# Snel or Snell, Willebrord

### law light constant refraction

(1580–1626) Dutch physicist: discovered law concerning refraction of light passing between media.

Snel studied law and mathematics in a number of European universities and in 1613 succeeded his father in the new university of Leiden, as professor of mathematics. He continued to publish translations of mathematical work, but also became involved in geodesy, and has been described as ‘the father of triangulation’. Starting with his own house, and with the spires of town churches as reference points, he soon mapped a substantial local area. His best-known discovery, Snel’s Law, or the second law of refraction of light waves, was probably made in about 1621 after much experimentation. In modern form, it states that for light passing from one isotropic medium to another, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for light of a particular wavelength; ie sin ? 1 /sin ? 2 = constant. The law can be deduced from principle. It is found that the value of the constant is equal to the ratio of the velocity of light in material (1) to that in material (2), and this is known as the relative refractive index.

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