1. Lasers. You know magic and stuff
Science Pg.30-37
By: Savannah Banks

2. Light-Emitting Diodes
A diode is an electronic device that allows electric charge to pass through it in only one direction.
Diodes are commonly used to convert alternating current(AC) to direct current(DC).
A photodiode is another type of diode that produces an electric current when light is incident upon it, which is essential to the operation of solar cells.
Light-emitting diodes (LED) work like photodiodes in reverse, they emit light when electric current is passed through them. 
A semiconductor is a material that can be made to conduct electricity under some conditions but not others. 

3. Laser operation
A laser is a device that emits a narrow beam of single-wavelength, coherent light as a result of stimulated emission. 
The term laser began as an acronym for "Light Amplification by Stimulated Emission of Radiation".
The output power of laser light can vary from a few thousandths of a watt (laser pointers) to several though watts (industrial laser cutters).
Every laser contains an active mediums, which is the source of atoms that will undergo cycles of excitation and de-excitation to release photons that will form a laser beam. Some types of lasers, the active medium can be a solid, liquid, or gas.

4. continued 
An atom can become excited from a lower to a higher energy level by absorbing a photon with energy equal to the difference between the energies of two levels. Eventually, the atom will return to a lower energy state by emitting a photon in a process known as spontaneous emission. 
Atoms can also transition to a lower energy level through a process called stimulated emission.
In order to sustain the chain reaction of stimulated emission event, the atoms must be prepared such that more are in an exited state than are in the ground state. Also known as population inversion.

5. Properties of lasers
Laser light is monochromatic, directional, and coherent. Ordinary white light is a combination of many wavelengths light, emits isotopically, and is a mixture of many out-of-phase wavelengths.
Single-frequency light are described as monochromatic.
Collimated- the light from a laser emerges as a very narrow beam with very little divergence or spread. This is also called Directional.
Light that is made up of waves that are "in-phase" relative to one another is said to be coherent.

6. Types of lasers
Gas lasers use a low-pressure gas mixture as an active medium. Most gas laser are excited by passing an electric current through the gas, delivered by electrodes placed at opposite ends of the tube.
Solid-state lasers use an active medium consisting of a solid crystalline or glass rod (the host) containing light-emitting atoms(active species). The first laser ever built was a solid-state laser using synthetic ruby, which is corundum (aluminum oxide crystal) with chromium as an active species.
Semiconductor diode laser, commonly known as laser diodes, operate using the same basic principles as light-emitting diodes, but with some important differences. Like an LED, a laser diode consists of two semiconductor layers.