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Switch On To Physics Australian Institute of Physics Education Sub-Committee Light Polariser You have heard of Polaroid sunglasses, and the claim that they eliminate the glare of reflected sunlight. These activities will show you how they work, and introduce some other applications of polarisers. A. Making the polarising viewer For this activity you need: two polarising filters, one large and one small polystyrene block, to act as a holder for the polarising filters sharp knife or blade, ruler and ballpoint pen felt pen incandescent globe or other light source 1. Take the polystyrene block and, using a ballpoint pen, mark two lines across it two centimetres from each end. Place the polystyrene block on a bench-top with a suitable protective surface. Using a ruler and the sharp blade provided, carefully make a fine cut along each of the lines, about 2 mm into the polystyrene block. BE VERY CAREFUL. Leave the polystyrene block on the benchtop while you are making the cuts, and keep your hands well away from the cutting blade. DO NOT move the blade towards you as you cut: move the blade from one side to the other ACROSS your body. Mark the front (F) and back (B) of the polystyrene block. 2. Select the larger square polarising filter. Use the felt pen to mark a small arrow in one corner, PARALLEL to one of the sides of the square. Mount this filter in the back slit of the polystyrene block, so that the arrow is in one of the top corners. 3. Hold the polystyrene block at the front, and view the light source through the filter. Rotate the polystyrene holder as you look at the light source through the filter. Do you notice any changes in the appearance of the light? Is the change sudden or gradual? 4. Place the second smaller polarising filter into the front slit in the polystyrene block. 5. View the light source again through both filters. This time, rotate the front filter in its slit as you look at the light source. Don’t move the back filter. 6. You should find that there is one orientation of the front filter, which almost totally prevents light passing through the two filters. This is referred to as extinction – the light is extinguished. At other orientations some light is visible. 7. Rotate the front filter to find the orientation at which the MOST light is visible. At this orientation the two filters are said to be parallel. With the front filter in this parallel orientation mark an arrow on it, at the same position and in the same direction as the arrow on the back polarising filter. SOS Light Polariser 1 8. When extinction occurs the two filters are said to be crossed. At extinction the arrows on the two filters should be at right angles. Check to see that this is the case. 9. With the two filters crossed, view the light source while rotating the viewer as a whole. Does this make any difference to what you see? Try the same with the two filters parallel. B. Investigating with the polarising viewer For the next two activities you only need one polarising filter in the viewer. Remove the front filter, and leave it in a safe place so it won’t get scratched or fall onto the floor. 1. Reflected light Find a shiny non-metallic surface that is reflecting light from the sun or an inside source. Suitable surfaces include laminex, polished wood, opaque plastic or a sheet of clear plastic, perspex or glass over a sheet of black paper. i. Look at this reflecting surface through the polarising filter. Don’t look straight down – look at an angle. If you rotate the filter, you should find that, as the orientation of the filter is changed, the amount of reflected light that passes through it varies. Find the orientation of the filter for which the least amount of light is transmitted. Place the filter in the viewer so that you can hold the filter comfortably at this orientation. ii. Keep the filter at the orientation you have just found, but change the vertical angle at which you view the reflected light. Can you find an angle for which almost no light passes through the polarising filter? iii. Now try looking at the light reflected from a metal surface. Is this light polarised? Does the amount of polarisation depend on the angle at which you view the surface, as it did in ii above? iv. What about light reflected (scattered) from a non-shiny surface such as matt white paper or cloth? Is this polarised? 2. Scattered light Shine a beam of light through a glass container filled with a cloudy solution. Most of the light will pass directly through the solution, but some of it will be scattered. This scattered light makes it possible for you to see the beam as it passes through the solution. i. Look at the scattered light SIDE ON to the beam, through the polarising filter in the viewer. Rotate the viewer. You should see that the brightness of the beam changes as you change the orientation of the viewer. Find the angle for which the least amount of light is transmitted through the filter. ii. Try viewing the beam from the other side, and from above the container. Is the orientation of the viewer that passes the least amount of light the same in every case? What about if you view the beam end on, from the side of the container opposite the source? iii. Get one member of your group to hold the second polarising filter between the light source and the solution, with the arrow pointing either up or down. Again view the scattered light from each side and above the container, and opposite the source, rotating the viewer in each position. iv. Repeat iii, but this time with the arrow pointing sideways on the polarising filter near the light source. 