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Snell's law (also known as the Snell–Descartes law, the ibn-Sahl law, [1] and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air.
Refraction of a thin planoconvex lens. Consider a thin lens with a first surface of radius and a flat rear surface, made of material with index of refraction .. Applying Snell's law, light entering the first surface is refracted according to = , where is the angle of incidence on the interface and is the angle of refraction.
This phenomenon, known as total internal reflection, occurs at incidence angles for which Snell's law predicts that the sine of the angle of refraction would exceed unity (whereas in fact sin θ ≤ 1 for all real θ). For glass with n = 1.5 surrounded by air, the critical angle is approximately 42°.
Due to Snell's law, the numerical aperture remains the same: NA = n 1 sin θ 1 = n 2 sin θ 2. In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light.
[47]: 229 The refractive index is used for optics in Fresnel equations and Snell's law; while the relative permittivity and permeability are used in Maxwell's equations and electronics. Most naturally occurring materials are non-magnetic at optical frequencies, that is μ r is very close to 1, therefore n is approximately √ ε r. [48]
This is an illustration of Snell's Law. A seismic wave coming with the path of the red line would refract when it passes through the surface of medium change. Seismic waves travelling at a critical angle (blue line) will be refracted critically with an angle of refraction equal to 90°. An illustration of seismic reflection and refraction ...
A ray trace through a prism with apex angle α. Regions 0, 1, and 2 have indices of refraction, , and , and primed angles ′ indicate the ray's angle after refraction.. Ray angle deviation and dispersion through a prism can be determined by tracing a sample ray through the element and using Snell's law at each interface.
Hand-held refractometer. A refractometer is a laboratory or field device for the measurement of an index of refraction (refractometry).The index of refraction is calculated from the observed refraction angle using Snell's law.