🔖 Topics

  • Thin film interference

🎯 Objectives

  • Describe the origins of interference effects in thin films
  • Calculate the positions of maxima and minima in a thin film

📋 Sequence

  • Origin of thin film interference
    • We need to think about the path difference as light travels through the film and phase changes due to bouncing off different media.
    • There are two sources of phase change, not just one!
  • Thin film diagrams
    • We ignore higher order terms
    • We are assuming light is approaching the film head-on (\(\theta = 0^\circ\))
    • Transmitted pulses have the same phase
    • Reflected pulses have the same phase if you are moving from a slower medium (large n) to a faster medium (small n)
    • Reflected pulses have a phase shift of \(180^\circ\) if you are moving from a faster medium (small n) to a slower medium (large n)
  • Example problems for thin film interference
    • Oil slick on water
    • Soap bubble in air
    • Newton’s rings

🖥️ Animations, Simulations, Activities

📝 Practice Problems

  1. Calculate the minimum thickness of an oil slick on water that appears blue when illuminated by white light perpendicular to its surface. Take the blue wavelength to be 470 nm and the index of refraction of oil to be 1.40.
  2. A film of soapy water (n = 1.33) on top of a plastic cutting board has a thickness of 233 nm. What color is most strongly reflected if it is illuminated perpendicular to its surface?

✅ Partial Solutions

  1. 83.9 nm
  2. 620 nm (orange)

📘 Connected Resources

  • Giambattista, Alan, et al. College Physics With an Integrated Approach to Forces and Kinematics. 5th ed., McGraw-Hill Education, 2020.
  • Tipler, Paul A., and Gene Mosca. Physics for Scientists and Engineers: With Modern Physics. 6th ed., W. H. Freeman, 2008.