What is the smallest thing that is possible to see with a microscope?

Dave - It depends on the kind of microscope. Light behaves a bit like a wave. It has a wavelength. It's very, very hard to see things smaller than the wavelength of the light because you essentially get interference effects. The waves interfere and it messes with your picture. With conventional microscopes it's very hard to see anything less than the wavelength of light which is about half a micrometer - roughly a 2000th of a millimetre. There are ways of doing things with light which means you can get a little bit smaller than that, using funky things called metamaterials, but they are not really common.

If you want to get much more magnification than that you need to use something with a much smaller wavelength. A common one to use is an electron, because although it appears like a particle it's also a wave and the wavelength is much, much shorter and therefore you can see much, much smaller things. You can actually see big atoms with a normal electron microscope. Other forms of microscope involve dragging a tip, it's called a scanning tunneling electron microscope, and you measure the electric current going between your tip and your object - with this, you can measure down to large atoms.

Chris - IBM iconically produced the letters "IBM", I think it was 1990-1991, by manoeuvring xenon atoms with a scanning tunnelling electron miscroscope, didn't they? I can't remember quite how many atoms they used - 40 or something - and it took them about 2 weeks to move these things around. People were saying this is the future of computing! There was also a story that got published in the middle of 2008. It was by a researcher at the University of California, Berkley, Jannik Meyer. This was a wonderful paper because they were able to see hydrogen atoms. That probably qualifies as the smallest thing you could see because that's the smallest entity at an atomic level in the Universe.

The way they did this was they had a sheet of graphene, which is a single layer, one carbon atom thick, of graphite and they could drop molecules onto that surface and then scan across it with the scanning tunnelling electron microscope and measure where it was interacting. The tip was interacting with the different atoms.

Because the graphene is like a little pattern of chicken wire - it's a very regular hexagon pattern - it's very easy to subtract mathematically from whatever signature you pick up, so they can see any atomic species that were dropped on there. They could even see these little white dots that turned out to be hydrogen atoms; if you put a bigger molecule, like a butane molecule - the stuff that you burn in your lighter - on there, you can actually see this zig-zagging chain of hydrocarbons. It's just absolutely phenomenal!