# Homework Help: Problem with electromagnetic waves

1. Mar 14, 2005

### andrew410

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. Mar 14, 2005

### Astronuc

Staff Emeritus
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.