Low-cost pendulum rig delivers sensitivies to observe radiation pressure for students

The concept of massless photons exerting a force continues to puzzle many physics students when learning about radiation pressure. Historically, weak forces have been measured using a torsion pendulum, including Cavendish’s measurement of gravitational attraction, as well as the original measurements of radiation pressure.

Researchers developed a rig in which laser diodes five milliwatts in power push on a torsion pendulum, providing an intuitive yet quantitative measurement of the optical force resulting from radiation pressure. They created homemade reflectors on a commercial balance that mitigate forces generated by convective forces as the reflectors heat up. By replacing the traditional lead masses from the Cavendish experiment with these new reflectors, the device by Russell et al. can capture radiation pressure with forces less 10 piconewtons.

“We wanted to see if we could measure the force due to an everyday light source, like a laser pointer, in the undergraduate lab, and we wanted to do so in a way that would be relatively straightforward to both implement and understand,” said author Jennifer Heath.

The response of the pendulum can be observed in a short enough time series to be used in a single lab session. In a longer, 9-hour experimental run, the group observed two piconewtons of white noise, and a discrete Fourier transform demonstrated multiple frequency components from the square-wave driving force.

The low-cost reflector design minimizes competing forces due to heat, such that this delicate measurement can be made in air, without requiring a vacuum system.

The group hopes this approach provides a compelling platform for further student-driven projects and stokes more interest in finding affordable solutions for experimental learning.

Source: “Measuring optical force with a torsion pendulum: A platform for independent student experimentation,” by Leland Russell, Ezekiel A. Rein, Anatalya Piatigorsky, and Jennifer T. Heath, American Journal of Physics (2025). The article can be accessed at https://doi.org/10.1119/5.0268585 .