Thin film interference occurs when light waves reflect off the top and bottom surfaces of a thin film and interfere with each other. This interference is caused by the phase difference between the waves that are reflected from each surface.

Reflection and Refraction:
When light waves encounter a thin film, they can be partially reflected and partially refracted at each surface of the film. The reflected waves interfere with each other, leading to the phenomenon of thin film interference.

Phase Difference:
The phase difference between the waves depends on the path length traveled by each wave. When light waves reflect off a medium with a higher refractive index, such as air to glass, there is a phase change of 180 degrees. This means that the reflected wave is shifted in phase by half a wavelength compared to the incident wave. On the other hand, when light waves reflect off a medium with a lower refractive index, such as glass to air, there is no phase change.

Interference:
When two waves with a phase difference interfere, they can either constructively or destructively interfere. Constructive interference occurs when the waves are in phase and their amplitudes add up, resulting in a bright spot. Destructive interference occurs when the waves are out of phase and their amplitudes cancel out, resulting in a dark spot.

Thin Film Interference:
In the case of thin film interference, the reflected waves from the top and bottom surfaces of the film interfere with each other. If the path length difference between the two waves is an integer multiple of the wavelength, constructive interference occurs, resulting in bright fringes. If the path length difference is a half-wavelength or an odd multiple of half-wavelengths, destructive interference occurs, resulting in dark fringes.

Conclusion:
Thin film interference is caused by the interference of light waves that reflect off the top and bottom surfaces of a thin film. This interference occurs due to the phase difference between the reflected waves, which depends on the refractive indices of the media the waves travel through. The phenomenon of thin film interference can be observed as bright and dark fringes in various applications, such as soap bubbles, oil slicks, and anti-reflective coatings on lenses.