magenta.

Why not? That's how a computer monitor does it.

 

[Drakkith]

Fair enough. But I cannot see your distinction between how a lens produces an image and the way that, for example, a Zone plate will also produces an image. No one would claim that a zone plate doesn't work by diffraction - it produces points on a plane where the light from a particular point on an object adds constructively (in phase) but nowhere else from that source point. Precisely how a lens operates. You have chosen to look at the operation of a lens in terms of Rays but it can also be treated in terms of what it does to a spherical wave front, emanating from one point on an object.

Indeed. I believe one could say that diffraction is just wave interference as well. But we tend to define different terms to talk about specific situations when it suits us.

I know it is a lot of questions but since the heading contains both light and colours here goes.
Blue + red = magenta
But just combining rays would not give me magenta.At least that's what I think.
So,what are the criteria?
I think an object which gives out magenta will absorb every color except blue and red.
But is this all?

There is much more to color perception, but that is the basic idea, yes.

Oh, come on !
You all going to leave me hanging??!?

You posted your previous question right after most people who use the forum went to bed (your post was at 11:38 PM my time). Give them time to get their morning coffee at least!

 

[sophiecentaur]

But just combining rays would not give me magenta.At least that's what I think.

Combining "Rays" is additive mixing. which is how TV produces the colours. Red light plus Blue light will produce Magenta. But when you refer to an"object" that is describing the colour due to Subtractive mixing - the object absorbs all wavelengths except red and blue and will be described as Magenta.
You need to google Additive and subtractive colour mixing. They are the 'opposite' idea to each other. Subtractive mixing is a much more difficult way to produce wanted colours which is why ALL colour film and printing fails for even simple colours and Spot Colours have to be used if you really want colour fidelity.

 

[sophiecentaur]

we tend to define different terms to talk about specific situations when it suits us.

yes, that is common usage but the reality is that whenever there is more than an ideal point source, diffraction takes place. The 'quality' of the image is just a matter of how well the various waves manage to arrive at the right time in the right place.

 

[Drakkith]

yes, that is common usage but the reality is that whenever there is more than an ideal point source, diffraction takes place.

Diffraction always occurs though. Even with an ideal point source. A "diffraction limited" telescope would image a point-source as an airy disk, just as expected from diffraction laws.

The 'quality' of the image is just a matter of how well the various waves manage to arrive at the right time in the right place.

Certainly. Geometric (ray) optics cannot adequately describe the final image quality in many optical systems.

 

[shihab-kol]

You posted your previous question right after most people who use the forum went to bed (your post was at 11:38 PM my time). Give them time to get their morning coffee at least!

Oh, by my time it was pretty early!

 

[shihab-kol]

Combining "Rays" is additive mixing. which is how TV produces the colours. Red light plus Blue light will produce Magenta. But when you refer to an"object" that is describing the colour due to Subtractive mixing - the object absorbs all wavelengths except red and blue and will be described as Magenta.
You need to google Additive and subtractive colour mixing. They are the 'opposite' idea to each other. Subtractive mixing is a much more difficult way to produce wanted colours which is why ALL colour film and printing fails for even simple colours and Spot Colours have to be used if you really want colour fidelity.

So, when i emit only blu and red or blue and green they will mix just like that but when I reflect it , I absorb other colours and reflect only those two.
Ok, got it.
Thanks.

 

[shihab-kol]

It seems the members won't answer to my 'geodesic' queries .
Why? Because its not mentioned in the tags?
Ok, look again, its there!
Now, please reply!

 

[sophiecentaur]

Diffraction always occurs though. Even with an ideal point source.

Sorry, I didn't mean it that way. I meant that it's only a point source that creates a pattern (itself) that is diffraction fringe free.
Unless you are a glow worm or bioluminescent squid, you have colour due to subtractive mixing from white light

 

[russ_watters]

It seems the members won't answer to my 'geodesic' queries .
Why? Because its not mentioned in the tags?
Ok, look again, its there!
Now, please reply!

Can't you Google that?

 

[sophiecentaur]

It seems the members won't answer to my 'geodesic' queries .
Why? Because its not mentioned in the tags?
Ok, look again, its there!
Now, please reply!

That topic is nothing to do with the colour black. Start a new thread (when you have done some googling) with an appropriate title and you may get some answers. The right title will grab the right people.
(And try to make your comments a little less demanding than that post. You need good will from PF.)

 

[shihab-kol]

Sorry,I did not realize I was demanding.
I will do as told.
Thanks for your help!

 

[sophiecentaur]

Sorry,I did not realize I was demanding.
I will do as told.
Thanks for your help!

You have to realize that some of the people answering you are probably older than your grandfather and we like the Etiquette of the PF environment.
You never get called a **** or a ***** or even a ****** here. "Demanding" is about the worst thing you can be called and for the member to remain a member