Slowing down light in milk and white paint.

Slowing down light in milk and white paint

A glass of milk presents a formidable obstacle course to a beam of light. Microscopic globules of fat and protein, evenly dispersed throughout the liquid, continually change the light beam's direction, forcing it to follow a zigzag path. This "scattering" is so efficient and effective for all colors of light that milk appears both opaque and white.

Dutch physicists have now discovered that the scattering process is more complicated than researchers thought. The obstacle - whether globules in milk, water droplets in clouds, or tiny beads of pigment in white paint -- not only change the light's direction but also significantly retard its speed.

"We were looking at the propagation of light in disordered materials," says Ad Lagendijk of the University of Amsterdam and the FOM-Institute for Atomic and Molecular Physics in the Netherlands. "I was astonished by this reduction in the speed of light. To me, it was unbelievable." Lagendijk described his group's discovery this week at an American Physical Society meeting in Cincinnati.

The researchers stumbled upon the effect while studying the scattering of light by concentrated white paint -- microscopic beads of the pigment titanium dioxide suspended in a liquid. While interpreting their data from diffrent experiments, they found to their surprise that they always got contraditory results.

Further experiments and analyses revealed that the problem lay in the conventional assumption that scattering occurred instantaneously, or equivalently, that light traveled at its normal speed through the medium. By adjusting the speed of light in their calculations, the Dutch researchers found that they could get all their experimental data to agree.

Lagendijk and his colleagues developed a new theory to account for this unexpected behavior. They propose that light not only scatters but also enters an obstacle and stays trapped inside for a while before it escapes and shoots off in a new direction. In other words, the obstacle themselves significantly delay the scattering of light. In milk or white paint, this happens so often that light appears to travel at only one-tenth its usually assumed speed in these materials.

"This phenomenon is particularly strong when the dimensions of the obstacles...are of approximately the same size as the wavelength of light," Lagendijk says. In effect, an obstacle acts as a "resonator," or cavity, which must be "filled up" with light before it can fully redirect a beam.

The finding affects scientists' interpretations of a variety of experiments involving the propagation of waves through disordered materials. "If you fail to take this delay into account, you come to totally different conclusions about the propagation of light in that medium," Lagendijk says.

In particular, the delay phenomenon affects the determination of a key parameter known as the mean free path -- the average path a wave travels before it's scattered. Researchers use this quantity to characterize a disordered material and to make predictions about the material's behavior.

"If you use the wrong speed, you get the wrong mean free path," Lagendijk says. "It makes it harder to predict what happens in such materials."

The discovery that light slows down much more than expected when traveling through materials such as milk and white paint demonstrates that even a mature field of physics may yet harbor hidden treasures, Lagendijk says. "It is surprising that such fundamental experiments can still be done in the 1990s with essentially no more knowledge about light propagation than at the beginning of this century."