Register to reply

Is there any relation between wavelength and brightness?

by tris_d
Tags: brightness, relation, wavelength
Share this thread:
Drakkith
#55
Nov17-12, 12:47 PM
Mentor
Drakkith's Avatar
P: 11,884
Quote Quote by tris_d View Post
How do we measure light flux?
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.

Why can not we measure brightness of point light sources, would not image itself (photo/eyes) be a measure of its brightness?
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.


I mean light flux, the one you say we can measure and drops off with the square of the distance. How many different light fluxes there are?
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

I don't think units of flux can be be just Watts. Flux is generally flow rate per unit area, which seems to me is just about the same as what light intensity is.
Click the link above and it's right there.
Drakkith
#56
Nov17-12, 12:58 PM
Mentor
Drakkith's Avatar
P: 11,884
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
#57
Nov17-12, 01:15 PM
P: 162
Quote Quote by Drakkith View Post
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.
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
#58
Nov17-12, 01:23 PM
Mentor
Drakkith's Avatar
P: 11,884
Quote Quote by tris_d View Post
I know what nm means, I sad I do not know what size are you referring to.
My mistake then. I misunderstood your reply to Sophie. What "size" are you asking about? The size of the airy disk?

Would it not be easier for you to just tell me straight then misunderstanding me in order to criticize me?
I have been telling you straight.

No one is forcing you to talk to me. If you think my questions are stupid just ignore me please.
Calm down please.
tris_d
#59
Nov17-12, 02:00 PM
P: 162
Quote Quote by sophiecentaur View Post
How can intensity be "number of photons" when photons for different wavelengths all have different energies.
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?


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'?
"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!


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).
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
#60
Nov17-12, 02:03 PM
P: 162
Quote Quote by Drakkith View Post
I have been telling you straight.
Yes, you have. Don't stop!
tris_d
#61
Nov17-12, 02:37 PM
P: 162
Quote Quote by Drakkith View Post
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.
Then flux CAN be defined in terms of 'number of photons'.


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?
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
#62
Nov17-12, 04:21 PM
Mentor
Drakkith's Avatar
P: 11,884
Quote Quote by tris_d View Post
Then flux CAN be defined in terms of 'number of photons'.
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.

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.
I already explained it in my other post then. The "brightness" the reference you quoted uses is exactly how I used it.
sophiecentaur
#63
Nov17-12, 05:36 PM
Sci Advisor
Thanks
PF Gold
sophiecentaur's Avatar
P: 12,185
Quote Quote by Drakkith View Post
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.
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
#64
Nov17-12, 08:14 PM
Homework
Sci Advisor
HW Helper
Thanks
Simon Bridge's Avatar
P: 12,961
The photon and em wave models are both drilled into students - I'm wondering if the links need to be clearer earlier?

Light intensity is number of photons per unit area per unit time?
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
#65
Nov17-12, 08:19 PM
Mentor
Drakkith's Avatar
P: 11,884
Quote Quote by Simon Bridge View Post
(4)Drakkith seems to be using "brightness" to mean the number of photons emitted, per unit solid angle, from a light source (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
#66
Nov18-12, 04:42 AM
Sci Advisor
Thanks
PF Gold
sophiecentaur's Avatar
P: 12,185
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
#67
Nov18-12, 10:21 AM
P: 162
Quote Quote by Drakkith View Post
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.
...if the image of the star is much less than the size of the airy disk.
What you describe, is that point light source?

Brightness, as I defined it in my post you are quoting, cannot be measured...
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
#68
Nov18-12, 11:02 AM
Sci Advisor
Thanks
PF Gold
sophiecentaur's Avatar
P: 12,185
Quote Quote by tris_d View Post
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!
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.
sophiecentaur
#69
Nov18-12, 11:09 AM
Sci Advisor
Thanks
PF Gold
sophiecentaur's Avatar
P: 12,185
Quote Quote by Simon Bridge View Post
(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.
To avoid confusion, I have to point out that it is the other way round. The stars with the lowest visibility are given the highest magnitude value. Magnitude 1 corresponds to the apparent magnitude of Vega. The Sun, therefore, has a large Negative Magnitude. It makes sense as the stars with the highest magnitudes hadn't even been seen when the magnitude scale was first constructed.
Drakkith
#70
Nov18-12, 11:14 AM
Mentor
Drakkith's Avatar
P: 11,884
Quote Quote by tris_d View Post
What you describe, is that point light source?
Yes, we can treat the star as a point source.
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?
Yes.

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?
The way we've been using brightness, yes. But be aware that brightness is a very bad term to describe light with. There are just too many different ways people use it. For example the way wikipedia uses it in your post is different than the way we've been using it.
tris_d
#71
Nov18-12, 03:49 PM
P: 162
Quote Quote by sophiecentaur View Post
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.
I find your condescending remarks are funny. I recommend you stop talking about me, it's unnecessary. Just address what I say, directly, point out what you believe is wrong and tell us what you think is correct... or ignore it.


This stuff would never have been sorted out if it had been approached in a careless and uninformed way.
http://en.wikipedia.org/wiki/Intensity_%28physics%29
...intensity can mean any of radiant intensity, luminous intensity or irradiance, depending on the background of the person using the term.

- "And so God scattered them upon the face of the Earth and confused their languages, so they would never ever, ever get back together." -- This stuff is indeed handled 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.
Aha. Let me help you understand then. I am developing a simulator to visualize these relations between light source, emitted light, lens, image and its consequent brightness. In order to do that I must model light as photons. Ok? Now, there is simply no other way to go about it but to define intensity, flux and brightness in terms of 'number of photons', and I will do it with or without your help. I guarantee you that this can be done if simplify the scenario by having the light source emit photons of the same energy, and if you help me it will happen sooner rather than later. My friend, it's take it or leave it. Your comments about my person are inappropriate, it's all up to you whether you are going to help or not, so suit yourself and please stop whining about it already.
tris_d
#72
Nov18-12, 04:15 PM
P: 162
Quote Quote by Drakkith View Post
The way we've been using brightness, yes. But be aware that brightness is a very bad term to describe light with. There are just too many different ways people use it. For example the way wikipedia uses it in your post is different than the way we've been using it.
That is why I keep saying brightness should not be defined as a property of light but as a property of an image. Then it will fit the definition from Wikipedia.


Register to reply

Related Discussions
De Broglie Relation for Freq of Electron wave VS wavelength associa with part of mass Quantum Physics 5
Relation between wavelength,gravity,density and velocity of water General Physics 0
Relation between mass and wavelength Quantum Physics 6
Relation between diffraction and wavelength General Physics 12
Wavelength & Distance Relation in MICROWAVE General Physics 7