3. Scattered sunlight If you can, go outside to look at the sky, or look through a large clear window. Look at an area of blue sky through the polarising filter in the viewer. Don’t look at the sun itself - stand so that the SOS Light Polariser 2 sun is shining from your side. Rotate the viewer to find the orientation that passes the least amount of light. 4. Optical activity Shine a beam of light through a clear container of water. View the light emerging from the water with one of the polarising filters in the viewer. Hold the second polarising filter between the light source and the container. Rotate the polarising filter in the viewer until the emerging beam of light is extinguished. When this occurs the arrows on the two filters should be at right angles. Check that this is the case. This shows that the water does not affect the polarisation of the light: extinction occurs for the same orientation of the filters, with or without the water present. Now try the same thing with a container of concentrated sugar solution. You should now find that the filters have to be in a different orientation for the emerging light to be extinguished. The sugar solution changes the polarisation of the light: the stronger the solution, and the longer the path of the beam of light in the solution, the more the polarisation of the light is changed. C. Polaroid art For this activity you need to cut a piece 4–5 cm square from a sheet of overhead projector transparency. i. Remove the two the polarising filters from the polystyrene block, and place them on top of one another so that their directions are crossed. Now place the transparent sheet between the crossed filters to make a sandwich. Look at the light source through the sandwich. Rotate the transparent sheet, while keeping the polarising filters crossed. Many brands of transparent sticky tape also produce the same effect. ii. Take a microscope slide, and place two short lengths of sticky tape on it so that they overlap partly, but not completely. Place the microscope slide between the crossed polarising filters. Look at the light source through the sandwich and rotate the microscope slide. You can produce some artistic stained glass effects using patterned layers of sticky tape – try it! iii. Reassemble the polarisers in the viewer, with their directions crossed. Hold a clear plastic object (such as a ruler, or setsquare, a CD cover etc) between the polarisers. You will see some particularly interesting effects around any cracks or holes in the plastic. Rotate the plastic object between the crossed polarisers. iv. Try holding some plastic wrap or cellophane between the crossed polarisers, and rotating it. What difference does it make when you stretch the plastic wrap? Fold some of the plastic wrap in layers, and view it between the crossed polarisers. Try scrunched up plastic wrap or cellophane. v. Place a piece of waxed paper or other translucent material between the crossed polarisers. The translucent material scatters and depolarises the light. How can you tell that the effect you see is due to depolarisation of the light, not birefringence? This activity was designed by David Mills, Keith Thompson (both of Monash University, Department of Physics), and Dan O’Keeffe (Camberwell Grammar School), and written by Christina Hart, for the Education sub-Committee of the Australian Institute of Physics (Victorian Branch). Layout and graphics by Bronwyn Halls. The Australian Institute of Physics (Victorian Branch), 1999. SOS Light Polariser 3 Switched On To Physics Australian Institute of Physics Education Sub-Committee Light Polariser Explaining polarisation Electromagnetic waves You probably already know that light is said to be a form of wave energy, but to understand polarisation you need to know a little bit about the kind of wave that physicists have in mind when they say this. As with any form of energy, light can be produced in one place and have an effect in another place, which may be very distant from where it was produced. In this respect it is similar to a tennis ball that carries energy from the place where it is hit to the place where it lands. A sound wave also carries energy from the place where it is produced, (such as your throat) to the place where it is heard (your friend’s ear). But whereas the tennis ball moves with the energy it carries, the air particles do not travel with the sound wave as it moves. The air particles jiggle around as the sound energy moves past them, but the sound is the disturbance that passes through the air, not movement of the air itself as in a gust of wind. In the same way, a ripple momentarily disturbs the water’s surface, but the water does not move along with the ripple. One of the big questions in physics, since the time of Isaac Newton (1642-1727), is whether light is made up of a stream of particles that (like the tennis ball) carry energy with them as they move, or whether (like sound and water ripples) light travels as a disturbance or wave. There were several reasons why, by about 1850, physicists eventually decided that light must be a form of wave energy1. However, unlike sound, light can travel through a vacuum, and this makes it a bit difficult to see what is ‘waving’, or being disturbed, as the light travels. In fact, this was one reason why Newton himself thought light must consist of a stream of very tiny, very fast moving, particles. James Clerk Maxwell, whose eminence in physics is comparable with that of Newton and Albert Einstein, found an answer to this question in 1864. In doing so, he also showed mathematically that light must belong to a much larger family of waves, called electromagnetic waves, that are produced by vibrating electrons. Other members of the family include