One of the consequences of light having a wave-like aspect is exemplified by its apparent ability to jump gaps. For instance, light penetrating through a block of glass at a shallow angle is effectively trapped within the glass by the barrier of air at the far side, unless a second glass block is placed close to it (but not touching). Because of the spread-out nature of the wave, some of it penetrates the air barrier and if encounters more glass beyond it can continue, thus apparently jumping the air gap and escaping its prison.A similar thing happens at the sub-atomic scale, when alpha particles try to escape from unstable nuclei during radioactive decay. The particles are effectively held in the nucleus by the nuclear forces and, in principle, should not be able to escape. However, escape they do, using a process known as quantum tunnelling, which makes use of the wave-like aspect of the particles, but also of a more general phenomenon known as “uncertainty” (which we will look at in more detail below).