When light waves encounter an interface between two different media, such as from a denser medium to a rarer medium, there can be a phase change of 180 degrees upon reflection. This phenomenon is known as the phase change or the inversion of phase upon reflection.
To understand why this occurs, we need to consider the behavior of light waves at the interface. When a light wave travels from one medium to another, its speed and direction can change due to the difference in refractive indices between the two media. The refractive index is a measure of how much the speed of light is reduced when passing through a medium compared to its speed in a vacuum.
When light waves encounter an interface, a portion of the incident wave is reflected back into the original medium, and another portion is transmitted into the new medium. The behavior of the reflected wave depends on the polarization of the incident light (i.e., the orientation of its electric field).
For light waves that are incident on an interface from a denser medium to a rarer medium (higher refractive index to lower refractive index), there is typically a phase change of 180 degrees upon reflection. This means that the reflected wave is shifted by half a wavelength relative to the incident wave.
The phase change occurs because of the way the electric and magnetic fields of the light wave interact with the charges in the material at the interface. Without going into the complex mathematical details, the reflection of light can be understood in terms of the behavior of the electromagnetic fields and the boundary conditions at the interface.
The phase change of 180 degrees upon reflection is a consequence of the wave nature of light and the properties of the electromagnetic fields. It is a fundamental property of light and can be observed in various optical phenomena, such as reflection from a mirror or the behavior of light waves at the surface of a pond.
It's important to note that this phase change of 180 degrees is specific to the case of reflection at an interface from a denser medium to a rarer medium. In other cases, such as reflection from a rarer medium to a denser medium, or when the incident light is polarized differently, the phase change upon reflection may be different or even nonexistent.