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Posts Tagged ‘Polarising filters’

I was listening to BBC Radio 5’s “Dr. Karl” phone-in last Wednesday/Thursday night (25th/26th), which had Wendy Zukerman (@wendyzuk on Twitter), a science journalist, replacing the usual Dr. Karl as he is currently travelling in the USA. One of the many interesting questions asked by listeners was “Does polarised glass have to be tinted?”. The question made me think that it was high time I wrote a blog about what polarisation is, and why wearing polarised sunglasses cuts down on glare (what with the summer here and all….)

Light is an electromagnetic wave

Light is an example of a more general phenomenon called electromagnetic radiation, which also includes radio waves, x-rays, microwaves and the infrared. We can think of light as a wave which travels from its source, and in a vacuum it travels at the speed of light (it travels slower in other media, but in air it travels nearly at the speed of light in a vacuum). The light wave actually consists of a fluctuating electric field and a fluctuating magnetic field at right angles to each other, as the figure below shows. The blue wave is the changing electric field, the red wave is the changing magnetic field.



An example of an electromagnetic wave. The blue wave is the electric field, the red wave is the magnetic field.

An example of an electromagnetic wave. The blue wave is the electric field, the red wave is the magnetic field. They are at right angles to each other, and the variation of one produces the other. As illustrated on the right, in this example the wave is travelling to the right, but if we looked at it end-on it would be coming towards us (a circle with dot in the middle is the way physicists show that something is coming out of the paper/screen).



What is polarisation?

Normally light is unpolarised, which basically means that the direction of the electric field vector (\vec{E} as shown in the diagram above) can be in any direction and is always changing as light waves stream from the source. In the diagram below, the unpolarised light is shown with four different directions (vertically, horizontally, and at 45 degrees each side of the vertical), but in reality all directions occur for unpolarised light.

If we pass this unpolarised light through a polarising filter, only the light whose electric field is in the direction of the “slits” in the polarising filter will be allowed through. So the light emerging to the right of the polarising filter is polarised. This is nicely shown in the figure below.



A polarising filter will only allow the light whose Electric field vector is in the same direction as the lines in the filter to pass through

A polarising filter will only allow the light whose Electric field vector is in the same direction as the lines in the filter to pass through



If we now put a second polarising filter in the path of the polarised light, and the orientation of the second filter is the same as the first filter, then the light will be unaffected by the second filter because the light is already polarised. This is shown in the middle diagram in the figure below.

If, however, we put a second polarising filter in the path of that polarised light, and the orientation of the second polarising filter is not the same orientation as that of the first filter, then no light will emerge through the second filter. This is shown in the bottom diagram in the figure below.



Normally light is unpolarised, which means the electric field vector is at random orientations. When it passes through a polarising filter, only the vector in the orientation of the filter can pass through, all other orientations are blocked. If we now put a second filter at a different orientation, no light will get through the second filter.

Normally light is unpolarised, which means the electric field vector is at random orientations. When it passes through a polarising filter, only the Electric field vector in the direction of the slits in the filter can pass through, all other orientations are blocked. If we put a second filter at a different orientation to the first filter, no light will get through the second filter.



Why do polarised sunglasses reduce glare near water?

When light is reflected off of a surface, such as water in a lake, swimming pool or the sea, the reflected light is polarised. The reasons for this are more involved than I want to go into in this non-technical blogpost, but it has to do with the direction in which the electric field can jiggle the electrons in the surface of the water. It can jiggle the electrons in the plane of the surface, but not perpendicular to the surface. This leads to the reflected light being polarised, in the sense that the reflected wave only has a polarisation parallel to the surface of the water. This is illustrated in the figure below.



When unpolarised light reflects off of a surface, such as water, it becomes polarised. The only reflected part is the light whose electric field vector is parallel to the reflecting surface.

When unpolarised light reflects off of a surface, such as water, it becomes polarised. The only reflected part is the light whose electric field vector is parallel to the reflecting surface.



This is why polarised sunglasses cut down so much on the glare at an outdoor swimming pool, a lake or on the beach. Polarised sunglasses have the polarising filters arranged so that the only light allowed through the filter is with the polarisation in the vertical direction, that is in the same direction as your body if you are standing up. Even in itself, this will cut down on even the direct light entering your eyes, as it cuts out all the light with the electric field vector in other directions, only allowing the light whose electric field vector is in the vertical direction to pass through. So, polarising sunglasses darken the scene, even if you are nowhere near water or other horizontal reflecting surfaces.

But, even more usefully, if you are standing up and look out towards water and the light which is being reflected off of the water, the polarised sunglasses are going to cut out all the light which has been reflected off of the water’s surface! This is because that light is polarised parallel to the surface of the water, which is at right angles to the polarising filter in your sunglasses, so it does not get through. This significantly reduces the glare one would normally have if you looked at strong sunlight being reflected off of water.

Does polarised glass have to be tinted?

Going back to the original question, “Does polarised glass have to be tinted?”, I guess the correct answer is that it depends what you mean by “tinted”. Polarised glass will always cut down on the amount of unpolarised light getting through, so anything illuminated by unpolarised light will look darker through polarised glass. But, tinting is a separate thing from polarisation. Cheap sunglasses are tinted, in that they have darkened glass or plastic to cut down on the amount of light passing through to your eye (and, hopefully, something to filter out the damaging UV rays from getting to your eyes). But, tinted glass does not polarise the light.

It is possible to also add tint to polarised glass so that even the “clear” parts of the filter, which allow light through, will cut down on the amount of that light. But, polarised glass and tinted glass are really two separate things. If you can afford it, polarised sunglasses are better, but they are also a lot more expensive than tinted sunglasses. Personally, I break (usually by sitting on) any sunglasses within a few months of buying them, so I have never bought an expensive pair of sunglasses, nor am I very likely to. But, if you are less clumsy than I am, polarised sunglasses are very nice and kind on the eyes.

So, a rather complicated answer to a simple question 🙂

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