how do powerful lasers clean rust etc if lasers are just a form of light?
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Noticed a few misleading and not quite complete answers here. This is long, but hopefully helps
A few people have mentioned the sun. Yes, the sun is a source of light. And yes, this light can do several things. It has been mentioned that the sun can burn you, and can be used to set grass on fire. Both true. Sunlight is a form of electromagnetic radiation. Some of this radiation you can actually see…this is sunlight. There is also a part of this radiation that you can’t see… this is called “ultraviolet light” and this part of sunlight is what causes a sun burn. It is important to note that a sunburn (from UV light) is not the same as the heat given off from a hot stove or hot metal surface. You can also feel this heat…but this is infra-red radiation (IR) and has a different effect than UV. And yes, you can’t directly see IR radiation. Sunlight is also broad spectrum light…that is to say, it has many frequencies of light mixed together. UV light, and visible light are mixed in. You can see the different colours separated out when you see a rainbow, or use a block of glass called a prism. There is a lot of energy in sunlight, and if you focus it to a point (with a lens) you can concentrate the light in a small area, producing heat. This is how sunlight can be used to burn grass. Ok? Lasers are also sources of light…but the light is very special. The nature of this light lets it do many interesting things. Sunlight sort of spreads out in all directions at once…it’s very random and disorganized. Laser light is the opposite of this…it is all very organized and straight. Think of running a comb through the light to straighten it out. The other thing is that laser light is generally monochromatic. Unlike sunlight which has many wavelengths (all those rainbow colours…) lasers are usually only a single colour…red, green, blue. Different wavelengths can be used for different purposes…depending on the materials the light needs to travel through or touch.
This is why you see lasers that make dots on the wall…all the light is moving in tight, orderly beam. And the single wavelength is why the dot is one colour (and not white!)
This also means that all the energy of the laser can be focused into a very tight dot…which can be used to dump all the energy into a very small area. This is how a laser is used to clean things. The idea is that the laser makes a very big pulse of light, and it hits a very small area…and then all the energy is very quickly absorbed by the material. The amount of energy is VERY high and VERY fast, so only the very surface of the material gets hot and a quick puff of smoke and it’s vaporized and gone! The key here is HIGH and FAST. The laser light itself is not hot…it’s the reaction of the material with the energy of the light that creates the heat.
The laser pulses off and on super quick…so it looks like it is just a constant beam, but it is just a series of on/offs. This is also why a laser can be used to remove a tattoo from human skin. Same mechanism. Remember what I said about wavelengths? Certain wavelengths of light can pass through skin…but are absorbed by pigments. So, for tattoo removal, the light goes through the skin and is absorbed by the tattoo ink, gets really hot (really fast!) and <poof> no more tattoo.
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