laser pointer

the laser pointer comes in pen and keychain varieties. They are generally class III regulated, which means they have a power output of 5 milliwatts nominal (I like saying nominal). The beam wavelength is measured in nanometers .. the smallest (and brightest) measurement I've seen is 645nm on a keychain laser. (as opposed to 1-2nm of cutting lasers)

Green laser pointers are also available. The one I own is a 532nm, 5 milliwatt device. Laser diodes of this wavelength are not currently available, so in the case of my laser pointer, a 808nm (IR) laser diode is used. The laser diode pumps a Neodymium doped yttrium orthovanadate (Nd:YVO₄) crystal. This results in the emission of 1064nm radiation. This in turn passes through a frequency doubler crystal, halving the wavelength to 532nm.

Enough mumbo jumbo! What does this mean ?

They don't actually have green laser diodes, but they use clever stuff so that you can still have a cool green laser pointer, instead of that boring red pointer your mate has. Green laser pointers are significantly more expensive than their red counterparts (Thinkgeek sells one for around $150, their red one sells for $20). At equivalent power ratings, green laser pointers appear much brighter than red ones. A nice side effect of this is that when using a green laser pointer in relative darkness the actual beam is visible (this isn't the case for red ones). This means that you can use it to point at objects far beyond the range of the pointer (constellations for example). And it's generally quite neat to switch on your pointer and see this green beam stretch out.

Aren't those things dangerous ? Especially those super bright green ones ?

At the end of the day 1mW of power is 1mW of power, whether it is green, red, IR or blue radiation. However your eye's natural preservation reflexes don't work nearly as well on wavelengths you can't see. Some wavelengths may be absorbed by the lens or the cornea and therefore damage those instead of your retina. In some ways a green laser pointer is less dangerous than a red one, since it appears brighter, people may be less likely to stare into the beam.

Laser pointers of course use visible light so if the beam shines in your eye, your eye will very quickly narrow the pupil, cause you to blink and tell you to stop pointing a laser beam in your eye you freak!

Permanent damage would require looking straight into the beam at close range for several seconds, without blinking or turning away. This is unlike to happen by accident both because of your body's reflexes and because the pupil of someone's eye is a small target to hit at any distance. Try fixing a 2mm dot from several metres with your laser pointer for more than a fraction of a second. Stories of people having their eyes burned out by a laser pointer the other side of a football stadium are fake.
Even prolonged exposure is unlikely to cause total blindness. The laser beam, by its very nature will be focused on a small area of your retina, and only damage that part, resulting in a blind spot or slightly reduced field of vision. This means that if you stare into a laser beam and can still see fine, firstly you are an idiot, secondly if you hardly noticed any effects and look into the beam again you are an even bigger idiot. Damage may not be apparent and your retina can't feel pain.

Laser pointers can still be dangerous because even brief exposure will dazzle you. Don't shine a laser pointer in your eye while driving or operating heavy machinery.

Bottom line is use your common sense: don't point it at your eyes or anyone else's.

Sources:http://www.thinkgeek.com/stuff/gadgets/5a47.shtml
http://www.repairfaq.org/sam/lasersam.htm (massive amounts of info on all things laser)