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A very thin sheet of glass of nglass $51\mathrm{.}55$ floats

on the surface of water of nwater $51\mathrm{.}33.$ When

illuminated with white light at normal incidence,

the reflected light consists predominantly of the

wavelengths $5\mathrm{.}60$x$10-7$

m and $4\mathrm{.}00$ x$10-7$m

Determine the thickness of the glass.