What do human teeth, daisy petals, eggshells and white paint all have in common? Improbably, they can all be turned into lenses. This discovery might lead to cheap lenses that replace the pricey ones used in today鈥檚 optical microscopes.
Some microscopes use a focused beam of light to illuminate a particular area, for instance a cell, to locate molecules that fluoresce in response to the light. The finer the beam, the more precisely it can pinpoint molecules. Conventionally, such a beam is focused by curved glass lenses, but the best lenses 鈥� those that focus light to the smallest point 鈥� are expensive.
Now Allard Mosk of Twente University in Enschede, the Netherlands, and his colleague Ivo Vellekoop have shown that cheap, opaque materials can also focus light sharply 鈥� provided you fiddle with the light first. 鈥淎 layer of paint, an eggshell or a tooth may match the resolution of the most perfect microscope,鈥� says Mosk.
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The materials they chose are not normally transparent. But some light waves do pass through them, after being scattered in random directions and bouncing around inside the material. How the light bounces depends on the pattern of the light waves when they hit the material. The researchers wondered if by tweaking this pattern, known as the wavefront, they could control the bouncing so that the light passes through and then converges at a tightly focused point on the other side of the material, as it does with a conventional lens.
To test the idea, Mosk and Vellekoop used an array of liquid crystals to vary the wavefront of the light beam as they shone it on a daisy petal, a 0.4-millimetre-thick piece of eggshell, a layer of paint and a 1.5-millimetre-thick human incisor. They produced a wavefront that caused a tightly focused bright spot of light to appear on the other side of the opaque material.
鈥淎 bright spot of light appeared on the other side of the opaque material鈥�
With the paint, the light at this spot was 1000 times more intense than the light that emerged using an unmodified beam, indicating that the material was focusing the light (, vol 32, p 2309). The other materials produced focused light spots that were less intense.
鈥淭his is a clever experiment,鈥� says Roberto Merlin of the University of Michigan in Ann Arbor. Mosk admits that it requires electronics as well as the opaque materials to work, but is betting on that being cheaper than precisely curved glass.
