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## Does polarised glass have to be tinted?

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. 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

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 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.

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 🙂

## The 10 best physicists – no. 6 – Michael Faraday

At number 6 in The Guardian’s list of the top ten physicists is Michael Faraday.

Michael Faraday is known both as a physicist and a chemist. In the field of physics the contributions which put him in this list are mainly due to his groundbreaking work on understanding electromagnetism. Amongst other things, he introduced the idea of the electromagnetic field, he discovered the link between magnetism and light, and he discovered electromagnetic induction and invented the first electric motor.

Faraday was born in 1791 in Newington Butts which is now part of London, but at the time was in suburban Surrey. He was born into a strictly religious family, his father James was a member of a sect of Christianity known as the Glassites. His family was not wealthy, and Faraday only received a basic education. By the age of 14 he was working as an apprentice bookbinder. It was during his 7 years working as an apprentice bookbinder that Faraday set about educating himself.

At the age of 20, Faraday attended his first lecture given by Humphrey Davy at the Royal Institution. Davy had become one of the foremost chemists of his time, and was a superb lecturer and showman. Faraday was captivated by Davy’s lectures, and wrote copious notes on what he was seeing and hearing in these lectures.

Faraday lovingly collected the lecture notes he had taken, and bound them in a 300-page volume that he sent to Davy, at the same time asking for a job. Davy did not have a job to offer him, but in 1823 Davy suffered an accident in the laboratory, and offered a position to Faraday working as his assistant. This was the beginning of Faraday’s scientific career, and he became one of the best experimentalists physics has seen.

Faraday married Sarah Barnard in 1821, they had met through church. Both were devoutly religious, and his religion influenced Faraday throughout his life. He twice refused the offer to become President of the Royal Society, and also refused the offer of a knighthood. During the Crimean war of 1853-56, Faraday was asked to help develop chemical weapons but refused. In 1848 he was awarded and accepted a “grace and favour” house in Hampton Court, a house which became known as Faraday House. Faraday and his wife had no children, and he died in 1867 at the age of 75. He turned down the offer to be buried at Westminster Abbey, and instead is buried at Highgate Cemetery.

The list of Faraday’s contributions to physics is quite long, so I will only mention the highlights. In 1821, hearing about the work which the Danish scientist Hans Christian Ørsted had done on electromagnetism, Faraday and co-workers Davy and William Hyde Wollaston showed that a wire carrying an electric current set up a circular magnetic field about it. Unfortunately, in publishing this work, Faraday failed to mention the contributions of Davy and Wollaston, which strained the relationship between Faraday and his mentor Davy, and led to Faraday being assigned other work for several years, effectively stopping his work on electromagnetism for a period.

In 1831 Faraday conducted a series of experiments which showed the phenomenon of electromagnetic induction. Faraday wrapped two insulated wires around a iron ring, one coil on each side of the ring. Upon passing a current through one coil, he found that a current was produced in the other coil. This is the basis of how transformers work, and was a major breakthrough in our understanding of electromagnetism.

This work showed that a changing electric field produces a magnetic field,and a changing magnetic field produces an electric field. This is now known as Faraday’s law. Faraday went on from this work to produce the first ever electrical dynamo, a device which produces electricity and is the basis of the electric generators today used in power stations.

Later in his life, in the 1860s, Faraday showed that the plane of linearly polarised light could be rotated if the light passed through a magnetic field which was in the same direction as the direction of the light’s travel. This phenomenon, now known as the Faraday effect, was the first indications of a link between light and magnetism, and was a crucial step in Maxwell’s later work which showed that light was just a manifestation of electromagnetism to which our eyes are sensitive.

There is probably no other series of discoveries in the world of Physics which have had more of an influence on our modern everyday lives than those made in the first half of the 1800s in understanding electromagnetism. Almost every device we use today, our entire modern lives, relies on discoveries which were made in large part by Faraday. The unit of capacitance, the Farad, is named in his honour. He is the only physicist in this top ten list who had no formal education, and yet his contributions to our understanding of physics are immense.

Does Faraday deserve his place in this list of the ten best physicists?