A cloud chamber photograph showing the straggling of alpha rays from polonium. A circular chamber is filled with a water and alcohol solution which is vapourised. When particles with an electric charge are pased through this chamber, droplets (of water and alcohol vapour) are formed at the points where the particle pases. A light source at the side of the chamber illuminates the resulting trail of droplets (the particle track), which is then photographed.
© Science Museum / Science & Society Picture Library