Understanding light
Polarisation
Light is a wave that shakes sideways. Polarisation describes which way it shakes — and once you can control that, you can build filters, 3D glasses, LCD screens and optical isolators.
The states
| State | What the field does | In Photonica |
|---|---|---|
| Unpolarised | Shakes in every direction at random — ordinary lamplight | The default source setting |
| Linear | Shakes along one fixed line | Set an angle from horizontal |
| Circular | The direction rotates steadily, left or right | Right or left circular |
Polarisers and Malus's law
A polariser passes only the component along its transmission axis. Feed it light already polarised at an angle θ to that axis and the transmitted intensity is:
Unpolarised light loses exactly half its power passing the first polariser, whatever the angle — there is no preferred direction to keep.
Wave plates
A wave plate is a slice of crystal that slows one direction of shaking slightly more than the perpendicular one. The delay is measured in wavelengths:
- A quarter-wave plate at 45° to incoming linear light turns it circular.
- A half-wave plate rotates linear light — by twice the angle between the light and the plate's fast axis.
Reading the state
The Measure panel reports polarisation as a Stokes vector plus a degree of polarisation:
| Value | Means |
|---|---|
| DOP | 1 = fully polarised, 0 = completely unpolarised |
| s₁ | +1 horizontal, −1 vertical |
| s₂ | +1 at +45°, −1 at −45° |
| s₃ | +1 right circular, −1 left circular |
The ray inspector shows the same four numbers after every surface, so you can watch a reflection or a wave plate change the state step by step.