What is a laser?

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation.

A laser beam is a large group of light waves of the same frequency going in phase in the same direction.

A group of waves in phase will add up together into basically one big light wave with the frequency of the original waves but with a higher amplitude (intensity or brightness).

Because all the light waves in a laser are going in the same direction the resulting coherent beam is extremely intense compared to incoherent light of the same power source.

Laser beams are usually created in a closed medium (e.g., a cylindrical box) where both ends of the cylinder are partially reflective mirrors. Inside the cylinder a certain type of material is placed. That material has such properties that when its electrons are excited they can hover in excited states for unusually long periods of time. When one electron falls from its excited state it gives off a wave which moves through the material hitting other electrons which in turn give off more waves moving in phase in the same direction. This means that by the time the final group of in phase light waves hits one of the mirrors, it has grown to be a wave with very high amplitude.

If the wave hits the mirror head on when it comes out of the material, it will bounce right back into the material repeating the above process and growing stronger.

Since the mirrors are only partially reflective, some of the light waves in the internal beam will leak outside the system but will remain coherent and in phase. That beam is what you see when you shine a laser pointer on a wall.

Why are lasers dangerous?

Since the laser beam consists of only parallel light, it focuses into a small point. When this point is shined into your eye, it could burn the retina. That's why even laser pointers can be very dangerous and should be treated with caution.