Physics Photo of the Week

April 22, 2011

Polarized Reflection

These two images were taken one after the other.  There is a piece of glass propped onto the wall on the on the right side of each photo.  The glass is all but invisible in the right hand photo.  Look carefully to see the edges of the glass.

The drastic difference in these two photos is attributed to the use of a polarizing filter - a filter that can be rotated relative to the camera lens.  In the image on the left, where the reflection of the scenery is planely visible in the glass, the filter is oriented such that the plane of polarization is vertica
l - parallel to the glass surface.  The reflected scene originates from the left side of the image - unpolarized.  The reflected light from the glass is polarized.  At a particular angle, the reflected light is totally polarized such that the electric field vectors are parallel to the glass surface (vertical in this case).  If we rotate the orientation of the filter 90 degrees so its axis is horizontal, the vertically polarized reflected light is blocked.  This was done for the right hand photo.  The same effect can be seen in windows on houses and cars with polarizing sun glasses.  When driving, we generally wear our polarizing sun glasses with the axes vertical because most of the reflected light off road surfaces is horizontally polarized.

The animated image here shows the two images alternatin
g in one second intervals to compare the two.  Notice also that the scenery shows subtle contrast changes between the two polarization orientations.

The diagram below contains a more detailed explanation of the polarization by reflection - a challenge for physics students:

 Polarization by reflection.  Light from the above left is unpolarized.  It shows not only the light vibrations as green arrows perpendicular to the light ray, but also has polarizations perpendicular to the picture (parallel to the glass) represented as dots on the ray.  Similar vibrations of the light are shown in the refracted ray within the glass.  The reflected light is actually light that is radiated by the vibrations of the electrons in the glass molecules.  The electrons vibrate as the dots and arrows within the glass.  When electrons vibrate, they radiate light in a direction perpendicular to the vibrations, and the resulting vibrating fields are parallel to the vibrations. However, the vibrations of electrons in the direction of the arrows within the glass cannot radiate in the direction that the electrons vibrate.  Thus the reflected light is polarized with the vibrations parallel to the plane of the glass-air interface.  This is analogous to watching a mass on a spring vibrate.  If the vibration is perpendicular to your line of sight, you see the mass move back and forth.  If the vibration is in the same direction (forward and back) we cannot see the movement nearly as well.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu.

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.