Science Buddies: "Ask an Expert"

When doing the experiment "Measuring Sugar Content of a Liquid with a Laser Pointer" the ray seems to reflect at the faces of the prisms. Now, this is not totally unexpected, as I know from class that light most often refracts and reflect on the same surface. However, my question is whether this happens every time and if does not what are the reasons why it would reflect and refract simultaneously? Lastly, do these multiple reflections have any influence on the results of the experiment?

Hi Muggo,

Most important answer first: the scattering/reflecting of the laser by the prism surface (at the air/solid interface) won't affect your results. Of course you'll have less photons traveling through your sample due to this partial reflection, but in this project you only care about the vector of the beam after it passes through your system.

In many other experiments the intensity of the beam is critical (absorbance, for example) so you'd take special precautions to correct for scattering and reflection. Here assume that the laser beam is shooting in a straight line.

A quick way to consider this reflection is to think of the two limits: total reflection of the beam (like a perfect mirror) or total transmission of the beam (the air/solid interface is 'invisible' to the laser, zero reflection). Real surfaces are typically somewhere in between or else we'd be crashing into glass doors and windows a whole lot more.

You've discovered an interesting concept here by noticing the partial reflection: In laser spectroscopy scientists regularly use "partial mirrors" to split a laser beam into two beams in a controlled manner (for example a 50% mirror) and the behavior of these interfaces and indices of refraction of the materials involved are used to make anti-reflective coatings on common, everyday items like eyeglasses.

-John