radio waves, microwaves, ultraviolet-waves and x-rays. The theory that Maxwell proposed is rather a difficult one to explain fully; it is usually studied in university physics courses. But it depends on the idea that all charged particles, including electrons, have an electric force field associated with them. Any other charged particle nearby experiences an electric force from the force field. (In some ways the electric force field is like the gravitational force field that exists all around the earth. But while anything in the earth’s gravitational force field will feel a gravitational force from the earth, only charged particles can feel one another’s force fields.) When an electron vibrates, it produces a disturbance in the electric force field - in other words a wave. (It also produces a magnetic field disturbance, which is not important here, but which explains why the waves are called 1 Interestingly, by the first decade of the 20th century, several important experiments had shown that sometimes light did not behave like a wave, but was more like a particle. The Quantum Theory was developed to account for these apparently conflicting behaviours of light. SOS Light Polariser 4 electromagnetic waves.) Figure 1 represents the electric field disturbance, or electromagnetic wave, that is produced by an electron vibrating up and down along the vertical axis. All electromagnetic waves travel at the same speed as light waves, but some have a much longer wavelength. The wavelength of a wave is the distance between crests of the wave (see figure 2). Radio waves have a much longer wavelength than light waves; x-rays have a much shorter wavelength than light waves. wavelength Figure 1: The varying electric force field produced long wavelength wave by a vibrating electron. short wavelength wave Figure 2: Long and short wavelength waves Radio waves are produced by electrons vibrating up and down inside a radio transmitter. The electromagnetic radio wave travels away from the transmitter and can be picked up by a radio aerial, as illustrated in figure 3. When the wave arrives at the aerial, it makes the electrons in the aerial vibrate, and the rest of the radio receiver translates this electrical signal back into a sound wave. Electrons vibrating up and down inside a radio transmitter produce an electromagnetic wave, which travels outwards in all directions from the transmitter Some of these waves may reach a radio-receiving aerial, in which case the electric force field disturbance will make electrons in the aerial vibrate up and down radio transmitter radio aerial (receiver) Figure 3: Radio waves produced by a transmitter, and picked up by an aerial SOS Light Polariser 5 The electric field disturbance is in the same direction as the motion of the electrons up and down the transmitter (and the receiving aerial); in other words the electric field disturbance is perpendicular (at right angles) to the direction in which the wave is travelling, as shown in figure 4. The same is true for all other forms of electromagnetic waves, including light waves: the electric field disturbance is in the same direction as the vibration of the electrons, and is at right angles to the direction in which the wave travels. Direction in which the radio wave is travelling Direction of the electric force field Figure 4: The direction of the electric field variation is perpendicular to the direction in which the wave is travelling Electrons in the surface of the filament may each be vibrating in different directions In an incandescent light globe the electromagnetic wave (light) is produced by vibrating electrons near the surface of the tungsten filament. The vibrations of these electrons are very small compared to the vibrations in the radio transmitter, and different electrons will each be vibrating in different directions. Each electron produces a bit of light wave with an electric field disturbance in the same The electric field direction as that electron’s own vibration. disturbance can be in all Overall, as shown in figure 5, the electric directions that are field vibrations in the light from the perpendicular to the filament can be in any and every direction in which the light direction that is perpendicular (crosswise) is travelling to the direction in which the wave is Direction in which moving. Electromagnetic waves like the light travels to these are said to be unpolarised. the observer Electromagnetic waves like those from the radio transmitter, where the electric field disturbance is in one direction only, are said to be polarised. Figure 5: Light from an incandescent globe is When light passes through a polarising unpolarised filter, the filter only lets through the light waves where the electric field vibration is in one particular direction. So the unpolarised light coming from the light source is polarised by the filter. If two polarising filters are used, and their polarising directions are the same then the polarised light from the first filter can also pass through the second filter. If however the polarising directions are at right angles, as in figure 6, then the polarised light that comes through the first filter cannot pass through the second. SOS Light Polariser 6 The first polarising filter only lets through electric fields in this direction: it polarises the light in this direction The second polarising filter can only pass light polarised in this direction, so no light gets to the observer Figure 6: Crossed polarisers in front of a light source Explaining those Polaroid sunglasses When light falls onto a shiny, opaque, surface some of the light is reflected, and some is absorbed by the reflecting surface. The incoming light ray is