# Single Photons

1. Nov 12, 2004

### T.Roc

Questions:

1. Has an experiment been done firing one single photon through a prism?

2. Would you see the whole color spectrum? (or would it take a stream of photons to do that)

3. Explain how either way would technically work.

Troc

Last edited by a moderator: Nov 13, 2004
2. Nov 12, 2004

### imabug

An individual photon of energy E has a frequency of f, making it monochromatic. You would only get a spectrum of colours from light containing photons with a bunch of frequencies (the colours you get will be determined by the frequencies in your light beam)

3. Nov 12, 2004

### T.Roc

imabug,

So, you are siding with "it is a stream of photons that produces the visible spectrum"(through prism).

Can you counter Newtons' axiom that "white light contains all colors"?
Or experiments that produce white light from 2 and/or 3 colors?

TRoc

4. Nov 12, 2004

### imabug

why would I want to counter it? White light by definition (my definition anyway) is light containing photons with all frequencies in the visible spectrum. Passing that light through a prism causes the photons of different frequencies to separate out (different index of refraction at each frequency).

5. Nov 13, 2004

### T.Roc

imabug,
"...(my definition anyway) is light containing photons with all frequencies in the visible spectrum. Passing that light through a prism causes the photons of different frequencies to separate out (different index of refraction at each frequency)."

Then, by chance, EVERY time, when you shine light through a prism, 7 photons line up IN THE SAME ORDER every time, for a seamless color spectrum? Or how is it you think it works??!

TRoc

6. Nov 13, 2004

### imabug

why do you think 7 photons have to line up to produce a colour spectrum? a seamless colour spectrum is only produced by a beam of light containing photons with all the frequencies in the visible light range. The way they separate out is determined by the medium the light passes through, angle of incidence and it's index of refraction. The refraction angle for a particular frequency of light will be determined by Snell's Law, keeping in mind that the index of refraction (n) for the material is going to have some frequency dependence (the term is dispersion, if I recall correctly).

perhaps your questions could be more completely answered if you were to explain your understanding of the process.

7. Nov 13, 2004

### T.Roc

imabug,

OK. I understand the incident angle that the light hits the medium, and the mediums' shape creating different lengths of travel creates the diffraction - and the "spectrum" (R-O-Y-G-T-B-V).

So, to see those 7 colors, 7 photons have to go through 7 angle/distances in the medium. They would have to form a parallel line with the angle (so they would reflect together, in a line), almost touching each other so no gaps in the spectrum are made. They would have to be "in phase", and all traveling the same direction. And all of this would have to repeat anytime you preformed the experiment.

like this?

1>>>>>>>/
2>>>>>>/
3>>>>>/
4>>>>/
5>>>/
6>>/
7>/_______________

_______v-b-t-g-y-o-r
_______7-6-5-4-3-2-1
7=short wavelength, 1=long

It is my understanding that Newtons' statement that white light contains all colors is currently held to mean ONE photon has the potential to be any color.

That is why I asked if the experiment has been done. Like the double slit - both ways: ONE at a time, and in a stream. Another theory that has not been properly examined with cross-experimentation.

TRoc

Last edited by a moderator: Nov 13, 2004
8. Nov 13, 2004

### Staff: Mentor

Not so. As imabug explained, "white" light is a mix of all frequencies. A single photon of white light is meaningless.

9. Nov 13, 2004

### Integral

Staff Emeritus
Further, It would require a lot more then 7 photons to form a complete spectrum. Each color band is formed by a range of frequencies. If you fired only 7 photons you would have 7 spots, each spot would be in the region covered by that color.

Evidently Newton chose 7 colors more for reasons of numerology then physics. Where is that indigo band anyway?

10. Nov 13, 2004

### T.Roc

Doc AL & Integral,

["white" light is a mix of all frequencies]
certainly not only "all", when 2 or 3 will also do?

[A single photon of white light is meaningless.]
This helps me ! (is it really true?)

Yes, 7 is odd. I could do it mathematically with 13 though.

But, you are both saying, that just by chance, given the truly large # of photons coming, (at least) 13 of them line up (continually) in the "sweet spot" for as long as you hold the prism there? This seems, even with the law of large #'s applied, to be fantastic!

