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Homework Help: Problem with electromagnetic waves

  1. Mar 14, 2005 #1
    Lasers have been used to suspend spherical glass beads in the Earth's gravitational field. a) A black bead has a mass of 1 micrograms and a density of 0.200 g/cm^3. Determine the radiation intensity needed to support the bead. b) If the beam has a radius of 0.200 cm, what is the power required for this laser.

    I know that density = mass/volume, so I can get the volume from the density and mass. I know that the energy density = energy/volume. Also, I know that intensity = speed of light*energy density. How do I get the energy or am I doing this wrong? Please help...any help would be great! thx! :)
  2. jcsd
  3. Mar 14, 2005 #2


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    Well, the magnitude force needed to suspend the micro-ball must = mg.

    The magnitude of the force is related to Pressure x area, but remember this is a curved surface, and one needs the force operating anti-parallel to gravity.

    In another of your posts, there is a discussion of radiation (light) pressure. The pressure is simply due to momentum transfer. Consider the relationship between energy and momentum for light.
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