Interference fringes (peaks or troughs) can be used to determine the pathlength

Question

Interference fringes (peaks or troughs) can be used to determine the pathlength of an empty (containing air, n = 1.00) sample cell used for infrared spectrosoopy. These interference fringes are observed as waves (or rules) in the spectra, and are the result of light reflecting many times between the internal walls of the sample cell shile undergoing constructive and destructive inference, which depends on the ratio of the pathlength and the wavelength of the light. When this effect is observed, the pathlength of the call can be determined using the following equality for a empty cell, b = N/2(v_1 - v_1) where b is the pathlength, N is the number of peaks OR troughs and v_1 and v_2 are the begining and end points of the measured spectrum in cm^-1. An FTIR spectrum of an empty sample cell was obtained and 35 peaks were observed between 1871 cm^-1 and 694 cm^-1. For this sample cell, determine its pathlength in millimeters. pathlength =

Explanation / Answer

b = N/2[1 - 2]

= 35/2(1871 - 694) cm-1

= 0.015 cm

= 15 mm

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