Log1989 problem number 20.49

An interferometer is used to measure the refractive indices of transparent substances. Here S is a narrow slit illuminated by monochromatic light (289 nm); 1 and 2 - two identical tubes with air, the length of each L = 10 cm; D is a diaphragm with two slits. When the air in tube 2 was replaced with ammonia, the interference pattern on the screen shifted upward by N = 17 bands. Determine the refractive index n1 of ammonia, if for air n = 1,00029.