16.12:
Propagation of Waves
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.
16.12:
Propagation of Waves
When a wave propagates from one medium to another, part of it may get reflected in the first medium, and part of it may get transmitted to the second medium. In such a case, the interface of the two mediums can be considered as a boundary that is neither fixed nor free.
Consider a scenario where a wave propagates from a string of low linear mass density to a string of high linear mass density. In such a case, the reflected wave is out of phase with respect to the incident wave, however the transmitted wave is in phase with respect to the incident wave. Both the reflected and the transmitted waves have amplitudes less than the amplitude of the incident wave. If the tension in both the strings is the same, the wave speed is lower in the string with higher linear mass density.
In contrast, consider the propagation of waves from a string of high linear mass density to low linear mass density string. In such a case, the reflected wave is in phase with respect to the incident wave, as is the transmitted wave. Both the reflected and the transmitted waves have amplitudes less than the amplitude of the incident wave.
This text is adapted from Openstax, University Physics Volume 1, Section 16.5: Interference of Waves.