You have probably taken photographs using the smartphone on which you are reading this. If so, you have, perhaps unwittingly, made use of ‘light refraction’, a physical principle that was first studied in detail by Leiden Professor of Mathematics and Physics Willebrord Snellius.
Light refraction refers to the tendency of light to bend when, as physicists say, ‘it moves from one medium into another’. This is just a complicated way of saying that light transitions from water to air (or the other way around).
Everyone has had first-hand experience of light refraction. You can see this effect when you immerse your legs in a swimming pool and the light that reflects on your legs passes from water to air. If you look at your legs in the water, they will seem to ‘bend’. The same principle underlies all lenses, which in turn play a key role both in photography and in our ability to observe distant stars and planets through telescopes.
The law of nature used to describe light refraction is known to this day as ‘Snell’s Law’, a law that is now required material for all secondary school pupils. It was named after Willebrord Snellius, born in Leiden in 1580 as Willebrord Snel van Royen.
His father was a Professor of Mathematics at the University and this was a subject in which Willebrord also excelled. After his studies, he followed in his father’s footsteps and was appointed Professor in 1613. He quickly developed to become the scientific superstar of the then still young Leiden University. Snellius published on a diversity of topics such as pure and applied mathematics, physics, surveying, ship navigation, hydraulic engineering and astronomy.
In addition to his important contributions to physics, Snellius also made an essential contribution to surveying. As early as 1615 he was already known for his method for performing trigonometric measurements which made it possible to measure the circumference and radius of the earth. Despite the fact that he worked with much less advanced instruments, his measurements differ from their modern-day equivalents by a mere four per cent: an incredible performance. In addition, Snellius managed to calculate the famous number pi with greater precision, up to 35 decimal places.
Snellius discovered the law of light refraction in 1621. He died five years later. His tombstone, which he shares with his wife, can be found in the Pieterskerk. It says: ‘Hier leggen begraven Mr Willebrordus Snellius, in sijn leven professor matheseos, sterf op den 30 Octobris 1626, en de Maria de Lange sijne huisvrouwe sterf op den 11 Novembrius 1627’ (‘Here lie buried Mr Willebrordus Snellius, in his life Professor of Mathematics, who passed away on 30 October 1626, and Maria de Lange, his wife, who passed away on 11 November 1627’).
Snellius lives on in his now famous refraction law, but also in the Snellius Building that currently houses the Mathematical Institute of Leiden University, which is named after him.
