This powerpoint gives a brief description of thin film interference. It differentiates between destructive and constructive interferences, and outlines the primary formulas used to solve equations. In addition to the description of the concept are practice problems and detailed solutions.

1. Physics 101 Learning Objective 9:
Thin Film Interference
CINDY SHEN 03/21/15

2. Examples of Thin Film Interference?
• “Thin film interference” is seen when streaks of colour are
present in soap bubbles, oil films, peacock feathers, and types
of butterfly
• Ultimately, what we see is the reflection of the “film”... Or the
interference of light, created by a thin film
• Depends on INTERFERENCE and REFLECTION

3. Examples
SOAP BUBBLES MIXTURE OF OIL & WATER
http://graphics.ucsd.edu/~iman/SoapBubbles/
http://en.wikipedia.org/wiki/Diesel_fuel

4. How does Thin Film Interference occur?
• Multiple light rays “hits” a thin translucent film
• The light rays are either reflected or transferred through the
translucent film
• Each time the light ray hits a different interface, the reflection
and transfer process occurs
• Once the reflected light rays overlap in their reflections,
different colours can be seen
• The colour is determined by the wavelength

5. Standard Formula for Thin Film Interference
• 𝝀n = 𝝀/n
𝜆n represents the wavelength of the wave in a medium
𝜆 wavelength
n is the index of refraction
• can be used to find the wavelength inside film

6. Constructive or Destructive Interference?
• At the surface of the film, in thin film interference,
constructive or destructive interference can occur
• The type of interference depends on:
- PHASE CHANGES that occurs during reflection
- THICKNESS of the film

7. 180° phase changes in reflections…
• Light reflected from the top of films undergoes a 𝜋 phase shift
• When there is a 𝜋 phase shift in the reflected light,
DESTRUCTIVE INTERFERENCE (minimal reflection) occurs
• 2t = [m+(1/2)]𝝀n, m = 0, 1, 2, 3 …
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/thinfilm.html

8. No phase changes in reflections…
• Light reflected from the bottom
of films
• When there is no phase change
CONSTRUCTIVE INTERFERENCE
(maximal reflection) occurs
• 2t = m𝝀n, m = 0, 1, 2, 3 … http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/soapfilm.html

9. Question 1: Preschoolers are playing with bubbles in the park while the sun
shines above them. a) Which formula will consider the destructive interference of
sunlight created by the soap bubble films? b) If a bubble has an index of refraction of
n = 1.5, and the light of incident is 𝝀 = 800nm, how thick is the bubble?
1. You must use