Is there any relation between wavelength and brightness?

In summary: I used the "middle gray" of the smoke and compared it to the other colors of the rainbow to determine how many bits the digitizer was using for color resolution. In summary, The brightness of a color is determined by the sensitivity of the optical equipment used to measure it, as well as the colors and context surrounding it. Brightness is subjective and can vary depending on individual perception. Intensity is the amount of energy passing through a specified area in a specified amount of time and can be affected by frequency and the number of photons. However, brightness is not directly proportional to the number of photons. Instead, it is influenced by the way our eyes perceive color and the technology used to capture and display it.
  • #36
Drakkith said:
Why wouldn't it be fundamental to the question? I showed that before and after the optical system the results are the same. I am merely putting an optical system in because I am far more familiar with the workings of optics, sensors, and photons than I am waves and energy per area and such. I know we can't treat light as little particles traveling through space, but unless I've gravely misunderstood something I think my explanation works out the same either way. Please correct me if I'm wrong on something.

I like the way you explained it, that's how it naturally comes to me to think about it. I think we understand each other just fine now. And if there is nothing wrong about it, then I'd prefer we stick with photons and pixels. Individual photons can be considered to have wavelength and frequency, I guess, so we could include those concepts as well, and that I believe should then cover the whole subject and explain all the related phenomena.
 
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  • #37
Drakkith said:
Why wouldn't it be fundamental to the question? I showed that before and after the optical system the results are the same. I am merely putting an optical system in because I am far more familiar with the workings of optics, sensors, and photons than I am waves and energy per area and such. I know we can't treat light as little particles traveling through space, but unless I've gravely misunderstood something I think my explanation works out the same either way. Please correct me if I'm wrong on something.

I can see you are trying for a practical approach but I can't help feeling that, when you end up discussing one pixel getting a photon and the adjacent pixel not getting it then then it's a whole new ball game. The pixels and the photons you are discussing are not real ones - they are you own 'home brewed' variety. That's the beauty of discussing things in terms of continua. This is particularly relevant, I think, when you are discussing very low numbers (which you seem to be). Would you use individual electrons or mA in circuit theory? Same argument applies and, once you actually start to consider very low numbers in circuits and noise, things get harder and you would need to add some complexity. Is it really that difficult to use Watts and metres squared? After all, it was what you were taught first.
 
  • #38
tris_d said:
I'm interested in brightness, and I believe we established brightness is a property of an image rather than property of the light source or light itself. Therefore, since the image is digital object rather than analog, since it is a collection of discrete pixels, I think we also need to quantize the light so we can talk about the relation between the image and the light, and then we could perhaps define brightness as the number of photons per pixel.

That's where my original question came from. I was wondering if there is anything else beside the number of photons per pixel, like wavelength, that would define the brightness of a pixel.

I would agree that image brightness and object brightness are two different things. However, 'brightness' is a quantity that I see, all over the place, which relates to an object - as in luminosity of an object. Just Google "brightness" and you will see what I mean. So I don't think that your particular take on the term is "established" at all.

Alao, an image is NOT "a collection of pixels. An image is an image and you can sample it with an array of sensors which can only measure Energy.
 
  • #39
sophiecentaur said:
I can see you are trying for a practical approach but I can't help feeling that, when you end up discussing one pixel getting a photon and the adjacent pixel not getting it then then it's a whole new ball game. The pixels and the photons you are discussing are not real ones - they are you own 'home brewed' variety.

I don't understand what you are getting at here. What do you mean by "home brewed"?

That's the beauty of discussing things in terms of continua. This is particularly relevant, I think, when you are discussing very low numbers (which you seem to be). Would you use individual electrons or mA in circuit theory? Same argument applies and, once you actually start to consider very low numbers in circuits and noise, things get harder and you would need to add some complexity.

Now you are really losing me.

Is it really that difficult to use Watts and metres squared? After all, it was what you were taught first.

I have never been taught anything about optics, radiation, or anything else other than some extremely basic electronic principles in the Air Force. Which I then never used over my 10 years of active duty, making me forget most of it and having to relearn it myself. Ironically I start "Basic DC Circuits" in a week or two in my technical training that I'm currently attending since I retrained to a new career field to join the reserves. Everything I know I have learned myself. (Hence my usual non-mathematical explanations!)
 
