THIN-FILM+INTERFERENCE

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Parallel films
State the condition for light to undergo either a phase change of π, or no phase change, on reflection from an interface.

PHASE CHANGE ON REFLECTION: When light reflects at a boundary off an optically denser medium, it experiences a phase change of pi. When it reflects from a less dense medium, there is no phase change.



Describe how a source of light gives rise to an interference pattern when the light is reflected at both surfaces of a parallel film. State the conditions for constructive and destructive interference. Explain the formation of coloured fringes when white light is reflected from thin films, such as oil and soap films. Describe the difference between fringes formed by a parallel film and a wedge film.

PARALLEL-SIDED THIN FILMS: For a particular thickness, one wavelength will satisfy the conditions for constructive interference, so when white light shines, that colour will be reinforced.

WEDGE-SHAPED FILMS: When the thickness of the film varies, different colours will be seen at different positions. It is possible for a mix of two colours to be seen at the same point due to more than one wavelength satisfying the conditions for reinforcement.

Describe applications of parallel thin films. Solve problems involving parallel films.

Explain the production of interference fringes by a thin air wedge.
 * Wedge films **

AIR WEDGE: Two sheets of glass create a wedge of air of uniformly varying thickness. Regularly spaced fringes are formed

Students should be familiar with the terms equal inclination and equal thickness. Explain how wedge fringes can be used to measure very small separations. Applications include measurement of the thickness of the tear film on the eye and oil slicks. Describe how thin-film interference is used to test optical flats. Solve problems involving wedge films.