Tomsk State University. Physical Department.
Laboratory of training physics experiment
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Phase change for various conditions of reflection

The experiment in which Young placed a drop of cassia oil between a crown lens and a flint-glass plate in a Newton's ring apparatus is well known.

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Using a laser one can easily demonstrate by means of projection the presence or absence of a 180° phase change by various conditions of reflection. It can be done by taking a slightly tapered crown glass plate (n = 1.5) and greasing the middle portion of its back surface with a drop of alphamonobromnaphthalene (n = 1.66) as it is shown in the Figure. It is helpful to press this surface against a frosted, or better, black surface. That reduces the light reflected from the second surface of the liquid layer.
A broad parallel laser beam obtained by using a microscope objective followed by an ordinary collimating lens is directed at the plate in order to obtain two series of interference bands; one of them along the edges, and the other one in the middle of the plate. The bands of these series are displaced with respect to each other by a half of a band clearly indicating the difference of the phase shifts on the reflection at a boundary with 1) air, and 2) liquid which is optically denser than glass.
Plate+alpha
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