unpolarised. So the electric field disturbance is in all directions perpendicular to the direction of the incoming ray. Figure 7 shows only the vertical and horizontal directions, but this is enough to show what happens. The light energy is absorbed by electrons in the atoms of the reflecting material. So these electrons start to vibrate and send out light energy. It turns out that the electrons which are set vibrating by the horizontal part of the electric field disturbance are slightly more likely to send out the light that we see Figure 7: Light reflected from a non-metallic as reflected. On the other hand, the electrons that surface is partially polarised are set vibrating by the vertical part of the electric field disturbance are a lot more likely to produce energy that will be absorbed by the reflecting surface. This means that nearly all of the vertically polarised light is absorbed by the reflecting surface, leaving the reflected light somewhat horizontally polarised. For one particular angle of reflection the reflected light is completely horizontally polarised – all of the vertically polarised light is absorbed by the surface. This angle is known as the Brewster angle, after the person who first found it experimentally. Polaroid sunglasses have lenses made from polarising filters, positioned so that horizontally polarised light will not pass through. This means that you do not see most of the sunlight that is reflected directly from the sand or water and most other horizontal surfaces. All sunglasses absorb some of the light coming to your eyes, but only Polaroid sunglasses are able to reduce the glare from reflected light more than they reduce the amount of light that reaches your eyes in other ways. SOS Light Polariser 7 When unpolarised light is shone into the cloudy solution, the particles in the solution absorb the energy from the electromagnetic wave. This energy makes the electrons in the atoms of the solution vibrate in the direction of the electric field force, and the vibrating electrons then send out light waves. Remember that the electric field disturbance must be perpendicular to the direction in which the light is travelling. So, the light shining on the solution can only make the electrons vibrate up and down, or from side to side, or anything in between, but not backwards and forwards in the direction of the light beam itself. The electrons that vibrate up and down will send out vertically polarised light sideways, while the electrons that vibrate from side to side will send out horizontally polarised light directly upwards. If the light shining into the solution is vertically polarised, then scattered light can only be seen Figure 8: Unpolarised light shining into a cloudy solution, compared with light that is vertically polarised from the side, not from above, as shown in figure 8. Horizontally polarised light can only be seen from above. The air molecules that make up the earth’s atmosphere scatter light from the sun in much the same way as the particles in the cloudy solution. So the light that comes from the sky is at least partly polarised. Human eyes are not sensitive to the polarisation of light, so you do not see that the light from the sky is polarised, unless you look at it through a polarising filter. However, photographers sometimes use a polarising filter on their camera, which prevents the polarised light from the sky reaching the film. This makes the sky appear just slightly darker than it would without the filter, and enhances the contrast with the bright clouds. When you look at the light scattered from the cloudy solution, the degree of polarisation, and the direction of the polarisation depend on where you look from. The same applies in the atmosphere: the amount and direction of the polarisation in the light from the sky depends on the direction in which you are looking relative to the sun. A solar compass can find the direction of the sun by analysing the polarisation of the light coming from the sky. This provides a means of determining direction, and is especially useful near the earth’s poles where a magnetic compass is useless. Many insects seem to navigate by the polarisation of light from the sky. The wiggle dance of bees, in which they let others in the hive know where food can be found, seems to depend on their being able to ‘see’ the polarisation of light. In fact, bees may see patterns of polarisation as a pattern of colours. Birefringence and optical activity Some substances - like the sticky tape and transparency sheet - change the direction of polarisation of light. This allows some light to pass through the crossed polarising filters. The direction of polarisation is changed by a different amount for different colours, which is why you see coloured effects. The thickness of the material also affects the amount by which the direction SOS Light Polariser 8 of polarisation is changed, which is why overlapping layers of sticky tape give rise to different colours. This effect is a rather complicated result of an unusual property of materials like the transparency sheet. Light travels at different speeds through these materials, depending on its direction of polarisation, and they are referred to as birefringent materials. Many biological substances, such as proteins and nucleic acids, cannot be seen with a normal microscope. However, they are birefringent, and so they can be seen in a polarising microscope, which has built in polarising filters. Some substances are not normally birefringent, but can be made so by stretching, twisting or otherwise stressing them. This provides a useful way of investigating the forces in machine parts. A plastic model of the part is