The Michelson Interferometer is an optical instrument that splits a beam of light into two paths, reflects them back, and recombines them to produce interference patterns, enabling precise measurements of wavelength, distance, and refractive index. This device is foundational in physics for experiments like the Michelson-Morley experiment, which provided critical evidence for the theory of relativity by demonstrating the constancy of the speed of light in all inertial frames.