Thin film Interference
Iridescence is a phenomenon wherein a surface illuminated by white light reflects various spectral colors. This effect is due to a phenomenon known as thin film interference and is found in soap bubbles, water films on glass,gasoline spills on water, and on mother-of-pearl ( the shiny surface on the inside of a clam shell0. Once again, a plausible explanation of this behavior can be extracted from a wave theory of light.

   Consider a soap film of uniform thickness. If monochromatic light is incident nearly normally on the soap film, light will reflect off the top and bottom of the film and then recombine constructively or destructively. The outcome is determined by two factors: path length difference and the Lloyd's mirror phase change. (Click here to learn about each.) One of the difficulties with this topic that students find challenging is that interference occurs for several thicknesses rather than just one. If the index of refraction of the film is larger than the index on both sides or smaller than both sides, case 1 in the box below applies. If the index of the film in smaller than the index of the material on one side but greater than the index on the otehr side, then the equations are applied as in case 2.

if index of film is larger than index on either side OR
if index of film is smaller than index on either side
case 1   
for m = 0,1,2...yields constructive interference
   
for m = 0, 1, 2, ...yields destructive interference
  n = film index lambda = wavelength of light
t = film thickness

if index of film is larger than the index on one side BUT
is smaller than the index on the other side

case 2
 
for m = 0,1,2...yields constructive interference
  
for m = 0, 1, 2, ...yields destructive interference
 n = film index lambda = wavelength of light
t = film thickness

 

 

thin film interference can be applied to good use

 

http://users.erols.com/renau/thinfilm.html
http://www.gg.caltech.edu/~zhukov/research/film/applet.html
http://noether.physics.ubc.ca/Teaching/Physics101/StudySheets/Thinfilm.html

go to homework probs - thin film interference

 

This page was last reviewed by mgosselin 10/09/2005