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Portable Optical Tweezers Kit Chapter 8: Teaching Tips
Page 74 MTN012639-D02
edge. Comparing a ray in the middle of the profile to a ray outside, the inner one shows
the maximal intensity while the outer one exhibits a lower intensity. This is demonstrated
by the thickness of the blue lines in Figure 54.
The entire laser beam is the sum of all rays. For simplicity, we will only consider two rays
in the following which are symmetrical around the beam axis. The insights we gain with
these two rays can be transferred to the entire beam.
First, we have a look at an unfocused (collimated) laser beam that is incident on a
polystyrene bead. We have to distinguish between two cases:

The center of the bead is on the beam’s main axis

The center o
f the bead is not on the beam’s main axis
Note: The size of the vector arrows in the following diagrams do not match from diagram
to diagram, and instead are enlarged where possible for clarity.
Unfocused Laser Beam, Particle on the B
eam’s Main Axis:
We consider two partial beams which are symmetrically arranged around the beam axis,
as shown in Figure 55. The beams are labeled
𝑠
and
𝑡
.
Unfocused Laser Beam Strikes a Polystyrene Bead Located on
the Beam Axis
Both partial beams have the same direction and intensity (since they both have the same
radial distance to the beam center with highest intensity) and carry the same momentum.
In Figure 56, the momentum vectors of the beams are depicted at some positions using