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Re: [ATM] Foucault Slit
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
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