Light That Doesn't Heat Water express
A water droplet on a leaf disappears faster under the sun than heat calculations would predict. A few years ago, a hypothesis emerged: light itself, without heating, could knock water molecules off the surface. This was dubbed the photomolecular effect. If it exists, clouds, climate, and even everyday clothes drying could work differently.
The authors of the new study decided to test the idea with precise measurements. They directed polarized light (waves oscillating in one plane) at water and compared the reflection from the surface with and without water. If light were lost at the liquid-air interface, the reflection would change. They used two types of substrates: lightly doped silicon (a semiconductor) and platinum (a metal). Their optical properties were measured beforehand using ellipsometry (based on the change in light polarization upon reflection). The result matched the classical model where the surface absorbs nothing.
So, in practice, light does not "tear off" water molecules in any noticeable way. The effect is either much weaker than thought or occurs only under special conditions — different humidity, water purity, or light intensity. For now, good old heating remains the main culprit in evaporation.
Original abstract
as written by the authors arXiv:2603.17977
The photomolecular effect has been hypothesized to enhance evaporation of water at visible wavelengths. This study develops a measurement technique to investigate its presence and magnitude at the liquid-vapor interface of water. The experiment detects surface absorptance by comparing polarized reflectance for substrates with and without a water layer. The reflectance ratio is used as an indicator of interfacial absorptance or attenuation. Lightly doped silicon and platinum were used as dielectric and metal substrates, and their pseudo-optical properties were measured using ellipsometry. Experimental results were compared to a multilayer reflectance model to determine theoretical sensitivity and guide angle selection. Measurements showed close agreement with a classical model assuming zero surface absorptance. The model was then extended to include surface absorptance using a spectral response based on Lorentzian-distributed Feibelman parameters derived from a recent estimate of 0.84% interfacial absorption. Although this model predicted a 1-2% reduction in reflectance near resonant frequencies, no such spectral features were observed experimentally. These results suggest that under ambient conditions, interfacial absorption is well below 1%, indicating that the optical signature of the photomolecular effect is either much weaker than previously reported or strongly dependent on conditions not present in this experiment.
Polarized light reflects off water differently depending on the angle: at Brewster's angle (about 53° for water), the reflected light is completely polarized, a property used in polarized sunglasses.