# Light

## The double slit experiment

#### Types of Light

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

## Interferometer

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.

#### Interferometer

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:

1. Disproved the Luminiferous Aether
2. Surface topography of samples.
3. Measured Newton’s Gravitational Constant, G.
4. Maybe gravitational waves???
5. (lookup LIGO - http://www.ligo.caltech.edu/)