Brewsta

Meet Brewsta, the angle at which glare goes quiet for one polarisation.

Light hitting a surface between two transparent materials partly reflects and partly refracts. How much reflects depends on the polarisation. For light polarised in the plane of incidence, called p-polarised, reflection drops to zero at one special angle. David Brewster described it in 1815: it is the angle where tan θ_B equals n₂/n₁, which is also where the reflected and refracted rays are exactly 90° apart.

For light going from air (n ≈ 1.00) into glass (n = 1.5), θ_B = arctan 1.5 ≈ 56.3° from the normal. The refracted ray then travels at about 33.7°, and 56.3° + 33.7° = 90°.

At this angle, reflected light is fully s-polarised. That is why polarising sunglasses, with a vertical transmission axis, cut glare from water and roads, and why laser tubes use Brewster-angle windows to avoid reflection losses.

Only p-polarised light is fully transmitted; the s part still partly reflects. The angle also varies slightly with colour, because n depends on wavelength.