Re: Why can't waves cause the photoelectric effect

From: Dr. Photon (brendan.roycroft_at_nmrc.ie)
Date: 03/11/05 

Date: 11 Mar 2005 10:03:09 -0800

bz <bz+sp@ch100-5.chem.lsu.edu> wrote in message news:<Xns9614552F7FAB3WQAHBGMXSZHVspammote@130.39.198.139>...
> Bjoern Feuerbacher <feuerbac@thphys.uni-heidelberg.de> wrote in news:d0mhhg
> $7ki$1@news.urz.uni-heidelberg.de:
>
> > IRC, I did some interference experiments with laser light, and
> > afterwards some measurements with incoherent light in order to
> > measure the coherence length.
> >
> >
>
> Does this imply that even light from incoherent sources, such as a light
> bulb, when properly filtered, displays some coherence, when studied at a
> particular wave length? I am assuming that you were NOT measuring the
> coherence lenght of white light from an incoherent source.
>

You can measure the coherence length of pretty much any source you
like using a Michelson interferometer. For most purposes the term
coherent or incoherent source only refers to narrow linewidth or broad
linewidth source, with no hard definition of the difference.

Intriguingly, coherence length *does* increase when you filter the
spectrum, and is not related to whether the source is coherent or
incoherent. So if you take a white light bulb, pass the light through
a 0.01nm notch filter, then you end up with quite coherent light. You
can increase the coherence further by narrowing the filter. The
penalty you pay is that the intensity drops, so a white light bulb
filtered down to 0.00001nm will be as coherent as a medium quality
laser, but hardly any light will pass!

> I guess white lasers must have 3 coherence lengths.
> [quote http://www.edinst.com/whitelaser.htm ]
> Wavelength (nm) red at 671nm, blue at 473nm, green at 532nm
> Transverse mode TEM00
> Operating mode CW
> Output power (mW) 100, 150, 200
> Operating temperature (?) 15-35
> Beam divergence, full angle (mrad) <1.5
> Beam deflection (mrad) 1
> Expected lifetime (hours) 10000
> [unquote]
>
> I want one!

There are other white light lasers which are continuous cover the
whole visible spectrum. They are pulsed lasers which attain the broad
spectrum due to emitting *very* short pulses (a few fs). I am not
certain, but that laser used in the "electron uncertainty in time"
measurement probably counts as a white light laser. Unfortunately it
would also be *much* more expensive to use and maintain than the
three-colour laser you refer to.

BR

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