John Carter, of Eltham, asks :-

How did we discover there are molecules, atoms, protons and electrons when they are too small to be seen with microscopes?

John Campbell, the author of "Rutherford Scientist Supreme" and producer of the documentary "Rutherford", responded.

With the naked eye we can "see" small objects down to the size of human cells. To do that, make a pin-hole camera by pushing a pin through light cardboard. Place a mm ruler on a distant wall which is uniform colour and well lit. Place the pinhole against one eye and stare at the wall with your eye relaxed. You will see "floaters", each the shadow diffraction pattern of a dead cell floating in the liquid in our eye. The biggest ones are diffracted shadows of cells furthest from the back of the eye where our light receptor cells are. Notice what width the cell coincides with on the ruler. The human eyeball is about 25mm in diameter. As the angles are equal, the cell width is 25mm times the floater width at the ruler divided by the distance between the eye and ruler.

You will find this to be about 2.5 micro-metres. We cannot see smaller objects unaided because the light receptors in our eyes are too big.

We cannot see small molecules directly but we can see their effect. In 1827 Robert Brown, a botanist, reported looking through a microscope at pollen grains in water. The pollen grains appeared to move through the water. That was later proof that objects smaller than cells existed. Water molecules were jostling the pollen grain. A few extremely large molecules can be seen by the naked eye, e.g. DNA and long polymers. Also physical discoveries such as the gas laws told us a lot about molecules.

It was noticed that if water had an electric current passed through it then one gas collected at one electrode at twice the rate a different gas collected at the other. These were hydrogen and oxygen. Hence molecules were made up of a very few (about 90) different types of smaller entities, atoms. An atom has a diameter of about 0.0001 micrometres so we cannot see one directly. The properties of atoms were inferred from their chemical reactions. Nowadays we can "see" individual atoms on surfaces using an atomic force microscope.

When Ernest Rutherford went to the Cavendish Laboratory in 1895, one curiosity of nature was how was it that a perfectly good electrical insulator, a gas, could be turned into a perfectly good electrical conductor, as in a gas discharge tube? In 1896 Rutherford was invited to work with the professor there, J. J. Thomson, on trying to understand this curiosity. In 1897 Thomson carried out the definitive experiment to show that the conducting component was a charged particle whose mass was about 1/2000th the mass of the smallest atom. There were objects much smaller than atoms. He had discovered the electron.

We can see the effect of individual electrons, for example when one hits the glass tube it causes a flash of light. To measure the size of an electron this is now done using scattering of electrons in large particle accelerators. Effectively it is a point with no size. It is now known that even an electron is made of quarks. We are still learning about those particles.