The color of light that we perceive is based on the frequency (or wavelength) of the light.

The amount of light visible is very sensitive to the path difference between two rays. This means we can make a device to measure very small distances.

The smallest length we can measure using this device will be given by the wavelength of the light used.

We know the path length difference is: $$\Delta r = 2 L_2 - 2 L_1$$ When $\Delta r$ is equal to a whole number of wavelengths, ie: $\Delta r = m\lambda$, then the interference between the two paths will be perfectly constructive. Any change in the amplitude of the final, detected light, implies a change in either the wavelength of the light during one arm of travel, or a change in the length of one arm.

Uses of interferometry:

- Disproved the Luminiferous Aether
- Surface topography of samples.
- Measured Newtonâ€™s Gravitational Constant, G.
- Maybe gravitational waves??? (lookup LIGO - http://www.ligo.caltech.edu/)