Is there any relation between wavelength and brightness?

sophiecentaur

Drakkith and I get on very well. He has actually acquired a lot of knowledge and he has my respect. I also justified my problem with the way Simon was describing something and I thik we cam to terms with that. I get the impression that you have a different attitude, though, which is unfortunate if you really want to get to grips with this.

Perhaps you could quote one of my "empty statements"? I will try to justify it, if I haven't already. Where does the idea of an "attack" come from? Why so insecure, I wonder?

tris_d

Oh, boy. I'm "insecure" because if I took your attitude I'd be banned straight away. I'm not claiming to know this stuff, I told you three times in a row that I DO NOT KNOW, and that's why I'm asking these questions. I'm not insecure about my knowledge nor do I feel ashamed about the things that I do not know. It's only you who associate some egoistical meaning to the amount of information stored in someone's head and like to gloat about it, which might very well mean you are insecure about some other things, in real life, and thus are compensating your frustration in your virtual life on this forum by criticizing other people with unnecessary personal comments. Just stop pulling my tail and concentrate on the topic please.


If individual photos each can have their own wavelength, what does addition of other photons (density) have to do with the wavelength of each individual photon?


Can you explain what and how would be different? -- It was not explained to me what "450nm" actually refers to. Drakkith was talking about focal point and airy discs, so I assumed "450nm" referred to angular size of projected image after it went through a lens and thus is relative to magnification. Please correct me if I assumed wrong.


How is light flux measured?


So flux is some flow rate per unit area? Flow of what, photons, no? How is that different to light intensity? Please explain it in terms I can understand, which means photons, like this:

Intensity is number of photons per unit area per unit time?

Light flux is number of photons per what per what per what?

sophiecentaur

If you will do me the courtesy of reading what Wikki and others have to say about Photons then you will Know what I am talking about and getting it from another source may help you believe it.
There are definitions of flux and flux density all over - relating to light, magnetism and other things. In my definition, I don't mention photons - I use the word Watts - which you know is Power. Don't you want to accept it from me? It's the truth.
How can intensity be "number of photons" when photons for different wavelengths all have different energies. Intensity is not defined differently for each wavelength, is it (look it up before you get cross) so it just can't involve a photon count.
If you really insist that the only information you get must be from me then you will be disappointed. Just try looking it up (with an open mind) and you will find all you want to know. Why don't you want to do it that way? Don't you see why I doubt your motives in this? Do you actually need 'links'?
Whilst you are at it, I suggest that you look up Wavelength of Visible light and you will see what Drakkith was talking about. Is your browser stuck on PF? This is a free service, you know and you can hardly demand an answer. When I was Paid to help students, I used to accept all sorts of lazy sods for tuition but this is done for 'love' (haha - of the subject).

Drakkith

Take a few images and count the number of photons that have fallen onto the sensor over time. It's not quite that easy, as we have to deal with all sorts of noise, but that's basically it. I can measure the light that has fallen onto the sensor from a star.

Brightness, as I defined it in my post you are quoting, cannot be measured if the image of the star is much less than the size of the airy disk. For example, even nearby stars have an angular diameter of hundreths or thousandths of an arcsecond. This is FAR below the 1 arcsecond resolution of my telescope. The airy disk would be about 1 arcsecond across, which would correspond to about 4-5 microns on the sensor. Do you know what an arcsecond is? Have you read up on what an airy disk is?

Also, the way I have used brightness may not be correct. I was merely trying to use it as the same way your quoted source linked earlier used it. Terms like radiant flux, irradiance, and other terms are far better to use if we want to get anything meaningful out of a discussion.


Light flux is nothing. It's not a unit of measurement. The correct term would be Radiant Flux or something else. Note that things like Luminance and Luminous Flux are based on the human eyes varying sensitivity to different wavelengths. See here: http://en.wikipedia.org/wiki/Intensity_%28physics%29
And here: http://en.wikipedia.org/wiki/Radiant_flux

Click the link above and it's right there.

Drakkith

Also, Sophie is correct in that there is a wealth of information out there that you need to read. A LOT of what we have explained cannot be understood if you don't know the bare basics, such as what "nm" means. (Nanometers)

It sucks, but you may need to hold off on asking questions like these until you read up on the very basics of light.

tris_d

I know what nm means, I sad I do not know what size are you referring to. Would it not be easier for you to just tell me straight then misunderstanding me in order to criticize me? No one is forcing you to talk to me. If you think my questions are stupid just ignore me please.

Drakkith

My mistake then. I misunderstood your reply to Sophie. What "size" are you asking about? The size of the airy disk?

I have been telling you straight.

Calm down please.

tris_d

Perhaps if we want to simplify or if the source emits photons of the same energy, ok? And then intensity would be directly proportional to the number of photons, wouldn't it?

http://www.cv.nrao.edu/course/astr534/Brightness.html
The number of photons falling on the film per unit area per unit time per unit solid angle does not depend on the distance between the source and the observer. The total number of photons falling on the film per unit area per unit time (or the total energy absorbed per unit area per unit time) does decrease with increasing distance. Thus we distinguish between the brightness of the Sun, which does not depend on distance, and the apparent flux, which does.


You either tell me that article is completely wrong and that I should forget it, or use your marvelous intelligence and rephrase your amazing knowledge in these same terms so we can talk the same language here. How about it? C'mon, you can do it! Or can you?


