A photo that I took looking out to Diamond Head while I was on my boat.
When I was out on my boat last week watching the sunset, I witnessed the scene above and felt that I needed to take a photo of it to capture the moment. Despite the fact that the photo does not look very bright, the light reflected off of the water was actually quite blinding. It was hard to focus on taking the photo with the large amount of glare, so my friend lent me his polarized sunglasses. Upon putting them on, the glare from the water decreased drastically and I was able to take the photo. Looking at the above scene, we can witness many different concepts related to the wave nature of light such as the Rayleigh Criterion, Brewster’s Angle, Interference, etc. For the purposes of this blog, however, I want to hone in on a topic that was essential for my friends sunglasses to block the glare from the water: polarization.
Before we begin the sunglass discussion, let’s answer the following question: What is polarization and what does it mean for light to be polarized? Light is a transverse electromagnetic wave that vibrates in multiple directions. Light in its natural state, emitted from the sun or a classroom light, is said to be unpolarized because its electric and magnetic waves are vibrating in multiple directions. The interesting thing about light is that, although it is originally unpolarized, light can be polarized through a variety of methods. That is, light can be manipulated such that it can vibrate in a single plane rather than in multiple planes. In this blog post, I will analyze the photo I have posted above to explain how light can be polarized by reflection and the reason why polarized sunglasses block glare.
It is common for light to be polarized after reflecting off of non-metallic surfaces like water. Non-metallic surfaces naturally reflect light in a way such that the majority of the waves reflected are polarized in a direction parallel to the surface. The light waves in the posted photo, therefore, are mostly vibrating in a horizontal orientation. Now that we understand how light is polarized by reflection, how do sunglasses drastically cut glare? Glasses use polarization filters which are able to select which type of polarized light to allow through. If the polarization filter is perpendicular to the direction of the polarized light, no light will be allowed through. If the polarization filter is parallel to the direction of the polarized light, all of the light will be allowed through. Given that the light from the water was mostly polarized in the horizontal direction and that my friend’s sunglasses cut a large portion of the glare, we can reason that the polarization filter in the sunglasses must have been somewhat angled against the polarization direction but not completely perpendicular to it.
This blog post is only a brief description of polarization and how it relates to the wave nature of light. The topic is so large and undoubtedly impossible to cover in one short blogpost, but I hope that I was able to better your understanding of physics!
A Day in the Life of Physics 