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if Monochromatic rays of some intensity fall on a metal plate placed on a rough horizontal surface at certain angle. Will the plate move if the plate is perfectly reflecting irrespective of the value of intensity??

Carrick
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  • Please...I need an answer to this question. – Carrick Jan 14 '18 at 18:08
  • Irrespective of intensity? Do a thought experiment: reduce your ray to just one photon. What happens then? – Cosmas Zachos Jan 14 '18 at 19:15
  • @CosmasZachos are you trying to say that the energy transferred by the light to the plate depends only on the frequency of the photons(energy of photons) and not the number of photons striking the plate...right?? – Carrick Jan 14 '18 at 19:23
  • ? Is that what your thought experiment netted you? If you bounce a pellet off a plate, you'd get the same momentum transfer as when you bounce off zillions? – Cosmas Zachos Jan 14 '18 at 19:27
  • @CosmasZachos isn't that what I said lol?? I said the exact same thing I guess – Carrick Jan 14 '18 at 19:28
  • @CosmasZachos energy transferred by light is independent of its intensity – Carrick Jan 14 '18 at 19:38
  • I have no idea what you are saying. Perhaps you could rewrite your question to make it meaningful. Each photon transfers a given energy, per quantum, but if you wish to produce coherent motion, you need to multiply by their number (intensity). I hope you are not confusing all this with the (incoherent) photoelectric effect threshold issue. – Cosmas Zachos Jan 14 '18 at 21:12
  • @CosmasZachos let me make it clear...according to you if we increase the intensity of light then surely at one point, the plate will overcome the friction and start moving right?? but the answer to this ques. is "Plate will not move if plate is perfectly reflecting irrespective of the value of intensity." this question was asked in one my tests and I got it wrong – Carrick Jan 14 '18 at 21:21
  • @CosmasZachos and yh I was kind of confusing it with the photelectric effect – Carrick Jan 14 '18 at 21:22
  • Your answer is right, because the momentum transfer is always downwards, so, of cOurse the plate won't slide. That's why I was puzzled by the wildly irrelevant friction bit. The downward force, however, will increase with intensity. – Cosmas Zachos Jan 14 '18 at 21:56
  • Not in space though... – QuIcKmAtHs Jan 14 '18 at 22:02
  • @XcoderX how come will things differ in space...I would like to know – Carrick Jan 14 '18 at 22:09
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    In space, where it is a vacuum, shining a torchlight can cause you to move in the other direction, due to photons emitted. – QuIcKmAtHs Jan 14 '18 at 22:12
  • @XcoderX how is it relevant to the question? – Carrick Jan 14 '18 at 22:13
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    The light will obviously cause the plate to move in space. I was just drawing a parallel above. – QuIcKmAtHs Jan 14 '18 at 22:15
  • Probably useful reading: https://en.wikipedia.org/wiki/Solar_sail – Kyle Kanos Jan 24 '18 at 11:20
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    Possible duplicate of How does the solar sailing concept work? – Kyle Kanos Jan 24 '18 at 11:21

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Let's say the plate is in the x-y plane, and the incident rays are in the x-z plane. If the plate is perfectly reflecting, all the incident light will maintain its x-momentum after reflection. The z-component of the momentum is reversed, so the plate experiences a downward force due to the light. But no force in the x or y directions. The plate will not move.

Ben51
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