photoelectric experiment idea holding frequency steady and varying intensity

Hi,

I was just wondering if this experiment has been done, for the photoelectric effect, the light source would be of sufficient frequency to cause one photoelectron per second on average (weak light source) then the light source (1/r^2) is moved further away from the photoelectron emitter until the photoelectron emissions stop completely.

If light is composed of photons, this should be impossible to do this experiment as the photoelectron emission will never completely stop, as there is a statistical chance a photon will hit the photoelectron emitter and trigger the photoelectron emission, but if light is classical then at some point the light intensity will be too low to trigger any emissions of photoelectrons.

I know that classical light and photons should both have a statistical photoelectron emission possibly, but I am curious to see if maybe the classical light doesn't have the ability to create a photoelectron below a certain intensity.

cheers, Jamie

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Jamie M
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Also since the photoelectron energy is proportional to the frequency, a high narrow frequency light source could be used to help discern any background photoelectron emissions (spectroscopic technique)

cheers, Jamie

Reply to
Jamie M

The photon thing works. You will get photoelectrons less often, but you'll still get them, at the same energy per.

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John Larkin                  Highland Technology Inc 
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John Larkin

Also if a material with a high work function (electron binding energy) is used this should also eliminate background noise, since the binding energy of the electron to the material will have to be overcome before a photoelectron is emitted, so a higher work function would give lower background noise.

Also it is possible that classical light at low intensity could trigger a photoelectron emission by building up resonance in the material over time, to prevent this the experiment could operate with two non resonant frequency light sources that are quickly switched so that only one is active at a time. This shouldn't affect the overall number of photoelectric events if light is quantized, but would give two bands in the photoelectron emission spectrum.

cheers, Jamie

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Jamie M

Hi,

Are you 100% sure!? :) I think it is possible that an experiment could be constructed that might not generate the expected photoelectrons assuming that light is quantized.

cheers, Jamie

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Jamie M

There might also be some unexpected non-linearities in the photoelectric effect if a femtosecond laser is used.

These non linearities might be expected with classical waves but not with photons, since the short pulses might have an effect on the effective intensity of light, that is a non linear effect when considering the number of photons in the pulse.

cheers, Jamie

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Jamie M

Get a 1P39 photo-diode tube. (~$50) And you can do the photo electric effect for yourself.

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Rather than LED's you can also try a laser, which will have a narrower spectral distribution. (maybe one from a blue ray player?)

George H.

Reply to
George Herold

It was precisely that aspect of the photoelectric effect, i.e. that the electron yield went to zero at some wavelength regardless of intensity, that really establised the quantization of light. Einstein's Nobel Prize citation was for that, not relativity.

Cheers

Phil Hobbs

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Phil Hobbs

Yes.

I think it is possible that an experiment could

High-end night vision gear, with microchannel plates, detect single photons. As the light on a scene gets dimmer, the image becomes infrequent speckles, single hits, but it's still there. You can do the same thing with a photon-sensitive PMT.

At gamma wavelengths, you're dealing with the equivalent of bullets. Every photon is a big deal that you can't miss.

Photographic film is not single-photon sensitive at normal wavelengths, so it will quit imaging for long, low-light exposures.

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John Larkin                  Highland Technology Inc 
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John Larkin

The presence of shot noise in a photodetector tracks photon theory nicely. You can observe that, and verify the math, with ordinary photodiodes that are not single-photon sensitive.

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John Larkin                  Highland Technology Inc 
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John Larkin

I always wondered how cheap Brownie cameras, doors and shutters, kept sunlight off film, and how slide film holders worked ditto. It's probably possible because of reciprocity failure, namely that halide film is not single-photon sensitive. Halide film was a remarkable invention/bit of luck.

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John Larkin                  Highland Technology Inc 
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John Larkin

Sure, BTDT. The photoelectric effects lets you measure Plank's constant. You can't do that with noise.... (Well OK if you get to really high frequencies... kT ~= h*freq.) (I should get a 1P39.. like I don't have enough to do :^)

George H.

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George Herold

There are really pretty Philips etc flat-face PMTs that show up on ebay.

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John Larkin                  Highland Technology Inc 
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John Larkin

Interestingly, though developed far too late in the day to make a difference, a hypersensitizing method for ordinary film gets it a QE of very nearly 1.0 and a gain of 2, i.e. one photon can expose two grains. All it takes is a bit of formate ion in the emulsion. (See J. Belloni et al., Nature V. 402, p. 865 (Dec. 1999).)

Multiple right-angle bends between tight-fitting black surfaces. Light-tight boxes good enough for PMTs are easy to make.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

I just ordered an RCA 935 on ebay... some free Friday I'll measure Plank's constant.

Hmm what are the (seven?) fundamental constants? e, k_sub_B, c, h, G...? (Big G is hard to measure.)

George H.

Reply to
George Herold

There was a Scientific American "Amateur Scientist" experiment once to measure G, back when Scientific American was a serious magazine.

A modern version would be fun, strain gauges and lockins or something. Laser interferometer maybe? Capacitance gap thing?

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John Larkin         Highland Technology, Inc 

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John Larkin

Hi,

Yes the electron yield goes to zero at some wavelength regardless of intensity, but does the electron yield stay non-zero at a fixed wavelength regardless of intensity?

cheers, Jamie

Reply to
Jamie M

Hi,

That's a really good paper, thanks!

cheers, Jamie

Reply to
Jamie M

Hi,

For the experiment I was thinking, I would need a detector that can distinguish a single photoelectron emission though I think, as well as mask out any emissions that aren't in the narrow spectrum expected from the light frequency used.

cheers, Jamie

Reply to
Jamie M

Yup.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

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