Ex10-1 An electro-optical instrument utilizing infra-red light with a wavelength of 0.860 m m has a modulation frequency at 252.6 MHZ. At the time of measurement the temperature was 25^o C and atmospheric pressure was 751 mm Hg. Calculate the modulated wavelength of the light under the given atmospheric conditions.

Solution : For light wave, the index of refraction n_g of standard air is

Neglecting the second term, the refractive index of air, owing to changes in temperature pressure and humidity, becomes n_a and is given by

The velocity of the infrared light (v_a) through the atmosphere is given by

Thus the modulated wavelength (l) in given atmosphere is given by

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