  • #40
Drakkith said:
I don't understand what you are getting at here. What do you mean by "home brewed"?
Now you are really losing me.
I have never been taught anything about optics, radiation, or anything else other than some extremely basic electronic principles in the Air Force. Which I then never used over my 10 years of active duty, making me forget most of it and having to relearn it myself. Ironically I start "Basic DC Circuits" in a week or two in my technical training that I'm currently attending since I retrained to a new career field to join the reserves. Everything I know I have learned myself. (Hence my usual non-mathematical explanations!)

@Drakkith
I know that you are worth listening to and that you make many good points in your posts. I am not out to antagonise you - it's the message not the messenger that I'm getting at!
OK, by "home brewed" I mean that your use of the Photon and Pixel is very specific and personal and you are being selective in how you treat your photons (you refer to the Airy Disc, which is a totally Wave idea, for instance).
The following scenario may make my point: Just imagine your incredibly rich Uncle died and left you his radiotelescope (in the garden of his vast estate etc etc, which he has also left you). I bet that you, as an enthusiastic astronomer, would want to go out and make use ot this radiotelescope, rather than scrap it or sell it. You would no longer be talking pixels and no one talks photons where radio is concerned - it's all watts and waves. The situation is exactly the same, in principle, when you want to build up a picture of an area of space at radio frequencies as with visible light except that you don't have a sensor array with pixels - you move the telescope about and measure the received signal levels. My point is that you should be able to discuss both situations validly in exactly the same terms - because they are the same in all senses but the frequencies involved. I'm sure you wouldn't have a problem with signal strength in Watts, power flux in Watts per squ metre etc. if the situation called for it. You have managed to learn a lot about pixels and photons, after all.

When you take on photons (the official model, that is) you should also take on a load of extra stuff. You say you appreciate that yet you would still rather look at this stuff as little bullets hitting small areas on a sensor array. Just how big do you thing these photons are? You wouldn't get an airy disc if they were little dot sized. Telescope resolving power calculations don't use a photon explanation do they? Waves are cuddly and don't involve Zen -type brain ache. Stick to them when you can.
 
  • #41
sophiecentaur said:
If you insist on using photons in your arguments the everything changes with frequency and that just doesn't help anyone.

How does everything change? Can we not associate frequency or wavelength to individual photons?
 
  • #42
Drakkith said:
But what about far away stars? Here we run into an issue. My telescope focuses the light down to a point called an airy disc. Let's say I'm measuring 500 nm light. With an aperture of 250 mm and a focal length of 1,000 mm my telescope will focus 500 nm light down to a spot that is 4.88 microns in diameter. But, what if my star image is even smaller than that? Like, much smaller? Well, in that case we treat the star as a "point source". At this point we cannot measure the brightness of the star, only the total FLUX. If we know the size of the star and it's distance we could calculate the brightness, however we cannot measure it.

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


Drakkith said:
If you mean Radiant Flux, that would be Watts.

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?

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.
 
  • #43
sophiecentaur said:
No, they are not. Look at two lightbulbs, side by side. They have equal intensities. Turn one of them off and the remaining one has the same intensity as before but the flux reaching you has dropped to half. Move the two bulbs (both on) together or apart (not too far, or the geometry may change) and the flux from them is the same. However, if you were to superimpose the two (having two similar filaments in the same frosted envelope) then the intensity would double but the flux would be the same as having the two side by side.

So the flux is number of photons per what per what? Just tell me the units please and that will explain everything.
 
  • #44
tris_d said:
How does everything change? Can we not associate frequency or wavelength to individual photons?

You can - but the density of photons would change as the wavelength changes. Whatever conclusion you came to about light of 450nm would be entirely different for light of 600nm. What use would that be? Even worse when you use a mixture of wavelengths. (You are aware that E = hf, I presume?)

The bottom line of this is to ask yourself why flux is not measured, conventionally, in terms of photons. Do you really think that your idea is better than the conventional one? Why did they make the choice in terms of Watts?

BTW, you need to make the distinction between Flux (Watts) and Flux Density (Watts/msqu)

I still feel quite justified in my comment that you just ignore or accept what is written here as it suits you. All this has been written previously yet you still question it.
 
  • #45
sophiecentaur said:
You can - but the density of photons would change as the wavelength changes. Whatever conclusion you came to about light of 450nm would be entirely different for light of 600nm. What use would that be? Even worse when you use a mixture of wavelengths. (You are aware that E = hf, I presume?)

