In thin film interference, phase shift occurs when light waves reflect off a medium with a higher refractive index. This phenomenon can be explained by considering the change in the optical path length experienced by the light waves as they travel through different materials.
When light waves encounter a boundary between two media with different refractive indices, such as air and a thin film, a portion of the incident light is reflected and a portion is transmitted into the film. The reflected light undergoes a phase change of 180 degrees (or π radians) when it reflects off a medium with a higher refractive index.
This phase shift happens because the electric field of the reflected wave is inverted upon reflection from a higher refractive index medium. This inversion occurs due to the difference in the speed of light in the two media and the properties of the electromagnetic waves at the interface. As a result, the reflected wave has a phase difference of π compared to the incident wave.
This phase shift is crucial in thin film interference because it affects how the reflected waves interact with each other and with the transmitted waves, leading to constructive or destructive interference patterns. These interference patterns determine the intensity and color of the light observed after the waves recombine, influencing phenomena such as iridescence in soap bubbles or oil slicks.
