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In practice, optical tweezers are very expensive, custom-built instruments. These instruments
usually start with a commercial optical microscope but add extensive modifications. In addition,
the capability to couple multiple lasers into the microscope poses another challenge. High
power infrared laser beams are often used to achieve high trapping stiffness with minimal
photo-damage to biological samples. Precise steering of the optical trap is accomplished with
lenses, mirrors, and acousto/electro-optical devices that can be controlled via computer.
Laser Tweezers and Transcription. A transcription complex is tethered between
two streptavidin-coated beads and kept in a continuous buffer flow. As a
transcribing polymerase moves along the DNA, it physically pulls the two beads
closer together. One bead is held in place with laser tweezers (red funnel) and
the other by a pipette. The separation of the beads is measured by video
microscopy and used to determine the end-to-end distance of the DNA.
If some external object, like a molecular motor, were to pull the bead away from
the center of the trap, a restoring force would be imparted to the bead and thus to
the motor. An example trace of a single kinesin motor taking 8 nm steps against a
5-pN force is shown
Group Problem on Polarization
• A vertically polarized light beam is incident
on two ideal polarizers P1 and P2
following one another with P1 oriented at
45o to the vertical and P2 oriented
horizontally. If the incident intensity is 1
unit, what is the transmitted intensity after
P2?
• What happens in the same situation if the
incident light of 1 unit is unpolarized?
4 x 10-11
4 x 10-7
3 x 104
Long waves
102
104
4 x 10-3
3 x 10-4
3
Radio waves
106
108
4 x 101
3 x 10-8
Infrared
1010
1012
4 x 105
1014
Ultraviolet
1016
Visible Light
Energy (eV)
3 x 10-12
X-rays
1018
Wavelength (m)
Gamma rays
1020
1022
Frequency (Hz)
actin (red), vinculin (an adhesion plaque protein;
green), and the nucleus (blue)