A wave may propagate from one medium to another, each having a different density. In such a case, the incident wave partly gets propagated and partly gets reflected. The wave propagation velocity would be relatively less in the denser medium than in the rarer medium. A quantity of refractive index compares the velocity of electromagnetic waves in different media. It is expressed as the ratio of the velocity of light in a vacuum to the velocity of light in a given medium. If the wave travels from the denser medium to the rarer medium, the reflected and transmitted waves are in phase with the incident wave, but their amplitude is reduced. In contrast, if the laser is placed on the other side, the wave propagates from the rarer to the denser medium. In that case, the reflected wave is 180° out of phase with the incident wave. The transmitted wave remains in phase with the incident wave.