On Polaroid Filters – Brief

Light is weird. Light or Electromagnetic Waves are well, waves. They are a result of a changing oscillating electric field and a magnetic field. Sometimes we call them Photons, massless high-speed subatomic particles, coming in packets called Quanta. Wave-particle duality is only the brief introduction of the enormous and extraordinary area within Physics called Quantum Theory.

A slinky is a nice little demonstration of how light travels. Light is a transverse wave so it vibrates perpendicular to the direction of energy travel. In Third Year of High School, my Physics teacher used a slinky as an example to illustrate this feature of a transverse wave and also the other, longitudinal wave, which is a wave in which its vibrations are parallel to the direction of travel. Two people held the slinky at the two ends and one begins to vibrate the slinky coils left to right.

Now, a supposed wave is seen travelling along the slinky and towards you, though if you look closely, it is in fact only travelling in one plane, that is whatever plane the student decided to vibrate it at the beginning, so, left to right. The slinky is not an electromagnetic wave, however, because with an EM wave, vibrations in one plane will not be seen, but instead unfiltered light will have a multitude of orientations perpendicular to the direction of their energy travel. This is unpolarized light and is what sunlight and many other light sources produce.

It is possible to adjust unpolarised light and make it polarised. That is, to cancel out the vibrations of photons in a certain number of planes. Often, polarised light is light that vibrates perpendicular to one plane only.

When I was at Explorathon at Glasgow Science Centre, I had a chance to meet a Physics Lecturer from the University of Strathclyde, while he answered my question on parallel universes, he also showed me the wonders of polarisation. He put a filter in front of a TV screen, it blocked out some of the light so the display dimmed in brightness.

He then asked me what I predicted to happen if he inserted another one of those filters in front of the previous filter and I replied with an intuitive thought – the display would decrease in brightness once again. And my prediction was true, in some cases. To my surprise, he smiled and turned the second filter 90 degrees clockwise, and placed it back on the first – the display went entirely black! He changed the orientation of the second filter back and forth, and the display went from being low brightness and black and vice versa.

The whole process baffled me so much that I thought I was perceiving a magic trick. Turns out, he went on to explain polarisation of photons by transmission which is the Polaroid filter method.

Light can be polarised using Polaroid filters, which contain many long chain molecules/polymers aligned in the same direction within the material, like prison bars. The alignment essentially results in a polarisation axis which only allows the photons that vibrate parallel to this axis to pass through. When unpolarized light passes through the filter, around 50% of photons is absorbed. The probability of a photon passing through the filter depends on how close the polarisation of it is to the angle. Chances of transmission are half and half due to the randomness of light orientation.

So by using two filters, all photons can be blocked out if the polarisation axes of the two are perpendicular to one another and photons orientated in all directions are absorbed.

Author – Susan Chen

Susan is a 5th year high school student currently studying three STEM subjects at Scottish Higher level-Mathematics, Physics and Chemistry (Crash Course). She particularly loves ideas in cosmology and hopes to embark on an academic journey in the area of theoretical physics.