Why do lasers melt stuff?

Ok, here's my theory, feel free to discuss:

Lasers melt stuff because photons are shot onto a surface, which has its particles accelerated/excitated by the photons, which creates friction and thus giving heat to that area, which eventually creates melting on the material.

This isn't really laser-specific, but here's an article on Wikipedia that explains "black bodies" and black body radiation. It's essentially the reason that lasers heat stuff up, and hence melt stuff (if you read the article, you should also understand why white matches are harder to light than red or blacked out matches with a green laser pointer)

http://en.wikipedia.org/wiki/Black_body

I know about light absorption by reflection of wavelengths. I'll read it anyway (maybe edit this later). Thanks

Another way to look at is that whatever substance the laser photons are hitting is unable to dissipate the energy of the photons. Technically photons don't have a temperature - a photon of light from a distant star could go whizzing thru the vacuum of space forever and be undetectable unless it strikes something.

This is also the same with incoherent radiation sources - you put something plastic too close to a lit fireplace or flame and it will melt.....more so if it's black or dark colored.

An object can either reflect electromagnetic radiation or dissipate it - or both. For instance, I can melt thru ordinary glass with my CO2 laser, which absorbs the radiation. But when I tried it on a piece of high temperature ceramic glass (a piece left over from the door of my old wood stove, makes a decent beam stop now) nothing happened. The glass still absorbed the beam's energy but was able to dissipate it.

nikokapo said:

Ok, here's my theory, feel free to discuss:

Lasers melt stuff because photons are shot onto a surface, which has its particles accelerated/excitated by the photons, which creates friction and thus giving heat to that area, which eventually creates melting on the material.

Click to expand...

I think you're right, minus the friction part.Heat is transfered to the object precisely through moving it's particles.How fast the particles are moving determines the temperature of an object(you know, brownian motion).There is no friction involved.

Schrecken are you sure that the colour of an object placed near a flame matters? :-/ I thought that it melts because it gets heated up by the hot air around the flame, not because of direct radiation.I mean, does "black" really absorb more than "white" when we're talking about >8000nm into IR? :-/

Well, that's my theory anyway, will someone please correct it if he can.

I was thinking about this, and I remembered something called a Feynman Diagram (which, incidentally, has nothing to do with this question, but started me on a tangential train of thought), and thought about what photons actually are.

Photons are Bosons, or messenger particles. Messenger particles carry information, and photons are the electromagnetic messenger particles. Photons are created by electrons moving from a higher energy level to a lower energy level, and are absorbed by an electron moving from a lower energy level to a higher energy level. The higher energy level means that the electron moves faster, hence a higher temperature.

If you also consider now that any given particle is affected to a greater or lesser extent by certain wavelengths of light, then you start to see why different colors affect absorption, and hence movement and temperature. (Remember that we can't see outside of the visible spectrum, hence its name "the VISIBLE spectrum", so a white or black object may or may not react to wavelengths outside of the visible spectrum)

If I missed something, feel free to add or correct.

Switch said:

[quote author=nikokapo link=1212689008/0#0 date=1212689007]Ok, here's my theory, feel free to discuss:

Lasers melt stuff because photons are shot onto a surface, which has its particles accelerated/excitated by the photons, which creates friction and thus giving heat to that area, which eventually creates melting on the material.

Click to expand...

I think you're right, minus the friction part.Heat is transfered to the object precisely through moving it's particles.How fast the particles are moving determines the temperature of an object(you know, brownian motion).There is no friction involved.

Schrecken are you sure that the colour of an object placed near a flame matters? :-/ I thought that it melts because it gets heated up by the hot air around the flame, not because of direct radiation.I mean, does "black" really absorb more than "white" when we're talking about >8000nm into IR? :-/

Well, that's my theory anyway, will someone please correct it if he can.[/quote]

I just know that when I go to load my wood stove there's a difference if I'm wearing a white t-shirt or a black (or other dark color) one. The dark fabrics seem to pick up a lot more heat than light colors.

That would make sense to me as flames are emitting visible (mostly orange and red light) radiation as well as both near and far IR. Visible red light and near-IR will of course heat dark objects more than light colored ones. But of course with mid-far IR color really doesn't matter, as with a CO2 laser.

But as air is a poor conductor of heat, I'd imagine that something melting by being close to a fire or an electric resistance (the kind w/o a fan) heater would in large part be due to both visible and IR radiation.

