Is there any relation between wavelength and brightness?

[tris_d]

Let's look at an example. Let's say that the Sun puts 1,000 photons per second onto a sensor of 100 pixels at the focal point of a telescope here on Earth. Then we take this sensor and move it twice as far away from the Sun as it was. The Sun now puts 250 photons per second onto the sensor. BUT, in both cases, each pixel that receives light receives the same amount of photons per second. The reason that there are 1/4 as many photons hitting the sensor is that the image formed at the focal plain is half the size as before in both the X and Y directions. So the surface area of this image at the focal plane is 1/4 what it used to be and only 1/4 as many pixels are even hit by light from the Sun. So it was originally 1,000 photons over 100 pixels is 10 photons/second/pixel. Now its 250 photons over 25 pixels, which is still 10 photons/second/pixel! This also means that the TOTAL amount of photons falling onto the aperture of the telescope has fallen from 1,000 to 250, so as you can see in both the focused and unfocused case the RADIATIVE FLUX, the photons per second, has decreased to 1/4 just by doubling the distance.

Great. I think you could/should make an internet article out of this stuff. It's fairly simple, but not very obvious if you don't consider there is a lens in between and that focusing is involved, to which I was completely oblivious previously.

Now, what about brightness? I am not familiar enough to figure out which of the many units (See here) to use, so I will have to explain it my way again instead. Let's say that brightness is the number of photons coming from an angular section of the sky, as that seems to be the only way it makes sense.

Let's say I measured the number of photons per second coming from an area of the Sun that is 15 arcminutes x 15 arcminutes. So the area would be 225 arcminutes. Since the Sun was putting a total 1,000 photons per second onto the sensor, and 15x15 arcminutes is 1/4 the size of the Sun from the Earth (the Sun is 30 arcminutes across), the number of photons per second from this area is 250.

Now, I move the telescope twice as far away. How many photons per second to I now get from this same 15x15 arcminutes? Well, if the Sun has had it's dimensions halved, it is now 15 arcminutes across instead of 30. Which means that the area is 1/4 what it was, which means the whole Sun now fits in this 15x15 area! And if the Sun was putting a TOTAL of 250 photons per second onto our sensor earlier, then it must still be doing the same thing now since we are at the same distance. So even though I've moved twice as far away and the total light from the Sun has decreased to 1/4 what it was, I still have the same amount of light coming from the same angular area of sky. (Note that I've simplified the explanation by using the area of a square, not a circle. However the result is the same.) So the BRIGHTNESS, which I mean as the number of photons per area of sky, is exactly the same. Note that this also happens if we move CLOSER to the Sun. At half the distance to the Sun the light is quadrupled, but the image of the Sun is now 4 times as large! So 4,000 photons, over 400 pixels is still 10 photons/second/pixel!

Would you agree that instead of defining brightness as a measure of "number of photons per area in the sky" would be better to say it is a measure of "number of photons per pixel", so that brightness be a property of an image rather than property of the light itself?

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.

Are most of the stars in our galaxy point light sources? Are most of the other galaxies point light sources? -- When you say "500 nm light" you refer to angular size of "airy disc"? I guess the size of airy disc depends on magnification, so how do you know it's 500nm and not 200nm, that is how do you know you focused it properly (if this question makes sense)? -- How do we measure the flux? You say we can not measure the brightness in case of point light sources, but would not image itself of such point light source be a measure of its brightness?

 

[sophiecentaur]

Light intensity is number of photons per unit area per unit time?

Light flux is number of photons per unit area per unit time, or what?

If you insist on using photons in your arguments the everything changes with frequency and that just doesn't help anyone. That's the whole point of using Watts - it works for any combination of wavelengths. Your attempted definition will not work for a mixture of frequencies.
Why bring photons into this at all? All this stuff was done and dusted before anyone came up with E=hf

 

[sophiecentaur]

Let's look at an example. . . . .

I am having great difficulty in seeing why you keep bringing photons, pixels and peculiarities of imaging systems into this particular question. I realize the bottom line for a practical astronomer is looking at resolvable, bright enough images but is all this fundamental to the actual question?
You will have read my reservations about the use of photons on account of the variable energy. Watts were good enough for all the original calculations on this stuff.

(You know I have a general aversion to explanations of things that bring in Photons when their actual nature is not specified initially. It is a potentially risky process and the raw results are suspect. Surely it isn't any harder to consider light as a continuum for basic optics than ignoring the fact that a massive object consists of atoms, in the context of mechanics)

 

[tris_d]

I am having great difficulty in seeing why you keep bringing photons, pixels and peculiarities of imaging systems into this particular question. I realize the bottom line for a practical astronomer is looking at resolvable, bright enough images but is all this fundamental to the actual question?
You will have read my reservations about the use of photons on account of the variable energy. Watts were good enough for all the original calculations on this stuff.

(You know I have a general aversion to explanations of things that bring in Photons when their actual nature is not specified initially. It is a potentially risky process and the raw results are suspect. Surely it isn't any harder to consider light as a continuum for basic optics than ignoring the fact that a massive object consists of atoms, in the context of mechanics)

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.

 

[Drakkith]

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.

 

[tris_d]

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.

 

[sophiecentaur]

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.

 

[sophiecentaur]

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.

 

[Drakkith]

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!)

 

[sophiecentaur]

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.

 

[tris_d]

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?

 

[tris_d]

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?

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.

 

[tris_d]

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.

 

[sophiecentaur]

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.

 

[tris_d]

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.

 

[sophiecentaur]

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.

 

[tris_d]

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.

 

[tris_d]

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.

 

[sophiecentaur]

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.

 

[Merel]

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?

 

[tris_d]

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.

 

[sophiecentaur]

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?

 

[tris_d]

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?

 

[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]

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.

 

[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]

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]

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]

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]

I have been telling you straight.

Yes, you have. Don't stop!