made, and viewed between crossed polarisers. Then the same kinds of forces are put on the model that the real part will get in actual use. The rainbow pattern that is seen can be used to work out where the forces are most concentrated, and therefore where the part needs to be made strongest. Other structures such as buildings and bridges can also be tested in a similar way. Some other substances are able to rotate the direction of polarised light. Such substances are said to be optically active. Although optical activity produces effects that are rather like birefringence, the two things are actually quite different. Molecules in optically active substances usually have a spiral shape, and the way the spiral is wound affects which way the molecules rotate the direction of polarisation. Most sugar molecules found in living things rotate the direction of polarisation to the right, while the proteins found in living things rotate it to the left. Some more activities with your polariser Polarisation of direct sunlight If you were not able to look at light coming from the sky during activity B3 above, try some other time. This activity shows that light scattered from the sky is polarised. If you want to investigate the polarisation of direct sunlight it is really important that you DO NOT LOOK DIRECTLY AT THE SUN. If you do look directly at the sun you may damage the retina of your eye. This will cause blindness, although the damage may not be immediately obvious. A safe way of investigating direct sunlight is to use a pinhole. To do this you will need a piece of cardboard (from the back of an A4 notepad for example), a large piece of white paper such as butchers’ paper, a sharp implement such as a meat skewer, and some sticky tape. Make a small hole in the centre of the piece of cardboard, a few millimetres in diameter. Use the sharp implement, but keep your hands well clear of the hole as you push the implement through. Stick the piece of white paper on the ground, in direct sunlight. Alternatively stick the paper on a wall that is in the direct sunlight. Stand with your back to the sun so that the piece of paper is slightly in front of you, and just to one side. Hold the piece of cardboard on the same side as the piece of paper, so that the sunlight can shine through the hole onto the piece of paper. If the image of the sun on the paper is too faint, make the hole in the cardboard slightly bigger. You can investigate the polarisation of the sunlight by getting a friend to hold one of the polarising filters between the holes and the piece of paper. Rotate the filter to see whether or not the light is polarised. (This method of looking at the sun is also safe to use during a solar eclipse. Even if the sun is obscured during an eclipse, is still dangerous to look directly at the light. You can also observe sunspots in this way.) SOS Light Polariser 9 Polarisation and birefringence outside the laboratory Use your polarising viewer to investigate the polarisation of light reflected from roads and pavements, water surfaces, car windows and car bodies, steel, chromium or enamel cookware, knife blades, aluminium foil, and anything else that interests you that you can view with safety. If you have an opportunity try looking at a rainbow with your polariser. You should find that the light reflected from the rainbow is polarised parallel to the rainbow arc. You can make your own rainbow with the garden hose: just remember that to see the rainbow you have to look at the water drops with the sun is coming from behind you. Look around for birefringence effects. Try car windows, glass kitchen ware, a chip of ice, clear jelly, plastic items such as CD covers and chocolate boxes and any other items mentioned in the Polaroid art activities that you were not able to investigate then. You can also look for optical activity in other kitchen substances. Try a salt solution, and turpentine (but remember turpentine is highly flammable!). If your school has suitable photographic equipment you can try taking photographs of clouds with and without a polarising filter on the camera lens. You need a sunny day when there are bright white clouds in a blue sky. The orientation of the filter on the lens will be important! Water reflections are a bit harder to come by, but a polarising filter can make a difference to photographs of such reflections. More applications of polarisation To find out more about the uses of polarised light have a look at the home page of the Polaroid Corporation, at www.polaroid.com. Most of the products now made by Polaroid have little to do with polarisation of light, but the Company name derives from the polarising material which was one of its first products. On the home page select “Polaroid Products”, scroll down to “OEM Products”, and then select “Applications”. The first three listed applications depend on circular polarisation of light. All of the activities in the SOS Light Polariser Session were examples of linear polarisation of light. Circular polarisation is a good deal more complicated than linear polarisation, so the last two applications will be easier to understand than the first three. However, you can get a good idea of these other interesting applications without necessarily understanding the details of circular polarisation. These notes were prepared by Christina Hart, with help from David Mills, Keith Thompson (both of Monash University, Department of Physics), and Dan O’Keeffe (Camberwell Grammar School), for the Education sub-Committee of the Australian Institute of Physics (Victorian Branch). Layout and graphics by Bronwyn Halls. The Australian Institute of Physics (Victorian Branch), 1999. SOS Light Polariser 10