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Laser Pointers and Ice

Miss_Kitt

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I thought the fastest way to get a real answer to this question, including the follow-on ramifications, was to post here and let the resident science guys get to work.

Suppose you wanted to look at a small opening sealed with ice--would using a laser pointer cause melting? Are some colors more likely than others to cause this? What distance(s) are likely to cause melting?

I lack the financial resources to just buy a cartload of laser pointers in various strengths and colors and experiment away. And I know that lasers are subject to variable performance due to humidity and temperature--is this from the equipment, or the scattering of light in the air under different air conditions?


Thanks in advance, Miss_Kitt
 
It takes a lot of energy to melt ice; heat of fusion of water is high. That said, one can do it in theory with a powerful laser and enough time..

I suggest an infrared laser as most efficient. Transparent to visible means ice tends to not absorb in the visible very well.

Is this a D and D question?
 
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Giordano said:
I suggest an infrared laser as most efficient. Transparent to visible means ice tends to not absorb in the visible very well.
Click to expand...

Yep. What are the edges of the opening made of? If they're something dense and heat-conductive (aluminum), they'll be conducting heat away from the ice, making it harder to warm it up. If they're made of something light-absorbing and low-density (wood, black plastic?) you can get around the "ice is transparent to visible light" problem by aiming the laser at the material adjacent to the hole, warming that up, and letting the ice warm up by conduction.
 
It's actually a question for a work of fiction. I like the "warm up the surrounding" aspect, but would that work with dark stone??

And thank you!
 
Yes but.........:)
Typical laser pointers output a few milliwatts and ice takes a lot of energy to melt.

Take a 5wM laser (the highest power output allowed to be sold as a pointing device in the USA) and one gramme (~1.1cm3) of ice at 0°C.
It would take:

334J/g (Enthalpy of Fusion of water) x 1g ÷ 0.005J/s = 66,800 seconds (~eighteen and a half hours)
​
to melt that one gramme of ice.

Even a 250mW laser would take 22 minutes per gramme.

Given that diode lasers are typically ~20% efficient, placing the laser in contact with the ice and relying on waste heat would be faster.
 
Miss_Kitt said:
I like the "warm up the surrounding" aspect, but would that work with dark stone??
Click to expand...
It would be particularly effective with dark stone, as darker stone will absorb more energy than lighter stone which will reflect more.
 
Yes, it takes a lot to melt ice. Maybe a torch would be more suitable.:) A laser capable of putting a significant dent in ice would be very dangerous to the eyes, and probably to the skin.

As kids, we used railroad flares to cut huge icicles hanging off a railroad tunnel, just to see them crash onto the ground below. It took forever to cut them by melting.

There are videos circulating around showing an attempt to melt snow using a lighter or a torch. The liquid water is absorbed by the snow or vaporized so there is no actual liquid water coming off. Another video showed the same experiment using ice, and there was water as it melted.


The explanation was that snow contains much less water than ice, and is easily absorbed back into the mass of snow.

ETA:: Just occurred to me that maybe one of those little "lock deicers" would work. I think they contain a pressurized liquid like perhaps methanol that is squirted into the lock to melt the ice.
 
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Olowkow said:
Yes, it takes a lot to melt ice. Maybe a torch would be more suitable.:)
Click to expand...

Just recently there was a show on either the Science or Discovery Channel where they took a large block of ice and asked people how long they thought it would take to melt the block using a large blow torch. (The flame on the torch was about two feet long.) Most people said about 15 to 20 minutes. After 40 minutes of torching the block, about 90% of it was still there.

Steve S
 
Miss_Kitt said:
I thought the fastest way to get a real answer to this question, including the follow-on ramifications, was to post here and let the resident science guys get to work.

Suppose you wanted to look at a small opening sealed with ice--would using a laser pointer cause melting? Are some colors more likely than others to cause this? What distance(s) are likely to cause melting?

I lack the financial resources to just buy a cartload of laser pointers in various strengths and colors and experiment away. And I know that lasers are subject to variable performance due to humidity and temperature--is this from the equipment, or the scattering of light in the air under different air conditions?


Thanks in advance, Miss_Kitt
Click to expand...

