Lasers work by focusing which of the following?

The concept is quite simple. Laser's tube concentrates light over and over in order to get a powerful beam. But this beam is something more than just a flashlight. The thing is that laser's light cannot be compared with an ordinary "weak" beam. The idea of a laser consists in the process of adding energy to the primal weak light to make the beam concentrated and powerful.

The ordinary light consists of rays of different wavelengths. These rays are out of synchronization, so this light might be called inhomogeneous or incoherent. Laser's light is known as some kind of a perfect phenomenon (and powerful concentration of energy as well) - its rays are absolutely equal in their length and coherent.