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Polarisation

Introduction
Public summary: 

Experiment with the properties of light as certain filters block light, but only in certain directions!

Discover how polaroid filters block light as you turn them round
Useful information
Kit List: 

- Blue light box
- Two polaroid filters.
- Some bits of clear polythene
- Rulers, set squares etc, for bending.
- Some pieces of calcite
- A partially dismantled LCD screen
- Card model for demonstrating birefringence
- Slinky
- A bunch of photographic CPL (see explanation)

PLUS only:
- Two circularly polarising glasses
- Michel-Levy Chart + diagram of LCD screen and of twisted nematic structure

Packing Away: 

Put everything in the box, including the cable.

Frequency of use: 
5
Explanation
Explanation: 

OVERVIEW


Demonstrating the use of polarisers with crossed polars, polythene strips, plastic rulers and a light box.

Possible activities:
- Demonstrate how crossed polars work by rotating two pieces of polaroid with respect to each other and looking through them to see how the light changes.
- Looking at stressed plastics under polarisers.
- Demonstrate the colour change when a strip of polythene is stretched when viewed under the polariser material.

Other things to talk about:
- LCDs and how they work.

Tips for demonstrating:
- Let the children experiment with different objects under the polarisers as this will keep their interest.
- Use paper strips with waves drawn on to help to demonstrate how polaroid filters work.

BASIC PROCEDURE AND EXPLANATION


1. Show them the light box and the polarising filters

**Top tip: scroll down if you're reading this online- there's photos of the various bits of kit!**

- The box gives out "normal" white light. Experiment generally works best if set on the floor/ somewhere low enough that everyone can easily see from directly above.
- Take the top piece of polariser and give it to them, get them to look at it in the light. What does this look like -> grey piece of transparent plastic.
- Put the polariser on the light box the way round so light can get through. Still look normal?
- Get them to rotate the polariser around, and see what happens... This normally gets their interest. Get them to turn it upside down.
- Ask what is happening: looks almost black one way round, turn it 90 degrees and it looks bright again

2. Introduce the idea of light as a wave

Do they know what a wave is?

- Ask them what types of waves they know about: sound, water, etc.
- Can they make a wave along their arms? (An awesome dance move if you can pull it off!)

Light behaves like a wave, this wave can be in all directions

- Explain that light is a wave using a wave sketch on a bit of paper
- Light can wobble in different orientations - polarisations. It can wobble up-down, side-to-side, and everything in between
- You can imagine this as a 2D wave like the one sketched on the paper
(Yes, we know that there's a second component of the wave, but if you consistently talk about the same component of the wave the explanation is right..)

Polarising filters are like grills that let light through in only in some directions

- Using your fingers as a grill demonstrate what the polariser looks like. Get them to try and get the piece of paper in the through the grill parallel and perpendicular to the grill, i.e. in the two different polarisations.
- Only half the light can get through - this is why the plastic looks slightly darker normally.
- Then add another grill parallel to the first which will allow the light from the first grill to get through. Rotate it by 90 degrees to demonstrate that light can't get through with any orientation. This makes the polariser look black.

- You'll have lots of hands available, you can show the effect of as many twice as many polaroid filters as you have people! You can show that if the light passes through the first filter if it stays at that orientation it can't get through the second one.

Demonstrating polarisation with a slinky

If you have the space and at least 3 people, then instead of using finger grills, you can illustrate polarisation using the slinky:
- Get someone to hold each end of the slinky.
- Tell one of them to shake their end up and down, sending waves along the slinky. Then have them shake the end from side to side, sending waves with a different polarisation.
- Get someone to stand with their legs on either side of the slinky, acting as a polarising filter. Now when waves are sent down the slinky, only up-and-down waves can pass through.
- You can then stand over the slinky as a second polarisation filter, and show that nothing changes.
- Then, sit down with one leg above the slinky and one below it, so that the filters are crossed. Now no waves can get through both filters!

