How are the properties of laser light used in laser eye surgery?
The most important property of laser light is that it can be precisely collimated into a parallel beam. What this enables is focusing of the beam at an extremely precise location in space, with subsequent divergence of the beam past the focal point. I can hear you grumbling about the relevance of this to laser eye surgery. Well, you see, if you can focus a high-intensity beam to a point in space, it will burn (or cut) at precisely that location, but not harm adjacent tissue since the energy of the beam is dispersed over a larger area than at the focal point. This allows the eye surgeon to precisely ablate the lens to correct astigmatisms and so forth.
Well, the other answer is right. Lasers can be well focused which allows them to be used to ablate precisely (though this is done to the cornea, not the lens, as the other person suggested). However, there are much more important aspects of lasers that make it suitable for laser eye surgery. First off, lasers are monochromatic (which is one of the reasons why they can be well focused).
But this is more important because it allows the surgeon to use a color of light that gets absorbed in the cornea without having colors of light that get transmitted by the cornea. This is important because you want all the light to get absorbed by the cornea and not get transmitted back to the retina, which would likely cause retinal burns, permanently blinding the patient.
The second, and by far the most important property of lasers that make them good for laser eye surgery is that they can be pulsed. With laser light, you can pulse the laser so that you get pulses of light with no light between the pulses (kind of like using a shutter to flash the light on and off). But the thing about lasers is that the when you do this correctly, the average power doesn't change. If you did this to a room light, you would simply get less total power, since some portion of the time, the light is off. But with a pulsed laser, you get MORE light during the pulse than you would get if you didn't pulse it. By making very short pulses, the total power in each pulse can be EXTREMELY high. In fact, the peak power in a pulse can be higher than the peak power of an entire electrical power plant. But the AVERAGE power remains fairly low. So why is having very high peak power useful? Well, as you start to heat something up, the heat transfers through the entire substance. So, if you heat something continuously, the entire object will heat up to that temperature, over time. If you wanted to get the cornea of the eye hot enough to remove material, you would have to heat the whole area up, which would end up cooking the person's eye (not good). But if you have very high peak power laser, the heating only occurs when the light is on, which is only for a tiny fraction of a second. During that time, there is an extreme amount of energy deposited in the target, but that energy doesn't have time to transfer to neighboring parts of the eye. So the target part of the eye becomes very hot, hot enough to remove material, before the rest of the eye heats up at all. And then the light turns off and the hot area cools off, without significantly heating the rest of the eye. This results in removed material without cooking the whole eye!
So, yes, lasers can be focused well, which makes them good for eye surgery. But they are also just one color (the eye surgery ones are infrared, which you can't see with your eye), which makes them easier to control and safer. And they can be pulsed, which makes the difference between successful surgery and a cooked eyeball.
