Researcher ORCID Identifier


Graduation Year


Document Type

Open Access Senior Thesis

Degree Name

Bachelor of Arts



Reader 1

Janet Sheung

Reader 2

Emily Wiley

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© 2023 Gabriella J Seifert


Photons carry momentum. When a tightly-focused beam of photons hit a particle, they transfer some of their momentum to the particle, exerting a force. Optical tweezers take advantage of this phenomenon to trap (or “tweeze”) a spherical bead just after the focus of a diverging laser beam, creating a potential well that pulls in beads. In this thesis, I predict the force exerted on trapped beads and measure the actual force using an optical tweezers setup that I built. To predict the force, I follow the path of all possible rays from a diverging beam incident on a spherical bead in an optical trap. I then sum up all force contributions from input reflection, input refraction, first output reflection, and first output refraction for each ray. This ray analysis in combination with the measured power level of the optical tweezers predicts a net force on the scale of 10^(-13) N exerted on the bead.

The actual force exerted can be calculated using the Equipartition Theorem, which relates the “stiffness” or spring constant of the trap to its temperature and the standard deviation of trapped beads’ position, which was determined by video analysis. The actual force exerted was also on the scale of 10^(-13) N, in good agreement with the predicted force.