A polarising filter is something every photographer is familiar with. It increases contrast and decreases reflections. But, does the same go for polarised sunglasses? What is the purpose of polarised sunglasses and how do they work? And above all: do your sunglasses really need to be polarised?
The human eye is capable of adapting to changes in brightness to a certain extent. However, if it gets too bright, we need some sort of aid, and sunglasses do just that. In extreme conditions, such as during glacier crossings, we need glacier glasses of the highest category. These glasses hardly let any light through and provide the eye with the protection it needs.
Many of these glasses happen to be polarised as well. However, this doesn’t have as much to do with protecting the eyes as it does with safety during activities in the mountains. What polarised glasses do is, increase the contrast we perceive. If you’d like to find out more about how polarisation works, keep on reading!
What are we really talking about?
We’re talking about light. In physics, light is described as an electromagnetic wave. A wave is an oscillation in space. The plane on which the oscillation takes place is called the polarisation plane of the wave. Polarisation is thus a property of a wave and any wave for that matter, since every wave can be traced back to an oscillation. Light is therefore always polarised.
Scattering and reflection
Scattering and reflection change the polarisation of light. Sunlight that reaches the Earth’s atmosphere is scattered by every molecule in the air and broken up and reflected in the tiniest of water droplets. And, the polarisation planes get mixed up in the process. The light on the ground is described as unpolarised. This is obviously not completely correct, since light is always polarised. However, the light on the ground has a non-uniform polarisation. And that’s what matters.
When light hits a surface, some of the light is reflected and some is absorbed. Take a body of water as an example. The light is reflected on the surface of the water, but some light penetrates into the water as well, which is something you’ll certainly be familiar with if you’ve ever been snorkelling or scuba diving. The same goes for a glacier or a window pane. Some of the light is reflected and some penetrates into the boundary.
What is reflected and what is absorbed depends in part on the polarisation of the light. The boundaries act kind of like a polarising filter because if a certain polarisation is preferentially absorbed, then certain polarisations are reflected as well. Thus, a “preselection” of sorts takes place.
Now let’s get back to our polarised glasses. Like the boundary surfaces mentioned above, polarised glasses are also polarising filters. They have a defined polarisation plane and only let light with the same polarisation plane through.
Think of it like this: if you throw a thin stick at a net consisting of only vertical lines, the stick will always fly straight through it, provided it is vertically aligned and you happen to hit the gap between two lines dead-on. If the stick is horizontal or diagonal, it’ll simply get caught in the net.
Direction of polarisation
The same is pretty much true when it comes to polarised light as well. When light reaches your sunglasses, it will only be able to pass through if it has the same direction of polarisation as the glasses themselves. Of course, this comparison is only partly true. For light with a different polarisation plane won’t be completely blocked but reduced down to the bit that does have the same polarisation as the glasses. This is due to the simple fact that we’re talking about electromagnetic waves and not a stick. After all, you don’t want me to bust out a bunch of complex formulas, do you?
So, instead of throwing sticks at nets, let’s find a wall with a slit in it to throw our sticks at. Once you’ve found one (I’m kidding), throw the stick at the slit. If you hit the target, the stick will pass right through. If the stick hits the slit at an angle, the part that touches the wall will be cut off whilst the part that hits the slit will still be able to go through. Just as the stick ultimately decreases in size, so too does the light that hits our sunglasses decrease in intensity.
What do polarised glasses do?
For a start, they reduce the intensity of the light that reaches your sunglasses. In other words, they make things darker. However, this effect is less significant than you would think because our perception of brightness is not linear. Simply put, when the amount of light that reaches our eyes is cut in half, we don’t perceive it as being half as bright. This is due to the composition of the human eye. Our eyes can perceive differences in brightness much better in the dark than they can in bright light. But that’s a different topic altogether. Even though most of us only wear sunglasses when it’s really bright out, the dimming effect is not the sole effect of polarised glasses.
Much more important for us outdoorsy folk is the ability of polarised sunglasses to help us better perceive contrasts. To illustrate this fact, imagine you’re doing a glacier walk. Here the sun is shining brightly, right in your eyes. But the sun doesn’t stop there: the sunlight hits the ground and is reflected off the surface as well. As was already mentioned, the amount of reflected light depends on both the polarisation of the light as well as the the makeup of ground itself (rock absorbs more light than snow and is thus darker) and the angle of incidence.
The opposite is also true. The intensity and polarisation of the reflected light depends on the surface and the angle of the reflection of the light.
For example, if there is a step covered by snow on the ground right in front of you, the light reflected by that plane has a different polarisation than the light reflected by a slope. These different polarisations are then filtered by the glasses to varying degrees with the result that you perceive these zones with varying degrees of brightness. The step is thus seen more clearly with polarised sunglasses than with an unpolarised pair of glasses, as the latter only makes things darker. That’s not to say that you wouldn’t see the step with unpolarised sunglasses. It’s just that the amount of light that is not let through is the same for all zones. Polarised sunglasses reduce the intensity of light differently depending on the angle of incidence.
Areas of use
Polarised sunglasses make a major difference on water. They filter the light that is reflected off the surface of the water differently, resulting in us perceiving waves more clearly. Thus, polarised sunglasses are beneficial in places where reflections need to be perceived differently. In other words, they’re perfect for bodies of water and the mountains. Whether or not you really need polarised glasses is obviously up to you. But, since they allow you to better identify the makeup of the surface you’re walking on, they will increase your safety, especially in the mountains. Glacier glasses should definitely be polarised, though!
Here’s a concluding remark on the perception of polarised light in general: The human eye is not capable of identifying the state of polarisation of light. The only exception is the phenomenon of Haidinger’s brush where many people can see a visual pattern in light after looking at completely polarised light for a longer period of time and then looking at a surface that is as neutral as possible.
It’s a completely different story when it comes to insects. Karl von Frisch discovered that honey bees are able to detect polarisation patterns and orient themselves using this ability along with the position of the sun. Cool, right?