Q.The property of light which remains unchanged when it travels from one medium to another is ______.

Light's fundamental nature was debated for centuries. Newton's corpuscular theory (light as a stream of hard, elastic, massless particles) explained reflection easily but predicted light should speed up in a denser medium -- the opposite of what is measured -- and had no way to explain interference, diffraction or polarization. Huygens' rival wave theory (1678) correctly predicted light slows down in a denser medium, and once interference, diffraction and polarization were discovered experimentally in the nineteenth century, the wave picture became firmly established, since only a wave theory can produce these effects at all. …