This geometric principle states that the ratio of the sines of the angles of incidence and refraction equals the inverse ratio of the materials' refractive indices. Optical engineers use this equation to design lenses, predict total internal reflection in optical fibers, and trace beam paths through complex assemblies. It dictates exactly where a laser beam will point after passing through an angled window or a prism.
If you calculate the interface using Snell's law, you'll see why the beam is walking off the detector when we tilt the vacuum window.
Related terms
Total internal reflection
The complete reflection of a light ray at the boundary of a medium with a lower refractive index.
Critical angle
The minimum angle of incidence that produces total internal reflection.
Refract
To bend a light ray as it passes from one medium into another with a different refractive index.
Walk-off
The drifting apart of interacting beams in a crystal, either sideways from birefringence or in time from velocity mismatch.