why do lasers have colors and how can they have colors?


the title basically says it all but the point is what change when they have color? does this make them more powerful?
and one of my friends told me that green lasers were more powerful than other colored lasers. is this true?
and what is the component that makes it have a certain wavelenght, so a certain color?

I want to thank you all for your answers and you have all helped me a lot understanding lots of things about laser.
Once again, thanks everyone :)

iceng6 years ago
More Important Why or How Do You Perceive  COLOR  With Your Eyes.

Human color vision come in three types of cones as Short, Medium, and
Long sensitivity to wavelengths of light.  See the retinal layers below.

The laser is an optical oscillator of a particular wavelength which stimulates
a variety of your cones and your brain tells you it matches other memories
which you call a color.... . . . . A
The color of light
http://en.wikipedia.org/wiki/Color#Physics
is a matter of frequency, i.e. how fast the electric (and magnetic) fields associated with the light are changing.  Its also a matter of wavelength, the distance in space between places where the wave repeats itself.  Moreover those two numbers, frequency and wavelength, are related to each other by the speed at which the wave travels according to the familiar formula f*λ = c, where f is frequency, λ is wavelength, and c is the speed at which the wave propagates.

Actually I could tell a similar story about sound. Sound has frequency, wavelength, and a speed at which it moves through various materials.

Part of the reason I mention sound, is because I can make an analogy between light and sound, and I claim that the light produced by a laser is like the sound produced by a whistle, or a tuning fork.  It's mostly just one pure tone, just one frequency. In contrast, the light produced by the sun, or an incandescent lamp, is like the sound of rushing air. It's a wide distribution of frequencies, all jumbled on top of one another.

So the goal in making a laser is sort of the same goal as making a whistle. You want all, or most, of the emitted power to be at just one tone, just one frequency.

And you might wonder what is the point of building such "pure tone" light sources, and the answer to this is that different frequencies of light interact with matter in different ways.

For example, probably the most popular and widespread application for lasers is for reading optical discs, e.g. CDs, DVDs, Blu-ray(tm) discs, etc.  And the importance of color to optical disks, is the relationship between the wavelength of the laser, and the smallest sptatial detail on the disc that can be resolved. The pits on a DVD are smaller, and spaced closer together, than are the pits on a CD, and as a result you can fit more pits, and thus more bits of data on a DVD. But you need a shorter wavelength of light to "see" those smaller pits. In the case of DVD vs CD, it's a red vs a infrared laser. In the case of Blu-ray vs DVD, it's a blue vs a red laser.

Which makes you wonder why they didn't just invent Blu-ray first?  It seems like that would have saved everybody a lot of trouble.  But the legend is that somehow blue-lasers were technically more difficult to produce than infrared and red.  More cynical observers will say it was just so they could sell you the inferior tech first, then make you pay again, and again, to upgrade to the superior tech as it comes out.

BTW, the technical details are kinda crazy.  As other answerers have pointed out, the way different colors of lasers are made is via some very arcane materials science.
http://upload.wikimedia.org/wikipedia/commons/4/48/Commercial_laser_lines.svg
And for all of these the actual laser action is related to transitions between different electron orbitals, and the amount of energy associated with those transitions.  Which reminds me, now might be a good time to mention that in quantum mechanics, the energy and frequecncy of a photon of light are related by E = h*f = h*c/λ So that's how frequency is tied to energy in these quantum mechanical transitions.

Regarding your question about green light vs red light. I can think of two reasons why green light might be said to be "more powerful" than red light.  The first is that green light has more energy per photon, since the frequency is greater, and E=h*f.  The second reason is that the human eye is more sensitive to red than green.  If you have two lasers, red and green, with the same intensity, ie. power output in mW/cm2, the green laser will appear much brighter, because the eye is more sensitive to green, i.e. sees green more easily than red.

Back to the subject of how lasers are made, like I was saying it's kind of some arcane dark magic.  I wish there was sort of an easy analogy between the lasers are made and the way whistles and tuning forks are made. To make a whistles or a tuning forks produce a different tone, its usually just a matter of making it bigger or smaller.  Small whistles produce a short sound waves and a high pitched tone. Big whistles produce a longer waves and low pitched tone.

It makes you wonder why light cannot be produced in a similar way, and maybe it can? Maybe by engineering nano-sized cavities, or electron traps, or something.
http://en.wikipedia.org/wiki/Quantum_dot

Also there are free-electron lasers. I'm not sure exactly how they work, but I know they don't rely on the idiosyncrasies of exotic materials, certain atoms and their electron orbitals, because there are no orbitals involved. It's just a big beam of electrons in a vacuum.
http://en.wikipedia.org/wiki/Free-electron_laser
The colour comes from the exact method used to create the laser.
No, there is no magic about green lasers over anyother type.

The colour determining "element" depends on the laser type. In a semi-conducting laser, its the material the junction is made from, in a typical green laser, it's a special crystal which is "pumped" by an infrared laser.
sshuggi6 years ago
It depends on the material. You can think of it like regular lights. Those halogen headlights give off a very crisp, white light. Regular incandescents are a yellow color. Most florescent lights are a cooler, blueish color. Here's a chart from Wikipedia that show the material, wavelength, and intensity. As you can see, most of them are about the same, not just green. In fact, the most intense one is CO2 far in the mid-infrared. You couldn't even see that.