When we come across something unfamiliar in the ordinary everyday world, we might prod and poke it with a stick, or smell it or taste it to see what it is. There might for instance be little resistance on our stick if the object happens to be a jellyfish, or a considerable amount of resistance if it is a lump of basalt, and this resistance tells us something about the composition of the object. This is what we do with electrons; we throw a few electrons at the atoms and by noticing how they respond, we get some idea about what sort of ?things? atoms are. Electrons are our sticks, and they help us find out what makes atoms tick.In this book we theoretically calculate the probability of interaction between the target and the electrons by using a spherical complex optical potential.