3. Twisting the direction of the light

Demo with the filters

- Ask them what you would have to do to the light (wave drawn on paper) between the crossed polars to get it through. They normally tell you to twist it.
- You can introduce a third polar at 45 degrees to do this (current kit only has two though, unless you use the sunglasses)

Polythene (bags) can twist light

- The polythene normally doesn't affect the light but if you stretch it, it will twist the light.
- Let them have a go at stretching the polythene strip in the light box under the polaroid filter (this can take a while as they find it fascinating!). You should see many colours.
- The different colours twist the light by different amounts. So pick a red bit (best as they know what colour you get if you mix blue and green). You may rotate red by 90deg so it will get through, blue by 180deg so it won't, and green by 360deg so it won't...
- Now get them to look at a red bit while you turn the polariser through 90deg - it should now look turquoise... Now the polars are parallel so red is twisted 90deg so it doesn't get through, blue is by 180deg so it does... -> turquoise light gets through.

Plastic rulers can twist the light

- Put a ruler between crossed polars, you should see lots of colours. These are because they are made by injection moulding, so the plastic is effectively stretched in manufacture.
- The place where the most stretching happens is where the plastic was squirted in - you should be able to see this and you can probably see the rough bit where the sprue was attached.
- There are some rulers that have been cooked, and you should see the shape has changed most in the place where there was the most stress.
- You can also bend the ruler and see stress. A ruler with a crack in it should concentrate stress.

Calcite crystals can twist light

- The crystals of calcite twist light as it travels through, so can appear lighter or darker than the background when placed between the crossed polarisers.
- If v. keen can try and explain birefringence. (Can the person who added this try and explain as I don't know what it is - TW)

CPLs

- Photographic circular polarisers (CPLs) are directional. In the direction of photographic usage, they feature a linear filter (distal of the sensor), followed by a quarter-wave plate (proximal) The latter turns the (now) linearly polarised light into circularly polarised light.

4. Uses of polarisation

Possible uses of polarisation (you don't have to mention all of these!):

In physics: light reflecting from a surface or scattered from a material is partially polarised, and polarising sunglasses use this to cut out glare.

In chemistry and materials science: certain molecules rotate polarised light, and we can use this to identify and analyse substances.

In engineering: observing a material undergoing stress through crossed polars

In biology: some animals (such as certain insects) use polarised light for navigation, since the sky is naturally polarised, and even humans can observe polarised light with practice due to a quirk of biology: https://en.wikipedia.org/wiki/Haidinger%27s_brush

In geology: certain rocks give different colours when placed between crossed polars (see Michel-Levy chart in box)

3D cinema glasses (there's some in the box) use circular polarisers. More info: http://en.wikipedia.org/wiki/Polarized_3D_system#Circularly_polarized_gl...

5. Extension: LCD screens- how they use polarisation

The black object with a window and several buttons is an LCD which has had the polarisers removed (and the wiring completely mangled) so you can see that they work through polarisation. Look at it in normal light, then in between the crossed polarisers.

The display consists of two pieces of glass with a 'liquid crystal' in between. This consists of long rod shaped molecules which move around at random like a liquid, but are all aligned like a crystal. There are lines scored on the glass and the liquid crystals tend to align along them, the lines on the top are at 90 degrees to those on the bottom, so the molecules twist as you move through the liquid crystal.

If polarised light passes through the liquid crystal the light rotates by 90 degrees, however if you apply a voltage between the two glass plates by pressing the buttons, the rods rotate so they are end onto the light and stop rotating the light.

So by applying a voltage you can turn on and off the rotation of the light, which with 2 polarisers means you can make it go from clear to black, and by patterning some wires on the glass you can produce a display. which are used everywhere from watches to TVs. This is why if you look at a monitor through a polariser the image can disappear by rotating it.

PLUS Explanation

This explanation is intended to serve as an addition to the main explanation.

Additional points to include when demonstrating to taller than average children (use your own judgement as to how interested the students are, and which topics are appropriate to their subject area - sometimes it might be better to stick to the main explanation):

Malus's Law

For students interested in maths and physics, you can derive Malus's Law for the intensity of light transmitted through a polarising filter - which is that the transmitted intensity is cos^2(θ) relative to the incident intensity, where θ is the angle between the axis of the polariser and the polarisation of the light - using a fairly simple argument which doesn't require too much maths.

Show the students that you can write an arbitrary polarisation as the sum of two polarisations, one parallel to the axis of the polariser and one perpendicular to it, using arrows (draw a right-angled triangle). Since the length of the side parallel to the axis is cos(θ) times the length of the hypotenuse, the amplitude is reduced by cos(θ). Intensity is amplitude squared (possibly use the analogy that kinetic energy depends on velocity squared), so the intensity is proportional to cos^2(θ).

