Re: [ATM] Foucault Slit

[Mark Cowan <toolontop@yahoo.com>]

Wow, what a method!  And to think I've used just a
magnifier to examine the illuminated slit and eyeball
the width by a precision ruler.  ;)  

The only problem I see with this is that it doesn't
ensure the slit has the same width across its length,
which a shim stock solution (or alum foil) will
guarantee.  And it'd be tough to damage steel with
brass.  

However - a slightly rounded bevel on the "knife" edge
works better for minimizing diffraction effects and
will never cut you in the dark.  Easier if the slit
jaws are made from alum or brass.

10-30 microns is on the narrow side and will exhibit
diffraction effects that many people may have trouble
ignoring while not improving accuracy over the typical
50 micron width.

Cheers,
Mark Cowan
Salem, OR USA



--- "Wm. D. Hanagan, Jr., Ph.D."
<hanaganw@attglobal.net> wrote:

> Rich Ball wrote:
> > 
> > Is there any common object that can be used to
> calibrate the slit-width on
> > the Foucault light source?  
> > 
> . . .
> > --Rich Ball
> 
> 
> Hi Rich, 
> 
> Here's an easy method for measuring or setting the
> width of your slit using a
> laser pointer, a tape measure, and a calculator with
> trig functions.  Using this
> "Laser Diffraction Method" there is no need to risk
> damaging the knife edges of
> your slit.  
> 
> Aim the laser through the slit and allow the
> resulting single slit diffraction
> pattern to be projected on a light colored wall or a
> white board about 30" away
> and oriented perpendicular to the path of the laser.
>  I'll leave it to you to
> work out how to hold the slit and laser in position.
>  A strong rubber band or
> tape can be used to hold the laser trigger switch in
> the on position.    
> 
> Dim the lights.  If you can't see the first two dark
> side bands well, reduce the
> distance between the slit and the board or wall. 
> Measure the distance between
> the first two DARK bands appearing on either side of
> the central bright band. 
> Be sure to measure between the DARK bands, NOT
> between the bright bands.  Also
> measure the distance from the slit to the board or
> wall.   
> 
> The slit width, S, can be calculated from the
> following equation.  
> 
> S = W/[sin(tan-1(D/2P))]
> 
> where: 
> 
> S = slit width in microns
> W = wavelength of laser light, in microns (650nm =
> 0.650 microns)
> D = distance between the first two DARK bands in the
> diffraction pattern
> P = the projection distance = distance from the slit
> to the viewing surface
> 
> Any convenient unit can be used for D and P, but
> both values must use the same
> unit.  For example, D and P can both be entered into
> the equation in inches OR
> they can both be entered as millimeters.  
> 
> The warning tag on a laser ordinarily indicates the
> wavelength in nanometers
> (nm); convert that wavelength to microns before
> using it in the equation; 1
> micron = 1000 nm.  If your laser has no warning tag
> and it's deep red in color,
> the peak wavelength is likely to be about 650nm, or
> 0.650 microns.  If you have
> a true He-Ne laser, it's wavelength is likely to be
> 632.8 nm.  
> 
> If you keep the slit width above about 3.5 microns,
> you can use a simple
> approximation of the above equation, which appears
> below, while keeping the
> error due to the approximation below 2%.  For the
> Foucault test, there would be
> little cause for concern if the slit width was off
> target by 20% or more, so the
> error due to the approximation is trivial in this
> case.   
> 
> S = 2WP/D  (approximate)  
> 
> where parameters are as defined above.  
> 
> For setting a slit to a particular width, you can
> rearrange this approximation
> and solve for the distance D between dark bands:  
> 
> D = 2WP/S  (approximate)
> 
> where parameters are as defined above.  
> 
> Then, simply adjust the slit until you get the
> spacing you want.  The target
> range that I recommend is 10-30 microns.  
> 
> 
> Clear Skies, 
> 
> Bill Hanagan
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



		
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