Re: Newton Rings
SCHRAMMR@wlsvax.wvnet.edu
Thu, 22 Jun 1995 10:18:28 -0400 (EDT)
As a physicist, I have been enjoying the Newton's Rings discussion
greatly. Let me just add the following: It is correct to say that
Newton's Rings are caused by interference. This occurs when there is a
extremely this layer of a substance of one index of refraction located
between two layers of a different index of refraction. In this case,
the extremely thin layer of air located between the film and the glass.
As light passes through this "sandwich" it is refracted and reflected
at the boundries. Multiple reflections occur. Phase shifts occur. The
result is the the multiple light waves add up (bright areas) of cancel
out-subtract-(dark areas). This effect will not occur unless the thin
layer is very thin and of very uniform thickness. This is because the
light waves have a very,very,very short wavelength (about 5 x 10-5 cm.
for green light). Putting powder on the neg. or glass simply makes the
thin layer nonuniform and Newton's Rings will not occur. You could also
use non-glare glass. I'm not sure about putting oil on the negative.
I think it would work if the oil had an index of refraction similar to
the neg base and/or glass. You might try glycerin since it is water
soluable and will wash off easily. Perfect Newton's Rings are perfect
circles and are an indication of the uniformity of the layer. One can
use this to detect changes of thickness of the layer by as little as
1/4 the wavelength of the light. Monochromatic light produces Newton's
Rings of greater contrast. Newton's Rings are used to test the quality
of lenses and a number of other things that physics persons do.
Sorry this turned into a "lecture" but I can't help it. That's what
I do. ;-)
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Prof. Robert W. Schramm
Dept. of Physics or Art
West Liberty State College