Perhaps someone might have a way to test this idea, but it's hot as I don't know what here so I don't want to be firing up any heaters to try and prove it!

niko, what exactly are you looking for here? If you understand absorption, you already know the answer to your question. There are different ways to look at it though. Are you looking for a quantum mechanics explanation of how the photons cause the temp to rise? Personally, I always liked to think of it purely in terms of energy (conservation of energy and absorption explain it wonderfully). Friction ain't really the right way to go about it though. Its not something that really needs theorizing though unless you want to get real hairy with the quantum mechanics of it all - this part of science is pretty solid.

pseudonomen137 said:

niko, what exactly are you looking for here? If you understand absorption, you already know the answer to your question. There are different ways to look at it though. Are you looking for a quantum mechanics explanation of how the photons cause the temp to rise? Personally, I always liked to think of it purely in terms of energy (conservation of energy and absorption explain it wonderfully). Friction ain't really the right way to go about it though. Its not something that really needs theorizing though unless you want to get real hairy with the quantum mechanics of it all - this part of science is pretty solid.

Click to expand...

I dont know much about physics, so i try to guess (rationally) my own theories about stuff. so i wanted to let you know what i thought to see if i was wrong.

if it's not friction, what is it then?

(i read thru the whole article on black bodies and it doesnt really give me the answer)

It's not friction that makes things melt or burn from laser light or any focused light for that matter. Photons are quantized units of energy basically. Photons of a given wavelength have a certain energy associated. This energy is given by E = h*c/lambda where h is Planck's constant. This is why 405nm lasers are better at burning and melting than red lasers. Each photon has a higher energy. When these photons interact with the atoms of a material they transfer energy sometimes. The properties of the individual atoms determine how much of the energy is absorbed, how much is reflected, and how much just goes right through for a given wavelength. Things that appear black for example look black because they absorb most of the light that hits them; the opposite is true for objects that appear white. That's why a black match will be much easier to light than a white one. The material properties also determine whether is melts or burns. Wood for example burns, whereas plastic melts.

climbak said:

Photons of a given wavelength have a certain energy associated. This energy is given by E = h*c/lambda where h is Planck's constant. This is why 405nm lasers are better at burning and melting than red lasers. Each photon has a higher energy. When these photons interact with the atoms of a material they transfer energy sometimes. The material properties also determine whether is melts or burns. Wood for example burns, whereas plastic melts.

Click to expand...

You did it again. I knew you'd jump in and give me the answer I was looking for.


I just can't thank you enough

No problem. Glad I could help. I'd answer sooner, but I've just been busy lately with my two internships and stuff.

climbak said:

It's not friction that makes things melt or burn from laser light or any focused light for that matter. Photons are quantized units of energy basically. Photons of a given wavelength have a certain energy associated. This energy is given by E = h*c/lambda where h is Planck's constant. This is why 405nm lasers are better at burning and melting than red lasers. Each photon has a higher energy. When these photons interact with the atoms of a material they transfer energy sometimes. The properties of the individual atoms determine how much of the energy is absorbed, how much is reflected, and how much just goes right through for a given wavelength. Things that appear black for example look black because they absorb most of the light that hits them; the opposite is true for objects that appear white. That's why a black match will be much easier to light than a white one. The material properties also determine whether is melts or burns. Wood for example burns, whereas plastic melts.

Click to expand...

Hate to do this, but just a quick clarification. mW for mW, 405nm lasers are better for burning in many instances, but it isn't directly because each photon has more energy. mW is mW. A 100mW 405nm has the same energy per second as a 100mW 10600nm. Each photon of the 405nm may be more energetic, but it also has a lot less of them than the equally powerful 10600nm. UV is a good choice for burning for several reasons, but two of the most significant are its wavelength and absorbance.

The shorter your wavelength, the better your beam specs can be. That both means that the best collimated UV laser will be more intense (more power per area) than the best collimated greenie, and a focused UV laser can attain tighter spots than a focused greenie. To give you an idea of the importance: at 660nm for a DVD diode versus 405nm for a blu-ray, with the same divergence the best red would still have a spot diameter 660/405 times larger than the best blu-ray. Square that, and you see that the blu-ray can pack its 100mW into an area ~37.65% smaller than the red. Therefore, even though both lasers pack the same punch, the near-UV can pack it nearly 3 times more intensely thus giving you more burning power per mW.

