Can I make a laser from a flashlight?

 The colors of the laser rainbow on my skateboard.

     Since I was young, I always wondered how lasers worked, and if I could make one by poking a hole through a piece of paper and shining a flashlight through the hole.  I'm not sure if anyone has tried this, but I did.  What did I find out?  Wouldn't lasers be called "constrained flashlights" or something else if that worked?

     Laser stands for "Light Amplification by Stimulated Emission of Radiation."  Lasers use the principle of stimulated emission to produce a bright beam of light.  Stimulated emission is a process where incoming light passes by an excited atom, stimulating it to reproduce the light that is doing the stimulating.  This only works under certain conditions.  The gain medium that contains the atom must be pumped, or energized (electrically or optically), so that the atom is able to release some energy and reproduce the light.  Additionally, the light that stimulates the atom must have an energy that is just right to match the excited atom energy.

     How do excited atoms produce light though?  Electrons of an excited atom have more energy than they would like to.  These excited electrons transition from their high energy states to low energy states.  Imagine a ball placed at the top of a hill. The ball will want to roll down it!.  Similarly, electrons will want to release their energy by "rolling down a hill" (they actually tend to drop right to the bottom, but that's another story).   They can do this in a variety of ways, some of which produce phonons, or vibrations, and others that produce photons, or light.  The transitions that produce photons are the ones we are interested in because those transitions can be used for stimulated emission, and thus, a bright laser beam.

     Constructing a laser is not as simple as finding a suitable medium and shooting light through it.  There are always sources of loss in a laser system, and light can only be amplified if the gain from stimulated emission is greater than all of the sources of loss in the system.  One might ask, why does a laser emit in a straight beam if it just requires stimulated emission, which could be in any direction?  The answer is because one pass through the excited material does not typically produce enough extra light to make a bright beam.  Thus, a resonant cavity is used to allow multiple passes of the light through the excited medium.  This produces a large intensity, but the light must be in a tight beam to pass through the material many times to become so bright.  Otherwise, the light will not be reproduced and it will just leave the system.  Imagine driving down the highway in your car.  Wouldn't it be pretty rough if you decided to start driving on the shoulder of the road, and then even off-road?  You would have a hard time driving like that.  Similarly, light wants to travel through the easiest path it can, which happens to be straight through the gain medium, the light's "paved highway."  Therefore, the output of the laser is an intense, straight beam of light.

     Clearly the details of how a laser works are pretty complicated.  So to answer the question posed in the post title... No, a laser cannot be made by bottle-necking the output of a flashlight.  It's a bit more complicated than that.  And my experiment with the flashlight... let's just say it ended anticlimactically.  However, some claim that they can make a laser pointer from a flashlight as seen in the following video:

The flashlight hack shown does not use the flashlight bulb to create the laser beam.  Instead, it uses the housing and source of the flashlight to power a separate laser diode.  The resulting laser might not last very long because common laser diodes aren't designed to run at such high powers.  The DVD laser diode in the video, however, might be able to handle the power that flashlights run at.

      In case you have never seen a laser before... here are a collection of laser pointers I have acquired.  Each one of them has a resonant cavity in them, showing just how small we can make them nowadays (most of the space in the pen-shaped pointers is filled by two AAA batteries).  I used these laser pointers to take the photo at the beginning of this post.  As I usually end up saying in these posts, if you want more information, go to the Wikipedia laser page.