Laser light is not special in any way, except it is very well aligned. This means that you can focus it very precisely.

As ice is transparant, it does not have very good absorption of light, thus, it is hard to melt it with (even well focused) light.

According to this: http://www.atmos.washington.edu/~sgw/PAPERS/1984_Icemcx.pdf

You would want to look into ultraviolet.

Now, in a later post, you say this is for a work of fiction. With all respect for research and credibility of a work of fiction, I think you can get away with saying it is an ultraviolet laser. Such are in fact available.

http://www.wickedlasers.com/laser-tech/uv_lasers.html

Hans
 
Laser pointers are very low power - the reason the spot of light they produce is bright is that all the power is concentrated into a small area. For melting ice, a laser pointer would be pretty useless.

A laser that was powerful enough to melt ice in a reasonable time would be a dangerous thing to have around. Powerful lasers are used to cut wood, plastic or metal sheets and one of those would probably make short work of the ice, but I wouldn't want to be anywhere in visual range while it was happening.
 
Two more problems using lasers to melt ice are steam generation (if the laser was powerful enough). The cloud of steam scatters the beam.
And , of course, the inverse square law.
 
Inverse square law.

For non isotropic radiators such as parabolic antennas, headlights, and lasers, the effective origin is located far behind the beam aperture. If you are close to the origin, you don't have to go far to double the radius, so the signal drops quickly. When you are far from the origin and still have a strong new signal, like with a laser, you have to travel very far to double the radius and reduce the signal. This means you have a stronger signal or have antenna gain in the direction of the narrow beam relative to a wide beam in all directions of an isotropic antenna.
Click to expand...
 
Aside from various objections already raised, I think a problem with a very low powered laser source might be that it does not heat anything up fast enough to overcome the dissipating effect of surrounding air.

I happen to have right at hand a cheap but operable laser level. This is basically a bright laser pointer in a bar with a bubble on it. It's a bit more powerful than most little laser pointers, sufficiently that with a mirror attachment it make a long bright line on a wall instead of a bright spot. It's bright enough to illuminate a spot quite far in the distance. On a clear night, you can see it illuminate a spot 500 feet or more away. It carries a warning not to look at it directly, but if I aim it at my hand, I feel nothing. The batteries are hard to get out, or I'd sacrifice a couple and aim it at some ice for the duration of a pair. I don't think it's worth the bother.

If you're going to melt ice with a laser, I think you'll need much much more power than what a pointer delivers.
 
ceptimus said:
A laser that was powerful enough to melt ice in a reasonable time would be a dangerous thing to have around. Powerful lasers are used to cut wood, plastic or metal sheets and one of those would probably make short work of the ice, but I wouldn't want to be anywhere in visual range while it was happening.
Click to expand...

Amen to that. Having seen what 5 watts of green light will do . . .
 
Soapy Sam said:
Two more problems using lasers to melt ice are steam generation (if the laser was powerful enough). The cloud of steam scatters the beam.
And , of course, the inverse square law.
Click to expand...
And the reflection from the ice dissipating the beam.

John Jones said:
Agreed, except they don't sell infrared laser pointers.
Click to expand...
Well some of the crappy diodes found in cheap points put out fair bit of IR in addition to the visible component.
 
I seem to recall some home-brew projects using the lasers in DVD-R drives, but I don't remember the details. I think you can cob up something with a little more zing than a pointer, though. Maybe enough to pop the right color balloon.
 
Miss_Kitt said:
I thought the fastest way to get a real answer to this question, including the follow-on ramifications, was to post here and let the resident science guys get to work.

Suppose you wanted to look at a small opening sealed with ice--would using a laser pointer cause melting? Are some colors more likely than others to cause this? What distance(s) are likely to cause melting?

I lack the financial resources to just buy a cartload of laser pointers in various strengths and colors and experiment away. And I know that lasers are subject to variable performance due to humidity and temperature--is this from the equipment, or the scattering of light in the air under different air conditions?


Thanks in advance, Miss_Kitt
Click to expand...

This is not the place to ask such a question. Better to ask at Photon lexicon or Laser Pointer Forums.
 
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