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# thin film physics question?

when you look at the surface of a thin film solar cell, you might see a certain color due to the constructive interference between the light reflected from the front and back surfaces.

suppose that the color you see is blue lambda=475nm. if the light starts in air (n=1) the index of refraction of the silicon thin film is 4.5, and the index of refraction of the back surface material is 5.0.

what is the minimum thickness of the silicon thin film that will cause the reflected light to be this blue color. find two other thicknessess that meet the criteria for this problem

okay! so im trying to figure out what equations to use on this thing. the equation ive been looking at is t=(m+1/2)lambda . but then the only other equation i see would be the mlambda= change in l which i know doesnt really fit. any clarification and help would be awesome...thanks..

### 1 Answer

- Alexander GLv 41 decade agoFavorite Answer
The formula t=(m+1/2)lambda seem right. The light you see in air is 475nm, but the wavelength of light in the silicon thin film is shorter by lamdba/4.5. You want the thickness of the film to correspond to 1/2 multiples of the wavelength WITHIN the film. So use that new wavelength and the formula.

Different thicknesses can be found by using different m's: m=0, 1, 2.