here\'s the problem: A glass plate (n=1.61) is covered with a thin, uniform laye

Question

here's the problem: A glass plate (n=1.61) is covered with a thin, uniform layerof oil (n=1.20). A light beam of variable wavelength from air isincident normally on the oil surface. Observation of the reflectedbeam shows destructive interference at 500 nm and constructiveinterference at 750 nm. From this information, calculate thethickness of the oil film. I know in my book, it gives me 2 equations for destructive andconstructive interference to help solve for thickness based on thenumber of phase reversals and the order number n. What I don'tunderstand is which equation I should use given both destructiveand constructive intereference at certain wavelengths. Any help would be much appreciated. here's the problem: A glass plate (n=1.61) is covered with a thin, uniform layerof oil (n=1.20). A light beam of variable wavelength from air isincident normally on the oil surface. Observation of the reflectedbeam shows destructive interference at 500 nm and constructiveinterference at 750 nm. From this information, calculate thethickness of the oil film. I know in my book, it gives me 2 equations for destructive andconstructive interference to help solve for thickness based on thenumber of phase reversals and the order number n. What I don'tunderstand is which equation I should use given both destructiveand constructive intereference at certain wavelengths. Any help would be much appreciated.

Explanation / Answer

The general equation for destructive interference in thinfilms is 2nt = m *     (m =0,1,2,..............) n = 1.61,m = 1 and = 500 nm = 500 * 10-9m or t = (m * /2n) or t = (1 * 500 * 10-9/2 * 1.61) = 155.2 *10-9 m = 155.2 nm

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