Light waves become inverted when they reflect from the boundary of a medium that has a higher index of refraction than the original medium

• Light waves that reflect from the two surfaces of a thin film produce interference fringes that depend on:
  • The different path lengths travelled by the two waves
  • The wavelength of light
  • Any phase changes that occur from the reflection
  • The indices of refraction for the materials involved

If only one wave has a phase change:

t is the thickness of the film, $\lambda$ is the wavelength, and $n_{film}$ is the index of refraction for the film

If both waves have a phase change:

Air wedges:

Air wedges can be used to determine the thickness of very small objects through the relationship $\triangle x = L(\frac{\lambda}{2t})$, where $\triangle x$ is the distance between the fringes