Q.If the difference in the velocities of light in glass and water is $0.25 \times 10^8$ m/s, calculate the velocity of light in air. Given that refractive index of glass and water with respect to air are 1·5 and 4/3 respectively.

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. …