Experiment confirms apparently absurd thesis

Despite scattering: It does not matter how cloudy or clear a liquid is, because the light always covers the same distance. Although it may sound absurd, this thesis has now been confirmed by scientists. In experiments they were able to show that turbid liquids such as milk force the light to go wrong while the particles are being deflected. As a result, some paths become longer while others become shorter, keeping the balance on average.

In transparent liquids, light paths are straightforward - in turbid liquids they are complicated by scattering. © TU Vienna

Normally the light penetrates the water in a straight line, if it is clear water. With turbid substances, however, it behaves a little differently and the light has to go detours in the form of jigsaw patterns to be able to reach the outside of the vessel. This is what makes f.e. milk to appear in a white color.

Although one might suppose that there are situations where the light has to travel longer distances this is not the case: the scientific team around Stefan Rotter of the Vienna University of Technology, who had published a thesis years ago, have now proven this paths to be of equal length.

Nanoparticles in vitro

In the experiment, the scientists mixed water with nanoparticles to follow the behavior of light in turbid liquids. It turned out that the light was scattered all the more, the more particles the liquid did contain or the dimmer the liquid was.

When light is passed through this fluid, the scattering constantly changes because the nanoparticles move in the water, explains Rotter. This results in a characteristic glitter on the surface of the test tube, and if it is precisely measured and analyzed, it can be used to deduce the path the light has traveled in the liquid.

Always the same length

It was extremely interesting that it made no difference whether the liquid was transparent or cloudy. Measurements showed that the light always lays back the same length, which at first glance seems completely absurd. A seemingly invisible mechanism, which is responsible for preventing some particles from passing straight through the liquid and deflecting them outward after hitting the vessel.

It follows that the path that some particles travel back becomes longer, while the way of others seem to become shorter.

One can mathematically show that these two effects surprisingly cancel each other exactly, explains Rotter. On average, the average distance the light travels in the liquid is always the same.

Even in a description that considers light not only as a particle, but also takes into account its wave character, this relationship remains.

The distance traveled remains the same - regardless of how much the wave is scattered inside the medium, says the researcher.

Universal law

The experiments now confirm the mathematical calculations and could help in the future to better understand the propagation of waves in disordered media. There are many possible applications for this.

It's a universal law that basically applies to every type of wave, emphasizes Rotter.

Whether it's light waves in a cloudy liquid, whether it's sound waves scattered by objects in the air, or even gravitational waves that penetrate a galaxy, physics is the same in all cases, concludes the researcher.

Source: Science, 2017; doi: 10.1126