A pulse laser is basically a device for storing energy and then releasing it all at once to give a very intense beam of light. The heart of the laser is a crystal or tube of gas or liquid into which energy is pumped. This is usually done by surrounding it with a device to produce a powerful flash of light or an intense beam of radio waves or electrons. As pumping occurs, more and more of the atoms inside take up energy and are excited to high energy states. Suddenly an atom spontaneously returns to its first energy state and gives out a particle of light(a photon). This photon strikes another excited atom and causes it to produce another photon. Very rapidly, a cascade of photons develops. The crystal or tube is closed at both ends by mirrors and the photons bounce to and fro between them, building up the cascade. A proportion of this light is able to escape through one of the mirrors, which is half-silvered, and an intense flash of light emerges from the laser.