Re: How do you detech a photon passing through a slit?

From: Old Man (nomail_at_nomail.net)
Date: 10/31/04 

Date: Sun, 31 Oct 2004 15:19:36 -0600

"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]

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