The bottom line of this is to ask yourself why flux is not measured, conventionally, in terms of photons. Do you really think that your idea is better than the conventional one? Why did they make the choice in terms of Watts?

BTW, you need to make the distinction between Flux (Watts) and Flux Density (Watts/msqu)

I still feel quite justified in my comment that you just ignore or accept what is written here as it suits you. All this has been written previously yet you still question it.

I'm not arguing anything, I'm trying to understand. What do you expect, that I should just memorize your statements and leave it at that? Your attitude is very strange. You seem to assume that I consider myself an expert and that I am trying to put some point across, where I am simply asking questions and have no idea there would even exist such thing as "flux density". I thought flux IS description of light density. I have no idea how flux is measured, that's one of my questions. And I came across "E = hf", but no, I am not really aware or familiar with it. I do not know, man, hence my questions. You are so tense, like a tensor, and I think it would be better for everyone if you just relax a bit and be more like a spaghetti.
 
  • #46
If you don't grasp the significance of E = hf then you are really not in any position to have an opinion about photons at all and you certainly can't afford to be using them in any of the personal models you are building in your head. Go away and read about the nature of photons and where they come into the world of EM radiation. They are not the slightest bit like the little bullets you seem to be assuming. Don't ask for a reference - just google.

Likewise about a lot of the questions you are asking. The answers to all the definitions are present in a very reasonable form in many places. Your need for information is much better served by reading information as it is presented and not in these aimless 'question and answer' threads because you will see that many of your questions have been pointless and confusing - once you have informed yourself better. Forums like this can make one very lazy (I have been there myself). Reading a well thought out bit of pedagogic material can be much more fruitful.

I would suggest the 'hyperphysics' pages as a start.
 
  • #47
sophiecentaur said:
You can - but the density of photons would change as the wavelength changes.

I don't get it. 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?


Whatever conclusion you came to about light of 450nm would be entirely different for light of 600nm.

I don't understand. Can you explain more 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.
 
  • #48
sophiecentaur said:
If you don't grasp the significance of E = hf then you are really not in any position to have an opinion about photons at all and you certainly can't afford to be using them in any of the personal models you are building in your head.

Again, I DO NOT have any position, I DO NOT have made up opinion, I DO NOT have personal models. I am TRYING TO UNDERSTAND. You are harassing me, just let me be and mind your own business.
 
  • #49
tris_d said:
Again, I DO NOT have any position, I DO NOT have made up opinion, I DO NOT have personal models. I am TRYING TO UNDERSTAND. Just let me be and mind your own business.

So READ UP about it and stop asking to be spoon fed with each little bit of info on demand. I actually doubt that you really are trying very hard, rather than just enjoying a fruitless conversation. There is no excuse for not reading everything that Wiki and others have to offer. You will always have personal models in your head until you get some self discipline and start learning the real stuff for yourself from tailor made resources. (You clearly have the time available).

I will be quite prepared to respond once I can see that you are, in fact , making an effort for yourself. Good luck with it.
 
  • #50
Simon Bridge said:
The sensitivity of the optical equipment being used to measure it - i.e. the human eye has evolved to respond strongly to yellows and greens so these seem brighter and more noticeable.
How bright a color appears also depends on the colors around it and the context you are looking at it in.
..from google
con·text/ˈkäntekst/

Noun:
1.The circumstances that form the setting for an event, statement, or idea, and in terms of which it can be fully understood and assessed.
2.The parts of something written or spoken that immediately precede and follow a word or passage and clarify its meaning.

Does this mean...if you are a photon observing your-self..now like.. here https://www.physicsforums.com/showthread.php?t=138976
...does this mean that the context is the very dencity of some written text in that photon as yourself becomes 2d sheet ..then the text ..where does it comes from as it may look like information 'stored' in/on (..from where - antiuniverse?),,ect...?

and in that case ..what is photon..is light/ photon just a filter for something else?
 
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  • #51
sophiecentaur said:
So READ UP about it and stop asking to be spoon fed with each little bit of info on demand. I actually doubt that you really are trying very hard, rather than just enjoying a fruitless conversation. There is no excuse for not reading everything that Wiki and others have to offer. You will always have personal models in your head until you get some self discipline and start learning the real stuff for yourself from tailor made resources. (You clearly have the time available).

I will be quite prepared to respond once I can see that you are, in fact , making an effort for yourself. Good luck with it.

