Re: How do you detech a photon passing through a slit?
From: Mike (eleatis_at_yahoo.gr)
Date: 11/04/04
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Date: 4 Nov 2004 03:33:35 -0800
"Old Man" <nomail@nomail.net> wrote in message news:<OeCdnbjuPa53yxjcRVn-qQ@prairiewave.com>...
> "Wild Gnu" <WildGnu@hotmail.com> wrote in message news:27e869fb.0410302356.29d2279d@posting.google.com...
> > Hi.
> >
> > I'm not a physicist.
> >
> > I was reading Feynman's book "QED." It describes an experiment where a
> > photon is fired at a wall with two slits in it with a photomultiplier
> > behind it. Depending on the distance of the two slits from each other
> > the probabililty that the photomultiplier behind the wall detects a
> > photon is between 0 and 4% but if two detectors are placed at the two
> > slits to determine whether the photon passed through slit A or slit B
> > the probability that the final detector behind the wall detects a
> > photon becomes closer to 2% depending on how accurate the other two
> > detectors are.
> >
> > My question is, how can you make a device that detects a photon but at
> > the same time allows it to pass through unaffected. Does the photon
> > loose any energy when it passes through this kind of detector?
> >
> > Thanks.
> >
> > -wg
>
> In the two slit interference experiment, passage of photons
> through a given slit is guaranteed by blocking the other slit.
>
> A photon is detected via interaction with a charged particle,
> usually an electron. All of the kinetic energy and momentum
> of the photon are transferred to the electron. The photon is
> completely absorbed, but a new photon of less energy may
> (Compton scattering), or may not (photoelectric effect),
> be emitted. The photon detection process then involves
> measurement of the electron's recoil energy and momentum.
>
> [Old Man]
Hmmmm, the above two posts are identical, that is Koepke's and Old Man's.
Mike
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