If you put two similar striped patterns together, Moiré
stripes result. If the patterns are identical, Moiré stripes
don't occur, but if the patterns are just slightly different,
interference fringes based on that gap occur.
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| The refractive index of salt water
is dependent on its concentration. As the
concentration increases, the refractive index also
becomes larger. Therefore, a light source shone
horizontally into a stratified fluid will refract in
the direction of the concentration gradient. Using
these characteristics, by looking at a striped
interference fringe through a stratified fluid will
make the density disturbance in that fluid visible.
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First you paint a vertical striped pattern (G1) on a light
box, and then print a striped pattern with a thinner
pitch (G2) on an acrylic sheet, and place it a small
distance from the light box. Normally if you look through the
acrylic sheet at the light box, Moiré stripes become
visible, but if you look from a certain distance, the two
striped patterns match completely, and the Moiré stripes
disappear.
Having established this distance, place the water tank in
front of the acrylic sheet (G2) (placing it behind the sheet
is also OK) and make a stratified fluid in the tank.
Light passing through the stratified layers is refracted
downwards, however since the striped pattern is vertical, the
matching of the two striped patterns doesn't change. When
waves occur in the stratified fluid, the light is refracted
horizontally, and the striped pattern on the light box is
distorted to make Moiré stripes. These Moiré stripes are
isopleths (equal value lines) of the horizontal density
gradient, so if the amplitude and direction of the waves is
constant, the stripes are also lines denoting equal phase.
The sensitivity of this technique increases as you move
the two striped patterns apart and decrease their pitch.
However, in order to photograph the Moiré stripes, both
striped patterns must be in focus, so you cannot infinitely
raise the sensitivity.
When the walls of the water tank are curved, even when the
fluid is at rest, the refraction of light through the water
tank is inconsistent. Therefore it is almost impossible to
completely eliminate the Moiré stripes. Even so, if you
photograph one of the striped patterns twice at points where
the phase of a wave is 180 degrees different, and combine the
photos after, clear Moiré stripes can be obtained.
The most important thing about this technique is to
combine the two striped patterns with more precision than
their pitch. Combining two different frames on the same roll
of film accurately is extremely difficult. One way of getting
around this is to make the laboratory dark, leave the camera
shutter open for a half-period of the wave, and flash two
strobes once each, again a half-period apart, thereby
creating two images on the one frame. I f you use color film,
and change the color of the strobes by covering them with red
and blue film respectively, you can split the two images
using computer processing and thereby create Moiré stripes.