Pocket Polarisation Viewer

Polarised light is everywhere, but seeing it is difficult with the naked eye. You can of course use a simple sheet polariser as an aid. This pocket-sized visual tool goes one step further and provides a fun way to easily detect polarised light in the home and in the outdoor environment.

Plastic sheet polariser

- non-adhesive type, ideally about credit-card sized

Clear adhesive tape – e.g. Scotch Ultra Clear or similar

- not the matt “invisible” type

Polarised light is everywhere, but it's difficult to see with the naked eye. All around you the light reflected off walls and objects is at least partially polarised. In the outdoors too, much of the light we see is polarised. Some birds, bees, shrimps and many other animals use polarised light as a navigation aid - it's their superpower. You can of course detect the polarisation of light with a piece of polariser sheet or "Polaroid" sunglasses. You just have to look through and turn the polariser 90° to see if there's a difference in intensity. The visual tool described here just makes it easier, and a bit more fun.

The visual tool consists of a stack of adhesive tapes stuck on the polariser sheet. Place the polariser on the table and pull a wider length of tape off the roll, held at both ends. Try not to touch the surface. Stick the tape onto the polariser, about 45° to the polariser axes (edges, usually). Take a second long piece of tape and place on top of the first, shifted by two or three mm. Repeat this process until you have six or more layers. This gives a tape stack with a stepped thickness variation.

Now take a last piece of tape and stick in on top of the others, rotated to be not aligned with either the other tapes or the polariser axes. It’s not very critical. Make sure to leave a bare patch of polariser without any tape on it.

That’s it!

A great starting point for experiments with polarisation is the computer screen. LCD (not OLED) monitors have a linear polariser covering the whole area, so the light emitted is usually linearly polarised. Open a text editor showing a blank white page. Hold the tape-polariser stack against the screen with the polariser facing towards you. You should see coloured bands where the tape is present. If you rotate the tape stack, there will be an angle where the colours disappear in the region without the extra 45° tape. This is where the axes of the tapes are aligned parallel to the linear polarisation emitted by the screen. However, the tapes which are covered by the extra piece of tape will be coloured. Hence there will be coloured bands visible somewhere whenever the light is polarised, independent of how you hold the stack.

Along with sticky tape, most plastic films have been stretched in manufacture. This aligns the polymer molecules and renders the film linearly birefringent. i.e. the refractive index of the tape is different for light with electric vector along the tape and perpendicular to it. Alternatively stated, light polarised in the two directions travels at different speeds, suffering different retardation. The effect on a linear state of polarisation (SOP) entering at any intermediate angle is to convert the light into an elliptical SOP. In addition, the SOP depends on the wavelength of the light. The final polariser then absorbs some wavelengths more that others, selecting some colours from the white light input.

By using several pieces of adhesive tape, we have a range of birefringence (“retardance”) and hence a selection of different colours. Note that the colours are not the “pure” ROYGBIV colours of the rainbow, as the combination of retardance and polariser angle might pass wide or even several wavelength ranges at once.

Of course, if the input linearly polarised light is aligned with the tape or perpendicular to it, there will be no change into elliptical states and hence no visible colours. You can see this in the central image above. Therefore we added the extra piece of tape at an intermediate angle. This deals with this null-case, guaranteeing colours either where there is or isn’t the extra tape. The regions of polariser without any tape will be bright, dark or of an intermediate intensity as usual. The intensity is described by Malus' Law. There are elegant matrix methods to perform a general analysis of the elliptic SOP produced

We also know that unpolarised light is partially polarised on reflection and on transmission. This is the Brewster effect: light with electric vector perpendicular to the plane of incidence is reflected more strongly than light polarised in the plane of incidence. Hence what reflected light remains is (at least partially) linearly polarised. At one special angle, the “Brewster Angle”, which depends on the refractive index of the reflecting surface, light polarised in the plane of incidence has all reflection suppressed; the surface acts as a good polariser.

This pocket-sized viewer is related to a very old toy – a kind of polarisation kaleidoscope formed from random pieces of cellophane between two rotatable polarizers. It's also a kind of stress polariscope, commonly used to detect stress in clear plastic objects.

Now let's test some other light sources. Most mobile phones and tablets have LCD screens emitting polarised light. Check these out as before. Look also for coloured bands in sunlight reflected at about 55° incidence from a wood, glass or plastic table-top (non-metallic).

Now go outside – direct sunlight or even light from an overcast sky reflected off a rooftop, window, puddle of water or even from the wet roadway will show the colours.

If you are a diver, try underwater. Refracted sunlight is also partially polarised. Those shrimp must be using their superpowers somehow!

Lastly, look at the stack held up against a clear blue sky, not into the sun but at about 90° to it. This light is also partially linearly polarised by Rayleigh scattering, so you should see those colours. They may be weak, but can sometimes be very clear.

Actually, you don’t even need this little device to detect polarised light. Like bees, humans too can detect polarised light directly. It just needs a little practice. Stare up at the clear blue sky at 90° to the sun’s direction. Rock your head left and right. You should see a yellowish, symmetrical, two-headed “brush” a few fingers’ width at arms length. This is Haidinger’s brush.

If you can’t see it, practice the head-rocking in front of a white or pale blue LCD PC screen, or take a piece of polarising sheet and rock it quickly back and forth 45° in front of your eye. You should see the brush.

Once you have trained a little, it is quite hard NOT to see the polarisation of sunlight outdoors. Frequently I will notice Haidinger’s brush in the blue sky or on the wet roadway. However, so far it hasn’t been much help in outdoor navigation.

This little device, made with just a few minutes’ work, is useful to show how much of the world exhibits polarised light. If you want to delve deeper, there is plenty of nice maths, interest and application of polarisation across the electromagnetic spectrum, in natural and stress-induced birefringence, in birefringent filters, singlemode fibers and in quantum computing. In the outdoor environment, almost every visual phenomenon has aspects due to polarisation. If you can find a copy, M. Minnaert’s “The Nature of Light and Colour in the Open Air” is a fascinating read and an encouragement to us all to observe better.