Tweezra

Tweezra, a light-weaving fae, holds a tiny glowing bead at the focus of a cone of green light.

The Physics

Focus a laser beam tightly through a microscope lens and a small transparent bead is pulled towards the brightest point and held there. Light bends as it passes through the bead, and the change in its momentum pushes back on the bead. For a bead that bends light more than its surroundings, the net gradient force points up the intensity gradient, towards the focus. Arthur Ashkin accelerated and trapped particles with radiation pressure in 1970 (Phys. Rev. Lett. 24, 156), and with colleagues demonstrated the single-beam gradient trap, the optical tweezers, in 1986 (Opt. Lett. 11, 288).

F_grad ∝ ∇I

• F_grad: gradient force on the bead (N)
• ∇I: gradient of the light intensity (W/m³)

Ashkin shared the 2018 Nobel Prize in Physics for the optical tweezers and their use in biology. They now hold and pull on single cells, DNA molecules and molecular motors.

Honest caveat: The gradient force has to beat the scattering force, which pushes the bead along the beam, so a tight focus is needed. F ∝ ∇I is the small-particle limit; for larger beads it is a ray-optics calculation.

On the sticker

Tweezra holds out her hand as a green cone of light narrows onto a small glowing bead. Printed text: TWEEZRA with the line F ∝ ∇I. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.