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Re: Thomas Young's experiment



According to the high school physics text from which I first
learned about this experiment (that was in Poland, more
than half a century ago!) Young used pinholes, not slits.

Yes, I am guessing about 50 cm. The reason has to do
with the distance between fringes. To notice them in a
dark room (without using a magnifier) fringes should be
about 1 mm apart. If the distance between two pinholes
is 0.3 mm, and if I want fringes to be separated by one
millimeter, then, for visible light, the distance D (from set
of two pinholes to the screen) should be 60 cm.

Making the distance between the first pinhole and the
set of two pinholes too small might not be desirable,
unless two pinholes are so small that diffracted beams
they produce are very wide. That is why, I suspect, the
distance of 10 cm (from the first pinhole to the set of two
pinholes) might not be consistent with the best possible
utilization of the limited amount of light.
Ludwik Kowalski

On Thursday, Oct 30, 2003, Hugh Haskell wrote:

At 21:48 -0800 10/30/03, Ludwik Kowalski wrote:

Yes, using sun rays (reflected into a room by a mirror) would
increase the intensity a lot. But was the real experiment
performed by using the sun or a flame? That is the question.
Also was it a set of slits or a set of pinholes?

I couldn't find a reference to answer that definitively. Asimov's
Biographical Encyclopedia says it was slits, but that is not
necessarily an authoritative source, especially on small details like
that. But it does seem that a flame of the type available by the
early 1800s, focused by a concave mirror, would have given sufficient
intensity. Do you have some knowledge that the slits were about 50 cm
from the source hole, or are you guessing? I don't think that
distance would have been necessary, as long as the distance was large
compared to the source hole size. A few cm should have been enough,
and would have increased the final intensity considerably, compared
with a 50 cm distance. And if the two slits were close enough
together, one could be much closer to the slits than a meter, still
further increasing the intensity. So keeping the scale of the
experiment down to about 10 cm instead of 150 cm, could result in an
intensity gain of about a factor of 200, which might be enough to
make the interference pattern visible even in the feeble light of a
candle.

I have demonstrated the effect using a laser on a double slit
arrangement and been able to see the effect quite easily only two or
three cm away from the slits. Of course that isn't much good for
demonstrating it to a class full of students. But ti would be
acceptable if he had only one or two viewers other than himself.

Hugh
--

Hugh Haskell
<mailto:haskell@ncssm.edu>
<mailto:hhaskell@mindspring.com>

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