When a ray of light that is travelling through a piece of glass strikes the interface between the glass and the air, it changes direction according to Snell's law. If the angle of incidence is less than a critical angle, which is determined by the refractive index of the glass and the air, the ray is refracted and leaves the glass. However, if the angle of incidence is greater than this critical angle, the ray undergoes total internal reflection and remains in the glass.

In his classic book on optics Newton suggested that the light ray should be slightly delayed in the second medium before re-entering the first. Later, in 1955, the Hungarian physicist Eugene Wigner made a prediction for the value of this delay, but it has not been measured in an experiment until now.

Le Floch and colleagues began by placing a container filled with mercury along the hypotenuse of a glass prism. They then passed a femtosecond laser beam, which was polarised perpendicular to the plane of incidence, through the prism onto the surface of the mercury (see figure). Wigner delays are extremely short so they can only be measured with ultrashort light pulses.

Next, the physicists timed how long it took the light beam to be reflected back through the prism using an autocorrelator. Since reflection from a metal does not involve a time delay, this measurement defines the "absolute zero" in the experiment. The team then removed the mercury and repeated the measurement. The difference between the two results gave an absolute value for the delay from the glass-air interface.

Le Floch’s group found that the delays increased as the angle of incidence approached the critical angle of 43.48°, with the largest measured value being 28 femtoseconds. Moreover, when the experiment was repeated with light that was polarised parallel to the incident plane, the delays reached 57 femtoseconds. This implies that there must be two Wigner delays at total reflection for unpolarised light.

"Newton would probably be surprised by the existence of two different delay times because at the end of the seventeenth century the transverse nature of light was still unknown," team member Olivier Emile told PhysicsWeb.

The Wigner delays at total reflection could be used to study the new "left-handed" or negative refractive index materials, and materials with photonic band gaps. Moreover, they should also exist for beams of particles such as neutrons according to the Rennes team.