Given the finite # of elements present in the Sun, and their standard spectrometry, what frequencies of light do you think the Sun gives off?

Also related question - what frequency of photon is standard in the "double slit" experiment? (I always thought it was white - which there is no known frequency of)

TRoc

11. Nov 13, 2004

### Integral

Staff Emeritus
I really do not understand what you are trying to say here? Do you not understand that the frequency of the light determines the path taken thorough the material? The basic principal of a prism is that different frequencies of are separated because the "velocity" of light through the prism depends on the frequency.

The peak of the solar spectrum corresponds to what we call visible light. Is it a mystery that creatures which evolve in those conditions become sensitive to that energy band?

By the way, even 13 (where did that number come from?) photons will not form a continuous spectrum, but will be 13 isolated spots, the location of which depends on the frequency of the photons.

12. Nov 13, 2004

### DaveC426913

Not to put too fine a point on it, but...

There is no objective definition of white light. It is subjective and humano-centric. In fact, the "visible spectrum" itself is humano-centric (bees, for example see in UV as well). In fact, white light is person-centric, since we all have a range of sensitivities.

Humans have three receptors in their retinae that are each sensitive to a particular range of visible light - the sensitivity ranges are smooth curves - the peaks centring on - roughly - red, green and blue. When light of a frequency we call yellow hits our eyes, it stimulates both the red and green receptors, but not the blue. The resultant signal is interpreted by us as one single colour - yellow.

If *any* number (be it three or three thousand) of frequencies hit our retinae such that all three receptors are stimulated to the same degree, we perceive it as white light.

An *attempt* at pseudo-objectivity defines light as the sum of all frequencies in the visible spectrum but - without having fixed boundaries, what does "all frequencies" mean? We can set arbitrary boundaries and determine that they are all accounted for, but remember, that's still arbitrary.

"But, you are both saying, that just by chance, given the truly large # of photons coming, (at least) 13 of them line up (continually) in the "sweet spot" for as long as you hold the prism there? This seems, even with the law of large #'s applied, to be fantastic!"

White light is not a bunch of photons all lining up at the same time. It is the sum of a bunch of photons hitting in a time too short for us to measure. If you want to know all the frequencies of those uncountable photons, then stick a prism in the way, you'll see every frequency (but still not of individual photons).

"Given the finite # of elements present in the Sun, and their standard spectrometry, what frequencies of light do you think the Sun gives off?"

All these: http://www.astro.queensu.ca/~hanes/p014/Notes/Figures/Solar_Spectrum.gif
Sorry, couldn't find a colour picture. Try Googling "Sun spectral lines".

BTW, Newton blew it. There is no Indigo in a rainbow. He just really liked the number 7. True story.

Last edited: Nov 13, 2004
13. Nov 13, 2004

### T.Roc

Integral,

If the frequency is predetermined, then (I assumed) there would be symmetry between the rectangle (rainbow area) on the wall, and the surface of the prism. So an area of "specific" frequency photons would be reflected onto the wall. ??

Is there a formula for obtaining the value of a specific frequency's change as it passes through a prism (say the center)?
Maybe if I can understand what happens to 1 photon, then the rest will fit in.

Thanks DaveC426913 for the link. Very adequate variety of frequencies in the solar spectra. Large # randomness activating our 3 cones equally & simultaneously = white light.

TRoc

14. Nov 14, 2004

### Integral

Staff Emeritus
First of all start by doing a web search on SNELL'S LAW. This is the basic math behind bending of light in glass. Now I have a text that gives the following table for Light flint glass:

$\lambda$ = 361nm n = 1.614
$\lambda$ = 434nm n= 1.594
$\lambda$ = 589nm n= 1.575
$\lambda$ = 656nm n=1.571
$\lambda$ = 768nm n= 1.567

With that information you should be able to figure out why the spectrum spreads.

15. Nov 14, 2004

### Staff: Mentor

Although you can certainly do the experiment with "white" light---Young himself used a narrow beam of sunlight---today it would be done with coherent, monochromatic light (from a laser). Sunlight, being a mix of frequencies, adds dispersion to the diffraction pattern observed.