Interference

Nature of science:

Curiosity: Observed patterns of iridescence in animals, such as the shimmer of peacock feathers, led scientists to develop the theory of thin film interference. (1.5) Serendipity: The first laboratory production of thin films was accidental. (1.5)

• Young’s double-slit experiment
• Modulation of two-slit interference pattern by one-slit diffraction effect
• Multiple slit and diffraction grating interference patterns
• Thin film interference

Applications and skills:

• Qualitatively describing two-slit interference patterns, including modulation by one-slit diffraction effect
• Investigating Young’s double-slit experimentally
• Sketching and interpreting intensity graphs of double-slit interference patterns
• Solving problems involving the diffraction grating equation
• Describing conditions necessary for constructive and destructive interference from thin films, including phase change at interface and effect of refractive index
• Solving problems involving interference from thin films

• Most two-slit interference descriptions can be made without reference to the one-slit modulation effect. To what level can scientists ignore parts of a model for simplicity and clarity?

Utilization:

• Compact discs are a commercial example of the use of diffraction gratings
• Thin films are used to produce anti-reflection coatings

Aims:

• Aim 4: two scientific concepts (diffraction and interference) come together in this sub-topic, allowing students to analyse and synthesize a wider range of scientific information
• Aim 6: experiments could include (but are not limited to): observing the use of diffraction gratings in spectroscopes; analysis of thin soap films; sound wave and microwave interference pattern analysis
• Aim 9: the ray approach to the description of thin film interference is only an approximation. Students should recognize the limitations of such a visualization

• Students should be introduced to interference patterns from a variety of coherent sources such as (but not limited to) electromagnetic waves, sound and simulated demonstrations
• Diffraction grating patterns are restricted to those formed at normal incidence
• The treatment of thin film interference is confined to parallel-sided films at normal incidence
• The constructive interference and destructive interference formulae listed below and in the data booklet apply to specific cases of phase changes at interfaces and are not generally true

Data booklet reference: