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Hard lenses sit on the eye like a small rigid window. Soft lenses are different: they are a polymer net that holds water. That material, and a clever way to shape it, came from a Czech chemist working in Prague. This post covers what Otto Wichterle made, why the chemistry works, and a worked example of the spinning trick, which uses only Class 11 circular motion.
Who was Otto Wichterle?
An obituary in the Journal of Polymer Science calls him the inventor of soft contact lenses and a pioneer of new polymers for medical use. It records that he was born on 27 October 1913 in Prostějov, Czechoslovakia, and died in his sleep on 18 August 1998, aged almost 85.
The history page of the Institute of Macromolecular Chemistry of the Czech Academy of Sciences says the institute was set up in 1959 with Wichterle as its founder and first director. It names two of its most important practical results: a way of polymerising caprolactam, and hydrophilic (water-loving) polymers based on poly(2-hydroxyethyl methacrylate), developed under Wichterle's personal guidance. The second led to soft contact lenses in the 1960s and to intraocular lenses in the 1980s.
What was the gel?
In 1960 Wichterle and his colleague Drahoslav Lím published a short paper in Nature titled Hydrophilic gels for biological use. Their US patent, Process for producing shaped articles from three-dimensional hydrophilic high polymers, was published on 28 March 1961.
The idea is simple to state. Start with a monomer whose molecules carry an -OH group that attracts water. Polymerise it with a small amount of a cross-linker that has two reactive ends. The chains join into a loose three-dimensional net. Water soaks into the net and swells it, but the cross-links stop the polymer from dissolving. The result is soft, holds its shape, and lets water move through it.
Wichterle's later spin-casting patent gives a working recipe: ethylene glycol monomethacrylate with small amounts of diethylene glycol methacrylate and ethylene glycol bis-methacrylate. It notes that the finished lens must be soaked in sodium bicarbonate solution.
How did spinning shape a lens?
Cutting and polishing a wet, soft gel is hard. The spin-casting patent, with a priority date of 11 September 1963, describes another way. The liquid mixture is put into an open mould that rotates while the polymer sets. The outer surface takes the shape of the mould. The inner, concave surface takes an approximately paraboloid shape "determined by centrifugal forces and the surface tension of the polymerization mixture". Change the speed and you change the curve.
Worked example: how fast should the mould spin?
Ignore surface tension for a moment. A liquid rotating steadily at angular speed ω settles with its free surface at height
z = ω²r² / (2g)
above the lowest point, where r is the distance from the axis. You can get this in the rotating frame: the free surface is where gz − ½ω²r² is constant.
Near the centre, a curve z = r² / (2R) has radius of curvature R. Comparing the two gives R = g / ω².
Here is a sample calculation. Suppose we want the inner surface to have a radius of curvature of 8.0 mm at its centre.
Angular speed: ω² = g / R = 9.8 / 0.0080 = 1,225 s⁻², so ω = 35.0 rad/s.
Revolutions per minute: 35.0 × 60 / (2π) = 334 rpm.
What if we double the speed? R depends on 1/ω², so at 668 rpm the radius becomes 8.0 / 4 = 2.0 mm, a much steeper curve.
Real lenses also depend on surface tension and the mould, as the patent says, so this is only the leading idea.
Common mistakes: using rpm directly in ω²r² instead of converting to rad/s, and assuming the surface is a sphere. It is a paraboloid; a sphere only matches it near the centre.
To check your set-up on rotating-frame problems like this, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer. Our post on non-uniform circular motion covers the forces behind it.
Why does this matter for a student?
Two ideas you study separately meet here. Polymer chemistry decides how much water the material holds and how it feels on the eye. Mechanics decides the curve, and the curve decides the lens's focal power. Our post on defects of the human eye explains what that power has to correct.
What can a student take from Wichterle's way of working?
Start from the requirement. The eye needs a material that is soft, wet and stable. He designed the chemistry to meet that, rather than adapting an existing plastic.
Let physics do the shaping. A spinning liquid makes a smooth curve for free.
Publish and patent. The Nature paper told scientists what the gel was; the patents described how to make useful things from it.
Next step: find the rpm needed for R = 7.0 mm, then check by substituting your ω back into R = g/ω².
