Unwind a chromosome to see how it’s put together? Sort cells with a light beam? Make a model of a molecular motor? All these and more—welcome to the world of optical tweezers, where cells and even individual molecules are manipulated with laser light.

How does it work? When light passes through an object, the light refracts, that is, it changes direction. It does this as it enters the object, as it passes from one substance to another inside, and as it exits. Photons, the quanta of light, carry momentum, and the light’s momentum is changed by being bent as it passes through the object. To conserve the total momentum, the object itself acquires momentum equal to that lost by the photons, and this momentum can be used to move the object into a trap in the optical system.

To make a trap, a laser beam is set up with an intensity that diminishes moving out from the center of the beam, as shown in the diagram. Lenses bring the beam to a focus, the point of maximum light intensity.

The next diagram shows a translucent sphere placed in the laser beam between the lens and the focus. The solid and dotted line show the paths of a typical pair of photons that pass through the object. Because the two photons wind up going more sideways than initially, they lose momentum in the downward direction. To conserve momentum, the object acquires momentum in the downward direction, towards the focus.

Beam Intensity— A schematic drawing of the laser beam brought to a focus in optical tweezers. The beam intensity decreases going out from the beam axis.

Trapped Sphere — Two light paths in the optical tweezer laser beam passing through a translucent object. Note that the path that originates closer to the center of the beam will have the greater number of photons per second. The changes in momentum of photons on these paths gives the sphere momentum toward the beamaxis and also toward the focus .