Evans SPH 4U1

Unit 5: Waves

Note 11: Interference in Thin Film Concepts

Reference: Chapter 10.4/10.5

For this section you must refer to your class notes and diagrams and the figures in the textbook (pg. 512-515).

Most of the concepts for this section rely on the following principles:

1 ) When waves pass into a slower medium, the partially reflected waves are inverted. Whereas when the transition is from a slow to a fast medium the waves are NOT inverted.

2) When the path difference of two waves is half a wavelength (or any odd multiple of half a wavelength) apart the waves are out of phase.

Proofs for the following will be done in class.  It will be important for you to understand the proofs - not just memorize the result.

Reflected Light in Thin Films:

Destructive Interference occurs then the thin film has a thickness of

Constructive Interference occurs at a thin film thickness of

Transmitted Light through Thin Films:

Destructive Interference occurs then the thin film has a thickness of

Constructive Interference occurs at a thin film thickness of

Spectral Colours:    Since colours have different wavelengths, for red light is greater than for blue light.  Therefore blue light constructive interference will occur at a thinner film thickness than red light.  This is the reason when light light hits a thin film, the spectral colours are seen (each colour will have constructive interference at a different thickness).

For example a soap bubble has different colours at different thicknesses of the bubble (much like oil on water where the oil thickness will vary).

Air Wedges:

Air wedges are used to find the thickness of very small objects using the following

where t is the thickness of the object.

Applications of Thin Films ( Chapter 10.5):

Reading assignment for evening of Monday, April 26th.