"GOOGLE IT!" -- You could put that in your signature, then you would not need to bother answering any more questions but simply reply with an empty post and it will automatically answer all the questions anyone might have.

Those links do not define any of that in terms of photons, and that article does, and I like it, so I want to understand that article and thus I need explanation according to those same terms they use. And I like photons. I hate energies, they are so vague. Ok? Photons, photons, photons! Photons rule, energy sucks. Hah!


I have no idea why are you even talking to me. It would take less words to actually answer my questions than to explain in detail how and why I should not be asking anything until I learned everything. But if I do what you say, it would take me much more time and then when I have learned everything I would not have any need to come back here. Is that what you want, to get rid of me?

tris_d

Yes, you have. Don't stop!

tris_d

Then flux CAN be defined in terms of 'number of photons'.


If we can define flux and intensity in terms of number of photons, per something per something, then we will be able to define image brightness in terms of photons per pixel. That's what I want, to define everything in terms of 'number of photons', to Soph's utter disgust.

Drakkith

Technically no, as number of photons is not watts. However if we "hand wave" all the photons as being from the same frequency EM wave, then we can find the flux.

I already explained it in my other post then. The "brightness" the reference you quoted uses is exactly how I used it.

sophiecentaur

But of course, only if you happen to know the frequency of the EM you happen to be dealing with. That is why it makes such good sense to describe Flux in Watts. (Funny, I have read and written that several times before.)

We often have contributors who want to do their own version of things. They either grow wiser or poorer, in time.

Simon Bridge

The photon and em wave models are both drilled into students - I'm wondering if the links need to be clearer earlier?

I think "light intensity" and "light flux" are terms that could mean just about anything.

The "photon flux" is the number of photons through a unit area per unit time.

- which seems to be what is intended here by "light flux". The word "light" has a range of uses in physics as well as regular language. I suspect that sophiecentaur is probably on to something by insisting on a precise language here.

The "intensity of the light wave" would be the square of the amplitude of it's electric field? (sophiecentaur?) ... which would be related to the photon flux and the photon energy. (Photons are understood primarily as energy quanta ... though wavelength, momentum etc can also be used to characterize a photon.)

The "luminous intensity" of a light source would be the power per unit solid angle being emitted by a light source. So already, two different ways to define "intensity of light".

"brightness" is a subjective measure that means different things in different circumstances. In common language, we would understand one object to be brighter than another is it appears to glow more when you look at it. This is what I've been trying to talk about on the first page of this thread.

Some examples:
(1)Some colors look brighter than other colors (part of the original question) because they look more like the colors in fire - for example - so the common concept is to do with more that just the light itself.
(2)stars with a high visual magnitude will look brighter than those with a low visual magnitude. Originally the visual magnitude was a subjective measure related to the way the human eye perceives light. However, astronomy has objective ways to assign magnitude to stars (look this up for more). Astronomers may refer to "bright stars" informally, in this context.
(3) photographers may refer to brightness in terms of the tendancy of part of the picture to wash out other parts ... they use a light meter to help them work out exposure times. The meter usually measures power per unit area, averaged across the detection surface and this can be called the "brightness of the light" but more likely it will be called the "light level". I have seen "light flux" used in old SLR camera manuals.
(4)Drakkith seems to be using "brightness" to mean the number of photons emitted, per unit solid angle, from a light source (Drakkith?)
[edit] in order to be consistent with the linked article (earlier)

Because of the very wide usage, it is possible to come up with a reasonable sounding definition of "brightness" to contradict pretty much any argument. The term should be understood only in the context of a particular description. I don't think it is useful as a concrete general term.

I suspect that the persistence of OPs confusion in the face of repeated answers may indicate that we have yet to identify it's source. Perhaps sorting out more rigorous terms will help?

Drakkith

I'm merely using it the way I think the article linked was using it, which I believe is photons per solid angle.

sophiecentaur

You just can't beat a bit of "rigour" if you want to get somewhere in Science. It's not a matter of preferrence and one has to walk before running.

tris_d

What you describe, is that point light source?

http://en.wikipedia.org/wiki/Apparent_magnitude
- Note that brightness varies with distance; an extremely bright object may appear quite dim, if it is far away. Brightness varies inversely with the square of the distance.

It seems Wikipedia say further away star would simply appear as darker/dimmer 'airy disc' than closer away star with the same absolute magnitude. Would you agree?

But that does not apply when stars can be resolved to have some angular size, it applies only when the star is so far away that it becomes a point source. Ok?

sophiecentaur

That article is not "completely wrong" and, if you read it in total, you will see that they do not, anywhere, 'define' flux / brightness / intensity in terms of brightness. (Read what they actually say) They make the mistake, possibly, of introducing photons conversationally, to make the subject approachable . The fact that you picked up on that, to the exclusion to their formal definitions, shows that they chose an unfortunate way of putting things. You will not have been the only one to get an inaccurate message. But their main statement about brightness is in terms of Power - as it should be.

You really should not reject the idea of reading around a subject. Every stroppy post you are making is interfering with your self-education time. If you love Photons then you should learn what they really are. You will only find that out by reading and not asking the 'wrong' questions.

You are clearly in the early stages of learning about physics and I recommend you get the basics sorted out before coming to shaky conclusions. This stuff would never have been sorted out if it had been approached in a careless and uninformed way.

This is a discussion forum and not a free tuition service. Any help you may get is your good fortune and not a right.