Light Wave

[Little Bang]

Thank you, is there no way to measure the wave length of a single photon?

[Erasmus00]

Thank you, is there no way to measure the wave length of a single photon?

 

You can make inferences of the wavelength by making measurements of the energy. I can't think of any other way. You certainly can't build up the traditional double slit experiment with a single photon.

-Will

[CraigD]

... However, you aren't "seeing" the wavelength of the photon in the sense to which Little Bang was referring, but extrapolating from its energy.

The energy of a photon and its frequency are directly related (Energy = Frequency * Plank's constant). Although there are other sorts of frequencies that can be associated with signals composed of many photons - the frequency of an AM radio signal, or the baud rate of a pulsed signal - these are not directly related to any property of the photons involved.

 

I believe you’re correct, however, that what Little Bang is referring in post #1 is one of these latter kinds of frequencies. I’m guilty of not carefully reading this initial post.

 

In order to directly measure the frequency of many photons of a single or multiple frequencies, a device other than “a detector for light, say green light” connected to an oscilloscope would be required. The simplest such device I can imagine is a single-slit spectrometer – just a thin slit, and a piece of white paper with visible graduations to allow a naked eye measurement to be made. Such an instrument requires many photons /second to produce a detectable reading. Photographic film can improve this sensitivity , as can more elaborate arrangement using photomultipliers and detectors. I think such instruments can detect as few photons as the ambient “noise” levels allow, potentially as few as a single one.

[Little Bang]

Thank you guys, you have been a great help.

[Drosera]

Stupid question here, as I think I know what the answer will be already.

 

In principle, is it possible to take white light, shine it through a slit small enough so that some of the doesn't make it through, and some does? If possible, I'm guessing that the higher frequencies would be stopped first?

 

Take care!

[cwes99_03]

Man Craig. You are always one post ahead of me.

 

You have to separate your thinking here. A wave is a field, a photon is a particle. This is the nature of wave/particle duality. When talking about one you must talk about it's properties without crossing over to the properties of the other. Why?

 

What is the frequency of a wave? It is the inverse of the amount of time between the crest of a wave passing a particular point and the crest of the next wave.

What is the frequency of a photon? Well, I'll assume that question would mean how much time is between one photon impinging on a surface and the next photon.

We are talking about two different things here. A photon is simply a way of looking at light to quantize the energy. Thus a blue photon has more energy than a red photon. That is because their frequencies are different.

A ray of blue photons may have the same amplitude as a red ray of photons, that is to say if the same number of photons hit the same surface area in the same amount of time for each ray. Total energy to hit that surface will vary because of the different frequencies of light, but the amplitude of each wave will be the same.

[CraigD]

In principle, is it possible to take white light, shine it through a slit small enough so that some of the doesn't make it through, and some does?

Not exactly, but a slightly more complicated approach can. When white light shines through a thin slit, it’s diffracted into its component colors (frequencies). With a bit of clever, multilayered construction, it’s possible to use this to block specific colors while letting others pass thought, or do even more interesting things like cause the same colors to interfere with one another. Some brightly colored bird feathers and insect winds and shells use this effect to produce colors and patterns impossible with biological pigments, which produce colors by absorbing certain colors in white light

If possible, I'm guessing that the higher frequencies would be stopped first?

the longer the wavelength, the greater the angle of diffraction, so actually it’s the low frequency light that gets deflected, and thus stopped the most in something like the above. This is why iridescent things, like butterfly and parrot wings, usually tend toward blue, not red.

 

Check out the Wikipedia article "Diffraction grating" for pictures, explanations, and links.