8.1 Thin film interference

Interference

Thin films

Suppose there is a very thin film of dielectric and light is incident on it normally. Lets consider single reflections. (We make the small angle ofincidence approximation)

When destructive interference occurs then that value of $\lambda$ is not reflected. Note that this is a function of both $d$ and $\lambda$ . The next effect is that different colours of light get reflected at differentthicknesses of the film. This is why soap films or oil films on water give rainbow effects.

Note I have assumed that $n_3>n_{film}>n_{air}$ in the above, where $n_3$ is the material that the film sits upon.

Consider an interface between two materials with indices of refraction $n_1$ and $n_2$ . If $n_2>n_1$ . Then lets examine what happens to the phase of an electromagnetic wave uponreflection. For a transverse electric field, there is a phase change of $\pi \text{.}$ For the transverse magnetic field (or $E_{\parallel }$ ) there is not, if the light ray is close to the normal. However if $n_1>n_2$ then and the situation is reversed and the transverse electric field does not undergo a phase change and the transverse magnetic field does. In the exampleabove, their will be no relative phase change between the rays in either case. Either both will change by $\pi$ or neither will change, depending on the orientation of the E field.