How a beam can hold something in place
Light carries momentum, so it can push on matter. In a tightly focused beam, though, the force is not just a shove: the intensity is greatest at the focus, and the changing intensity across the beam creates a gradient force. For a tiny object whose refractive index is higher than the surrounding liquid, that force pulls it toward the brightest region. The beam’s forward push works against this pull. With the right focus and intensity, the forces balance and the object stays suspended near one point.
The balance is delicate, not a rigid clamp. If the trapped particle shifts slightly, the beam’s uneven intensity pushes it back toward the focus. Researchers can move the trap by steering the beam, carrying the particle through liquid without touching it with a solid tool.
From cells to molecular motors
For bacteria and other small objects in fluid, optical tweezers let researchers reposition single cells or measure tiny forces. A particle can be attached to a molecular motor—a protein that converts chemical energy into motion—and held in the beam while the motor tugs on it. Its movement reveals how much force the motor produces.
Atoms can also be trapped by light, but the details differ: an atom’s interaction with the light field supplies the trapping force, rather than the simple refractive-particle picture. In both cases, the practical achievement is similar: light makes a movable, contact-free handle small enough to work on individual microscopic objects.
