We show that a simple double-slit experimental setup can be used to measure the thickness of a transparent thin film. The phase difference between the light passing through one slit covered with photoresist film and that passing through the other slit without film was estimated using the simple Fraunhofer diffraction formula for a double slit. Our method gave error of a few percent or less for film thicknesses ranging from 0.7 to 1.7 μm, demonstrating that a laboratory double-slit experimental setup can be utilized in practical film-thickness measurements.
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