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Light carries momentum, so it can push. The push is far too small to feel in sunlight, but for a cell a few micrometres across it is large. Arthur Ashkin spent decades turning that idea into a tool that now holds DNA, proteins and living cells. This post covers his life, how the trap works, a worked example of the force, and what students can take from his way of working.
Who was Arthur Ashkin?
His Nobel facts page gives his birth as 2 September 1922 in Brooklyn, New York, and his death on 21 September 2020. He studied physics at Columbia, earned a PhD at Cornell in 1952, and spent his whole career at Bell Labs. He received half of the 2018 Nobel Prize in Physics "for the optical tweezers and their application to biological systems".
His Nobel biography says he was fascinated at ten by a Crookes radiometer, a toy with vanes that spin in light. It also records that he was 96 at the award, at that time the oldest Nobel recipient.
How can a beam of light trap a particle?
Ashkin's first paper, Acceleration and Trapping of Particles by Radiation Pressure (1970), reports micrometre-sized particles accelerated and held in stable traps using only the force of a continuous laser. His biography says internal reviewers at Bell Labs wanted to block it as nothing new; his boss Rudi Kompfner told him to send it in anyway, and the journal accepted it.
The Nobel Committee's popular information explains the surprise. A laser beam is brightest in the middle, so the light pressure on a transparent sphere pulls it towards the centre of the beam. Adding a strong lens to focus the light also holds the particle along the beam. In 1986 Ashkin and colleagues reported this single-beam gradient trap, holding particles from 10 micrometres down to about 25 nanometres in water.
How did he come to hold living cells?
The popular information says the move to biology was chance. Ashkin left samples of tobacco mosaic virus open overnight, and they filled with bacteria that were caught in the trap. His green laser killed them, but in infrared light they survived and even reproduced in the trap. The 1987 paper in Science reports trapping viruses with about 120 milliwatts of argon laser light, and live bacteria at a few milliwatts. A Nature paper the same year moved to infrared beams for single cells.
Worked example: how big is the push?
The NCERT chapter on dual nature gives each photon energy E = hc/λ and momentum p = h/λ. Sample numbers: a 10 mW infrared beam with λ = 1064 nm, fully absorbed by a particle.
Energy per photon. E = (6.63 × 10⁻³⁴ × 3.00 × 10⁸) / (1.064 × 10⁻⁶) = 1.87 × 10⁻¹⁹ J.
Photons per second. N = P/E = 0.010 / 1.87 × 10⁻¹⁹ = 5.36 × 10¹⁶ per second.
Momentum per photon. p = h/λ = 6.63 × 10⁻³⁴ / 1.064 × 10⁻⁶ = 6.23 × 10⁻²⁸ kg m/s.
Force. Force is momentum delivered per second: F = N × p = 5.36 × 10¹⁶ × 6.23 × 10⁻²⁸ = 3.3 × 10⁻¹¹ N, or 33 piconewtons.
Shortcut. N × p = (P/(hc/λ)) × (h/λ) = P/c. So F = 0.010 / 3.00 × 10⁸ = 3.3 × 10⁻¹¹ N. The wavelength cancels.
Compare with weight (sample numbers). A plastic bead 1 micrometre across with density 1050 kg/m³ has volume πd³/6 = 5.24 × 10⁻¹⁹ m³, mass 5.5 × 10⁻¹⁶ kg and weight 5.4 × 10⁻¹⁵ N. The light force is about 6,000 times larger.
This absorbed-beam estimate only sets the scale. A real tweezer works through refraction and the intensity gradient, as the popular information describes.
What mistakes do students make with light pressure?
Forgetting milliwatts. 10 mW is 0.010 W, not 10 W.
Confusing energy and momentum. E = hc/λ is in joules; p = h/λ is in kg m/s. Dividing power by c gives force, not energy.
Expecting λ in the answer. For a given power, the force does not depend on wavelength.
Photon momentum sits under "particle nature of light" in Unit 17 of the JEE (Main) 2026 syllabus; check your year's syllabus. For more on lasers, see our post on Charles Townes and the maser. If you want to check each step of a photon problem, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
What can a student take from how he worked?
Keep a childhood question. The radiometer that puzzled him at ten pointed to his life's work.
Do not stop at "nothing new". A paper his colleagues doubted became widely cited.
Follow the accident, then fix the problem. The stray bacteria opened biology; switching to infrared kept them alive.
Next step: repeat the worked example for a 100 mW beam reflected straight back, and compare the force with the bead's weight.
