Optical tweezers are a tool that use focused laser light to manipulate microscopic objects. Because cells have an optical density different than water, light bends as it passes through them. This...

Optical tweezers are a tool that use focused laser light to manipulate microscopic objects. Because cells have an optical density different than water, light bends as it passes through them. This results in a momentum change and an applied force. Unlike other manipulation methods, there is no risk of contamination, because the tool merely consists of photons carrying momentum. Determine the forces exerted by a typical laser beam (Figure 6.59) with a diameter of 1mm, power of 500 mW, and a wavelength of 1060 nm. The beam enters at a 45° angle from the horizontal and exits at a 78° angle from the horizontal.

(a) Using the equation pphoton=h/l, determine pphoton for a single photon passing through a cell. (Planck’s constant, h, is 6.626 10-34J.s.)

(b) Determine the number of photons passing through the optical tweezers beam every second, N, given the equation:

where P is the power of the beam, h is Planck’s constant, and f is frequency.

(c) Calculate the constant force exerted by the laser on the cell.