-1
$\begingroup$

I heard in videos that if you use a laser pointer (Green if I recall) and point it on a ice cube, that it would cause it to start melting, is that true?

I know that light carries energy and momentum, so would that cause the ice to melt despite the energy a laser pointer has and the ice blocks’ specific heat point?

$\endgroup$
  • 1
    $\begingroup$ The laser beam transfers energy, so why wouldn't it? Some of the energy of the beam would be reflected, so angle that it hits the cube matters as does how clear or opaque the cube is. But energy will transfer. How much energy does the pointer put out? $\endgroup$ – zeta-band Nov 5 '18 at 19:41
  • $\begingroup$ I guess 1 watt from what i quickly searched. $\endgroup$ – C. Jordan Nov 5 '18 at 21:47
  • 1
    $\begingroup$ It would melt a lot faster if the water that made it was dirty, that would absorb more energy. If the cube was outside at below zero it may not melt. Heat going in has to be greater than heat going out. Even glass will absorb green laser energy and get warmer, although very slightly. $\endgroup$ – PhysicsDave Nov 5 '18 at 22:05
  • 1
    $\begingroup$ @C.Jordan so figure out the mass of the ice cube, the specific heat of water, the fusion energy to melt ice at 0C and assume you have 50% power retention, how fast will the temp change? $\endgroup$ – zeta-band Nov 5 '18 at 22:32
  • $\begingroup$ Okay, I will try that out, thanks guys $\endgroup$ – C. Jordan Jul 19 at 4:01
4
$\begingroup$

Yes, in the sense that you are providing extra energy. No, in the sense that you will not notice much difference.

A green laser pointer has power $P$ of about 100-300 mW. Ice has heat of fusion $H= 333$ J/g: you need 333 J to melt one gram. So if all the laser pointer energy went into melting ice, it would melt $P/H=$0.0003 to 0.0009 g/s. So for a one gram ice cube you would have to keep the laser on it for about 20 minutes before it melted just due to that energy. Clearly the heating from the environment is going to be more effective.

This also assumes 100% of the energy is absorbed by the cube. Ice absorbs about 2% of light per centimetre, so the ice cube would presumably just scatter the remaining 98% of the laser energy.

$\endgroup$
  • $\begingroup$ Thank you for the answer, this will help a lot. $\endgroup$ – C. Jordan Jul 19 at 4:00
1
$\begingroup$

Yes. Light releases heat; therefore, light would speed up the process of ice melting in a similar way that it melts when left in sunlight.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.