Portable Optical Tweezers Kit
Chapter 7: Teaching Tips
Page 78
MTN024417-D02
Unfocused Laser Beam Total Momentum Change
In Figure 58 we observe that the total laser beam experiences a momentum change in the
opposite direction of its direction of incidence. Momentum conservation then demands a
momentum change of the bead in downward direction. This means that the bead is pushed
away by the beam, along the optical axis. The beam experiences no momentum change
orthogonal to the optical axis, so it stays in the center of the beam.
Unfocused Laser Beam, Particle is not on the Beam’s Main Axis:
In the following, we follow the same train of thought as above but with the particle outside
the center of the beam. This is depicted in Figure 59.
Unfocused Laser Beam Incident on a Polystyrene Beam Located Off the Beam
Axis
In this case, the two partial beams do not feature the same intensity. Therefore, they do
not have the same momentum, as shown in Figure 60.
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