Evans SPH 4U1
Physics Grade 12
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.