When this passes through the two slits, it is disturbed and hence secondary expanding spherical waves are produced at each slit. The superposition of the two secondary waves when they combine, results in constructive or destructive interference at the screen, resulting in bright or dark bands or fringes. A.D. measured the speed of light with a toothed wheel. The US scientist Michelson measured the speed of light in an experiment with his newly built Interferometer. He is credited for combining light, electricity and magnetism into one theory.
Light is the visible part of a very long electromagnetic wave spectrum.