Knudsen cosine law

Knudsen cosine law

[kə′nüd·sən ′kō‚sīn ‚lȯ]
(physics)
A law which states that the probability of a gas molecule leaving a solid surface in a given direction within a solid angle d ω is proportional to cos θ d ω, where θ is the angle between the direction and the normal to the surface.