If you wanted excessive detail about the structure of an eye, you’d probably visit Wikipedia (and you could even see a ridiculously high-resolution image of an eye). I promise to avoid making this article that long.

The retina is the part of your eye that actually responds to light. You’ve probably heard that it has rods (sensitive to light) and cones (sensitive to color). Nocturnal animals require and subsequently have more rods, whereas diurnal (i.e., active at day) animals don’t require as many rods. Despite all the rods that nocturnal animals have, the retina does not pick up all of the light that passes through it.

The tapetum lucidum (Latin for “bright carpet,” yeah, those scientists are clever…) is a reflective layer behind (though it can be within) the retina and is found in those animals that have reflective eyes. Essentially, the tapetum reflects light back to the retina for a second chance to pick it up. If humans had this special reflective layer, it would give us better night vision, but it would also decrease the clarity of our vision (since the reflected light interferes with the incoming light).

Interestingly enough, different cone cells respond to different wavelengths of light, giving us color vision. If we had an even greater variety of cones, we could pick up other wavelengths. Next to red, we would see infrared light and, next to violet, we would see ultraviolet. If we had a really wide variety, we could pick up radio stations, but the suicide rate would go up due to the DJ’s telling us how they never stop playing music but not understanding that we don’t want to hear their voices over the music…

What does “laser” mean?
First, laser actually stands for “light amplification by stimulated emission of radiation.” If you can understand that at face-value, you can pretty much figure out how lasers work. Since most people just skip over phrases with too many big words, I’ll break it down.

How is a laser different from a light bulb?
Photons (light energy) are released when an electron goes from a higher level orbit to a lower level orbit. That’s why heating something up causes it to change color; some of that heat is being released as light energy. I did a post about different types of lights and explained how they work in a fairly nontechnical manner. Essentially, they all work by exciting electrons, which go up in orbit and then release a photon when they drop in orbit. The difference is that “normal” lights do this pretty randomly, but lasers are more organized (amazing how much more you can accomplish if you are organized, isn’t it?).

What makes it so bright?
When photons are released, the energy level is what determines the color (frequency). If you can get all of the photons released at the same energy level, they build upon each other and are a much brighter, more focused beam of light. Inside a laser, this is done by having two mirrors. When a photon is released, it bounces back and forth between the mirrors, which causes other photons to be released if their electrons are stimulated at the same level. Essentially, it is like having one person on a boat running from side to side screaming, “Come on, this is fun!” Other people start to go along with him and the boat starts rocking. Eventually it capsizes and they drown, but that’s another story…

All these photons are rushing back and forth between the mirrors, convincing other electrons to let their photons out to enjoy the fun. Meanwhile, one of the mirrors is not completely a mirror (kind of like how you can just barely see through some one-way mirrors on the mirror side) and some of the photons escape. Enough of them keep bouncing around to continue causing other photons to do the same, but the ones which do escape are all of the same frequency, making them a very intense light.

How can lasers be used?
All kinds of devices use the laser in difference ways. For instance, CD players shine the laser onto the spinning CD, which has tiny grooves in some places. The parts that are flat (no grooves) reflect the laser back to an optical sensor, but the grooved parts do not. Since digital media only needs ones and zeros to represent data, the flat spots become 1’s and the grooved spots become 0’s.

What does it mean again?
Once again, the acronym is “light amplification by stimulated emission of radiation.” Now it should make a bit more sense. The light (photons) is amplified (because the photons are all at the same frequency) by stimulated emission (meaning the electrons are all stimulated to produce that same frequency of light) of radiation (which is simply the process of releasing energy as waves and/or particles).

Incandescent about 16 lm/W
Incandescent bulbs are still the most common type of household light in most cases. They work by putting current through a filament. Since the filament creates resistance, it begins to heat up. Long story short, the electrons start partying and spitting out photons. Since the heat would burn up the filament if oxygen was available, most bulbs are either vacuums or filled with inert gas. Basically, this kind of light is not efficient because it relies on heat to create the light. If you have ever heated your home with electric heaters, you know that they can become very expensive.

Fluorescent about 50-100 lm/W
Fluorescent lamps are more complex than incandescent bulbs, but they are also quite a bit more efficient. Here is a very simplistic explanation of their operation: electricity excites mercury vapor that is in a gas (such as neon or argon), emitting ultraviolet light. This causes to a phosphor to “fluoresce” (meaning that one spectrum of light is absorbed and another emitted). A phosphor is a substance that glows after being exposed to light. Phosphors are also used in CRT’s, which is what causes screens on old TV’s and computers to glow. Since the process is fairly complex, it’s more efficient (seriously, why do something more complex than incandescent bulbs if it isn’t more efficient?). Well, okay, the process is more efficient largely because so little heat is produced.

Halogen about 16 lm/W
Halogen bulbs are actually very similar to incandescent bulbs. Instead of glass, they enclose the filament in quartz and the “bulb” is very close to the filament. The fixture is filled with a halogen gas. The vapor from the filament combines with the halogen and is redeposited on the filament, keeping it from burning out quickly. This also allows the filament to be much hotter, giving off more light. Since it is still essentially using the same process as the incandescent bulb, it becomes very hot.

LED (Light Emitting Diode) about 35 lm/W (for the most common ones)
It’s interesting to see commercials for flashlights that use LED’s and make them sound like the “new cool thing.” LED’s have been around for about fifty years. Even after electronics courses, it’s hard to explain LED’s in a “simple” manner. A diode is created (doped) with specific “impurities” that create a cathode (n) and an anode (p). As electrons cross the diode, they release energy as photons. The difference in the materials that form the diode influence the type of light that it emits. The first LED’s that emitted white light (and were thus useful for more than just saying: “hey I’m on” or “look here, something is broken”) were made in the 1990’s.

Those aren’t the only types of lights, but I’m sure most people didn’t bother to read this far anyway. Congrats on being one of the few!