Researchers have devised a simple and sensitive laser-based experiment that proves a century-old prediction that light particles (photons) gain momentum and create nanoscale deformations when they enter air from water at a specific angle1. These results could be used to design better biosensors, lab-on-chip devices and molecular imaging tools.
When light passes from water to air at an arbitrary angle, part of it is reflected back while the rest of it travels through the water. Light is totally reflected from an air–water interface when it strikes the interface at an angle known as the critical angle ― a phenomenon called total internal reflection. The nanomechanical effects of photon momentum such as extremely tiny deformations when light passes through an air–water interface at angles close to the critical angle had not been previously studied.
The researchers shone a low-power laser beam on a water droplet placed on a glass prism. Scanning the air–water interface with a probe laser revealed that the water surface was slightly bent upwards. The researchers attributed this bending to the pressure exerted by light particles from the laser.
Detailed analysis revealed that the light particles gain momentum as they enter the air–water interface. When a light beam hits the interface, it transfers momentum to the interface causing it to bulge towards air, the researchers say.
“This sensitive technique will allow precise measurement of the properties of light as well as static or dynamic characterization of various fluids in a non-contact manner”, says lead researcher Kamal P. Singh.
