Brewster's angle
The angle is named after the Scottish physicist Sir David Brewster (1781–1868).
[1][2] When light encounters a boundary between two media with different refractive indices, some of it is usually reflected as shown in the figure above.
The fraction that is reflected is described by the Fresnel equations, and depends on the incoming light's polarization and angle of incidence.
The physical mechanism for this can be qualitatively understood from the manner in which electric dipoles in the media respond to p-polarized light.
One can imagine that light incident on the surface is absorbed, and then re-radiated by oscillating electric dipoles at the interface between the two media.
The phenomenon of light being polarized by reflection from a surface at a particular angle was first observed by Étienne-Louis Malus in 1808.
[3] He attempted to relate the polarizing angle to the refractive index of the material, but was frustrated by the inconsistent quality of glasses available at that time.
[4] This has implications for the existence of generalized Brewster angles for dielectric metasurfaces.
Thus reflected light from horizontal surfaces (such as the surface of a road) at a distance much greater than one's height (so that the incidence angle of specularly reflected light is near, or usually well beyond the Brewster angle) is strongly s-polarized.
Using a polarizing camera attachment which can be rotated, such a filter can be adjusted to reduce reflections from objects other than horizontal surfaces, such as seen in the accompanying photograph (right) where the s polarization (approximately vertical) has been eliminated using such a filter.
When recording a classical hologram, the bright reference beam is typically arranged to strike the film in the p polarization at Brewster's angle.
By thus eliminating reflection of the reference beam at the transparent back surface of the holographic film, unwanted interference effects in the resulting hologram are avoided.
Entrance windows or prisms with their surfaces at the Brewster angle are commonly used in optics and laser physics in particular.
The polarized laser light enters the prism at Brewster's angle without any reflective losses.
In surface science, Brewster angle microscopes are used to image layers of particles or molecules at air-liquid interfaces.
However any molecular layers or artifacts at the surface, whose refractive index or physical structure contrasts with the liquid, allows for some reflection against that black background which is captured by a camera.
Gas lasers using an external cavity (reflection by one or both mirrors outside the gain medium) generally seal the tube using windows tilted at Brewster's angle.
This prevents light in the intended polarization from being lost through reflection (and reducing the round-trip gain of the laser) which is critical in lasers having a low round-trip gain.
And many sealed-tube lasers (which do not even need windows) have a glass plate inserted within the tube at the Brewster angle, simply for the purpose of allowing lasing in only one polarization.
