The optical microscope is probably the only scientific instrument that children are familiar with, and possibly the only one that can be bought in toy shops. At school, most of us looked into such microscopes with great expectations, hoping to see unimaginable details of a leaf or dead fly. Often we were disappointed that we could not see more. Where were all those atoms and molecules that everything is supposed to be made of? If we complained to the teacher, we were usually told that one cannot see things that are too small in an optical microscope. Until recently the teacher was right, but not any more.
For over 100 years scientists have firmly believed that the resolution of a microscope bottomed out at about half the wavelength of the light used to illuminate the sample. The reason is that the wave nature of light - and not the laws of ray optics - dictates the formation of images. As a result, the image of a point object is a finite-sized spot, rather than a point. To make matters worse, the spot consists of fringes that are produced by diffraction at the edges of the lenses and other optical elements in the microscope. If we view two point objects as they move closer together, we see their images run into each other and there comes a point where we can no longer tell the two objects apart.
In the September issue of Physics World, Vahid Sandoghdar of ETH Zürich in Switzerland reveals how physicists are developing ingenious techniques to overcome the diffraction limit to view ever-smaller objects with optical microscopes. He also explains how single molecules have been used as light sources to examine objects as small as one hundred millionth of a metre in size.