angle of incidence

angle of incidence, in physics, the angle that an incoming wave or particle that is colliding with a surface makes with a line normal (i.e., perpendicular) to that surface.

Light rays change direction when they reflect off a surface, move from one transparent medium into another, or travel through a medium whose composition is continuously changing. The law of reflection states that, on reflection from a smooth surface, the angle of the reflected ray is equal to the angle of the incident ray. The angle of incidence equals the angle of reflection. The reflected ray is always in the plane defined by the incident ray and the normal to the surface. The law of reflection can be used to understand the images produced by plane and curved mirrors. Reflection at rough, or irregular, boundaries is diffuse.

When light traveling in one transparent medium encounters a boundary with a second transparent medium (e.g., air and glass), a portion of the light is reflected and a portion is transmitted into the second medium. As the transmitted light moves into the second medium, it changes its direction of travel; that is, it is refracted. The law of refraction, also known as Snell’s law, describes the relationship between the angle of incidence (θ₁) and the angle of refraction (θ₂), measured with respect to the normal to the surface, in mathematical terms: n₁ sin θ₁ = n₂ sin θ₂, where n₁ and n₂ are the indices of refraction of the first and second media, respectively. The index of refraction for any medium is a dimensionless constant equal to the ratio of the speed of light in a vacuum to its speed in that medium. There is also a critical angle of incidence at which, if the angle of incidence is greater than the critical angle, all the light is reflected and none is refracted.