No, you just get angry because you can not really explain what you are talking about. You make empty statements without any explanation behind them and you then blame me for not taking what you say for granted. You attacked Simon Bridge, you attacked Drakkith and you are attacking me just because we do not think like you do. Good luck to you too.
 
  • #52
tris_d said:
No, you just get angry because you can not really explain what you are talking about. You make empty statements without any explanation behind them and you then blame me for not taking what you say for granted. You attacked Simon Bridge, you attacked Drakkith and you are attacking me just because we do not think like you do. Good luck to you too.

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?
 
  • #53
sophiecentaur said:
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?

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.
You can - but the density of photons would change as the wavelength changes.

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?
Whatever conclusion you came to about light of 450nm would be entirely different for light of 600nm.

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.
The bottom line of this is to ask yourself why flux is not measured, conventionally, in terms of photons.

How is light flux measured?
BTW, you need to make the distinction between Flux (Watts) and Flux Density (Watts/msqu)

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?
 
  • #54
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).
 
  • #55
tris_d said:
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_(physics)
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.
 
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  • #56
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.
 
  • #57
Drakkith said:
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.
 
  • #58
tris_d said:
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.
 
  • #59
sophiecentaur said:
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?
 
  • #60
Drakkith said:
I have been telling you straight.

Yes, you have. Don't stop!
 
  • #61
Drakkith said:
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.
 
  • #62
tris_d said:
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.
 
  • #63
Drakkith said:
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.
 
  • #64
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 tendency 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?
 
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  • #65
Simon Bridge said:
(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.
 
  • #66
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.
 
  • #67
Drakkith said:
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?
 
  • #68
tris_d said:
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.
 
  • #69
Simon Bridge said:
(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.
 
  • #70
tris_d said:
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.
 
<h2>1. What is the relationship between wavelength and brightness?</h2><p>The relationship between wavelength and brightness is known as Wien's displacement law. It states that the peak wavelength of radiation emitted by an object is inversely proportional to its temperature. This means that as the wavelength decreases, the brightness increases.</p><h2>2. Why does the brightness increase as the wavelength decreases?</h2><p>This is because shorter wavelengths have higher energy levels, which means they carry more energy. As a result, when an object emits radiation at shorter wavelengths, it appears brighter to us.</p><h2>3. Is there a specific wavelength that is the brightest?</h2><p>Yes, the peak wavelength of an object is the brightest. This is known as the peak emission wavelength and it can be calculated using Wien's displacement law.</p><h2>4. Can the relationship between wavelength and brightness be seen in everyday life?</h2><p>Yes, the relationship between wavelength and brightness can be seen in everyday life. For example, the sun appears brightest at shorter wavelengths, such as blue and violet, while objects like fire emit radiation at longer wavelengths, appearing less bright to us.</p><h2>5. How does the relationship between wavelength and brightness relate to the color of light?</h2><p>The color of light is determined by its wavelength. As mentioned before, shorter wavelengths appear bluer and brighter, while longer wavelengths appear redder and less bright. This is why objects with higher temperatures, such as the sun, emit bluer and brighter light, while cooler objects emit redder and less bright light.</p>

1. What is the relationship between wavelength and brightness?

The relationship between wavelength and brightness is known as Wien's displacement law. It states that the peak wavelength of radiation emitted by an object is inversely proportional to its temperature. This means that as the wavelength decreases, the brightness increases.

2. Why does the brightness increase as the wavelength decreases?

This is because shorter wavelengths have higher energy levels, which means they carry more energy. As a result, when an object emits radiation at shorter wavelengths, it appears brighter to us.

3. Is there a specific wavelength that is the brightest?

Yes, the peak wavelength of an object is the brightest. This is known as the peak emission wavelength and it can be calculated using Wien's displacement law.

4. Can the relationship between wavelength and brightness be seen in everyday life?

Yes, the relationship between wavelength and brightness can be seen in everyday life. For example, the sun appears brightest at shorter wavelengths, such as blue and violet, while objects like fire emit radiation at longer wavelengths, appearing less bright to us.

5. How does the relationship between wavelength and brightness relate to the color of light?

The color of light is determined by its wavelength. As mentioned before, shorter wavelengths appear bluer and brighter, while longer wavelengths appear redder and less bright. This is why objects with higher temperatures, such as the sun, emit bluer and brighter light, while cooler objects emit redder and less bright light.

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