# Law of refraction

The law of refraction is a basic law of optics that governs the behaviour of light as it passes through a boundary between two different media, such as air and water.

The law of refraction states that the ratio of the sines of the angles of incidence and refraction of a light ray passing through the boundary of two media is constant, where the ratio is equal to the ratio of the refractive indices of the two media.

• The law of refraction (Snell’s law) deals with what happens to light as it crosses the boundary between two transparent media with different refractive indices.
• The law of refraction is useful because it relates the angle of incidence and angle of refraction of a ray of light to the refractive indices of the two media it is passing through.
• Transparent media all have different refractive indices (index of refraction) that measure how much the speed and direction of light changes as it passes out of one and into another.
• Factors that affect the refractive index of a medium include the wavelength of light passing through it, its optical density and its temperature.
• So the law of refraction explains the relationship between the angle of incidence as light approaches the boundary of one medium with a particular refractive index and the angle of refraction as it enters the second with a different refractive index.
• It derives a formula from the fact that when light of a particular frequency crosses the boundary between any pair of media, the ratio of the sines of the angles of incidence and the sines of the angles of refraction is a constant in every case.
• The formula is: n1 sin θ1 = n2 sin θ2, where n1 and n2 are the refractive indices of the two media, and θ1 and θ2 are the angles of incidence and refraction, respectively.
• Refractive indices can be measured experimentally using techniques like refractometry, and they are unique to each medium and can be used to identify unknown substances.
• There are only four terms in the law of refraction so if three are known then the fourth can be calculated.
• So, for example, it is possible to calculate the angle of refraction associated with the use of a lens or prism if the angle of  incidence and the refractive indices of the first medium (air) and the second (optical glass) are known.
###### References

As light crosses the boundary between two transparent media, the law of refraction (Snell’s law) states the relationship between the angle of incidence and angle of refraction of the light with reference to the refractive indices of both media as follows:

When electromagnetic radiation (light) of a specific frequency crosses the interface of any given pair of media, the ratio of the sines of the angles of incidence and the sines of the angles of refraction is a constant in every case.

• Snell’s law deals with the fact that for an incident ray approaching the boundary of two media, the sine of the angle of incidence multiplied by the index of refraction of the first medium is equal to the sine of the angle of refraction multiplied by the index of refraction of the second medium.
• Snell’s law deals with the fact that the sine of the angle of incidence to the sine of the angle of refraction is constant when a light ray passes across the boundary from one medium to another.
• Snell’s law can be used to calculate the angle of incidence or refraction associated with the use of lenses, prisms and other everyday materials.
• When using Snell’s law:
• The angles of incidence and refraction are measured between the direction of a ray of light and the normal – where the normal is an imaginary line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
• The wavelength of the incident light is accounted for.
• The refractive indices used are selected for the pair of media concerned.
• The speed of light is expressed in metres per second (m/s).