UV is also commonly used for industrial and scientific burning purposes because it is absorbed very well by a variety of things.

pseudonomen137 said:

[quote author=climbak link=1212689008/0#9 date=1212882787]It's not friction that makes things melt or burn from laser light or any focused light for that matter. Photons are quantized units of energy basically. Photons of a given wavelength have a certain energy associated. This energy is given by E = h*c/lambda where h is Planck's constant. This is why 405nm lasers are better at burning and melting than red lasers. Each photon has a higher energy. When these photons interact with the atoms of a material they transfer energy sometimes. The properties of the individual atoms determine how much of the energy is absorbed, how much is reflected, and how much just goes right through for a given wavelength. Things that appear black for example look black because they absorb most of the light that hits them; the opposite is true for objects that appear white. That's why a black match will be much easier to light than a white one. The material properties also determine whether is melts or burns. Wood for example burns, whereas plastic melts.

Click to expand...

Hate to do this, but just a quick clarification. mW for mW, 405nm lasers are better for burning in many instances, but it isn't directly because each photon has more energy. mW is mW. A 100mW 405nm has the same energy per second as a 100mW 10600nm. Each photon of the 405nm may be more energetic, but it also has a lot less of them than the equally powerful 10600nm. UV is a good choice for burning for several reasons, but two of the most significant are its wavelength and absorbance.

The shorter your wavelength, the better your beam specs can be. That both means that the best collimated UV laser will be more intense (more power per area) than the best collimated greenie, and a focused UV laser can attain tighter spots than a focused greenie. To give you an idea of the importance: at 660nm for a DVD diode versus 405nm for a blu-ray, with the same divergence the best red would still have a spot diameter 660/405 times larger than the best blu-ray. Square that, and you see that the blu-ray can pack its 100mW into an area ~37.65% smaller than the red. Therefore, even though both lasers pack the same punch, the near-UV can pack it nearly 3 times more intensely thus giving you more burning power per mW.

UV is also commonly used for industrial and scientific burning purposes because it is absorbed very well by a variety of things.[/quote]
Just furthering that point, climbak gave us the equation E = h*c/lambda. Energy (E) is measured in joules, and wattage is joules/sec, so as psuedo said, a 100mW red has the same power as a 100mW violet.

Good clarification. Thanks

mikewitt said:

[quote author=pseudonomen137 link=1212689008/12#12 date=1212934470][quote author=climbak link=1212689008/0#9 date=1212882787]It's not friction that makes things melt or burn from laser light or any focused light for that matter. Photons are quantized units of energy basically. Photons of a given wavelength have a certain energy associated. This energy is given by E = h*c/lambda where h is Planck's constant. This is why 405nm lasers are better at burning and melting than red lasers. Each photon has a higher energy. When these photons interact with the atoms of a material they transfer energy sometimes. The properties of the individual atoms determine how much of the energy is absorbed, how much is reflected, and how much just goes right through for a given wavelength. Things that appear black for example look black because they absorb most of the light that hits them; the opposite is true for objects that appear white. That's why a black match will be much easier to light than a white one. The material properties also determine whether is melts or burns. Wood for example burns, whereas plastic melts.

Click to expand...

Hate to do this, but just a quick clarification. mW for mW, 405nm lasers are better for burning in many instances, but it isn't directly because each photon has more energy. mW is mW. A 100mW 405nm has the same energy per second as a 100mW 10600nm. Each photon of the 405nm may be more energetic, but it also has a lot less of them than the equally powerful 10600nm. UV is a good choice for burning for several reasons, but two of the most significant are its wavelength and absorbance.

The shorter your wavelength, the better your beam specs can be. That both means that the best collimated UV laser will be more intense (more power per area) than the best collimated greenie, and a focused UV laser can attain tighter spots than a focused greenie. To give you an idea of the importance: at 660nm for a DVD diode versus 405nm for a blu-ray, with the same divergence the best red would still have a spot diameter 660/405 times larger than the best blu-ray. Square that, and you see that the blu-ray can pack its 100mW into an area ~37.65% smaller than the red. Therefore, even though both lasers pack the same punch, the near-UV can pack it nearly 3 times more intensely thus giving you more burning power per mW.

UV is also commonly used for industrial and scientific burning purposes because it is absorbed very well by a variety of things.[/quote]
Just furthering that point, climbak gave us the equation E = h*c/lambda. Energy (E) is measured in joules, and wattage is joules/sec, so as psuedo said, a 100mW red has the same power as a 100mW violet.[/quote]
So technically, a 100mW red has the same power as a 100mW violet, just that the violet is able to transfer energy faster (because the wave is shorter), hence it has more energy and is better at burning.