Another method visualizes the internal gravity wave using the moíre method. This
method utilizes the property of light that passes horizontally through a stratified fluid
is refracted to the opposite direction of the density gradient. This method is much more
inexpensive than the Shrearen method.
In principle, when a stripe image distorted by the density disturbance is overlaid with
an undistorted image, moíre stripes appear showing the pattern of the internal gravity
wave.6) In this experiment, however, the
curvature of the tank also distorts the image, and therefore, a simple application of this
method produces moíre stripes even when the working fluid is at rest. For this reason,
instead of using an undistorted image, a distorted image of the stripe pattern is overlaid
with an image in a different phase. This overlaying process
was performed with a computer after scanning the recorded film.