You are standing in air and are looking at a flat piece of glass ( n = 1.52) on

Question

You are standing in air and are looking at a flat piece of glass (n = 1.52) on which there is a layer of transparent plastic (n = 1.61). Light whose wavelength is 525 nm in vacuum is incident nearly perpendicularly on the coated glass and reflects into your eyes. The layer of plastic looks dark. Find the smallest possible nonzero value for the thickness of the layer.


You are standing in air and are looking at a flat piece of glass (n = 1.52) on which there is a layer of transparent plastic (n = 1.61). Light whose wavelength is 525 nm in vacuum is incident nearly perpendicularly on the coated glass and reflects into your eyes. The layer of plastic looks dark. Find the smallest possible nonzero value for the thickness of the layer.

Explanation / Answer

for the minimum nonzero thickness, m = 1.

destructive interference (because the plastic looks dark) --> 2nt=m*wavelength

t= m*wavelength/2n

= 1*(525*10-9)/(2*1.61) = 163.04 E-9 m = 163.04 nm

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