If they're especially keen you can get them to sketch this. To explain why the curve is smooth and not pointy when it hits the x-axis, you could talk about x vs x^2.

Birefringence

The box contains two pieces of card, with waves drawn on them, which slot together to form a model which can be used to demonstrate birefringence. By explaining that in certain materials one polarisation will travel slower than the other (possibly with reference to a drawing of a polymer structure, to explain why this happens), you can then demonstrate with the model that if one of these polarisations is shifted by half a wavelength the overall polarisation will rotate by 90°. You may need to explain that an arbitrary polarisation can be broken down into components parallel and perpendicular to the slow axis of the material.

You can then explain that, since the rotation depends on the second polarisation being shifted by a integer-plus-half multiple of the wavelength, only certain wavelengths of light will be transmitted through the second filter. This accounts for the colours observed - an extinction spectrum. You can use the Michel-Levy chart to show how the colour depends on the thickness of the material. The sellotape board is a good prop for explicitly showing this dependence, since the colours only change when pieces of tape cross over.

You can also expand on what sort of materials exhibit birefringence - typically these are materials with some sort of preferred direction, such as polymers in which the molecules are aligned in a certain direction (e.g. due to injection moulding - this can be seen in the pieces of ruler) or certain crystal structures.

The calcite is more strongly birefringent and thicker than the rulers: if you place the calcite over a line and look through it, you should see the line splitting into two (provided the calcite is clear enough!). This could be related to the difference in 'speed' between the two axes corresponding to a difference in refractive index. The difference in refractive index, through Snell's law, creates a difference in the angle of the light leaving the block, causing two images to be seen (Double Refraction).

LCD Screen

The box contains a diagram of a twisted nematic structure (as found in a liquid crystal), as well as the construction of an LCD screen - use this to explain how it operates.

The liquid crystal molecules in the screen, under normal conditions, take the structure of a helix with a 90° twist from top to bottom. This will rotate polarised light passing through by the same angle. However, if a voltage is applied to the crystal, the molecules instead line up with the electric field, breaking the structure and preventing the rotation of polarised light. This causes a liquid crystal sample between crossed polars to go dark. This can then be exploited to build a display.
You could comment on how the screen uses ambient light, and not backlit, being energy efficient.

Circular Polarisation

You can also use the model to demonstrate how circularly polarised light is possible - by shifting one of the pieces of card by 1/4 of a wavelength, you can show the direction of polarisation rotates around as you move along the wave. Use this to explain how 3D glasses work, and ask them why it's beneficial to use circular polarisation and not linear polarisation in that scenario.

Brewster's Angle

If there's time, and the conditions are right, you could comment on the Brewster Angle. At this angle unpolarised light incident on a reflective surface will be reflected with a plane of polarisation parallel to the surface. If there is a shiny horizontal surface (metal is best) in the area, you could ask them to look at it through a linear polariser (ideally polarising sunglasses if there are any to hand). With the polariser in a certain orientation, the glare should be greatly reduced. This is how glare reducing sunglasses work.

Risk Assessment
Date risk assesment last checked: 
Sun, 19/01/2020
Risk assesment checked by: 
Esmae
Date risk assesment double checked: 
Sun, 19/01/2020
Risk assesment double-checked by: 
jpichon
Risk Assessment: 
Hazard Risk Likelihood Severity Overall Mitigation Likelihood Severity Overall
Broken objects (i.e. rulers) Possible cuts from sharp edges. 3 2 6 Do not allow children to bend items to point where they are likely to break. Remove items which are broken.
Call first aider in event of injury. Stop experiment if required.
2 2 4
Light box and cables Box is a trip hazard if placed on the floor. Electrical cables also present a trip hazard. 4 3 12 Make sure equipment is safely and securely placed, at the side of the dark room out of the way of where people are walking. Do not allow the power cable to run across a walkway.
Call a first aider in the event of an emergency.
2 3 6
Light box Electrical hazard 4 4 16 See electrical parts RA 2 3 6
Light box (bulbs heating up) Possible burns due to contact with hot surface. 3 3 9 Turn off box between demonstrations to prevent excessive heating, or otherwise monitor for overheating and be prepared to take a break and let it cool down.
Call a first aider in the event of an emergency.
2 3 6
Fast moving slinky Someone could get hit by the slinky or trip over it. 3 2 6 Don’t have the slinky across somewhere people will be walking, and make sure that no one is standing where they could be hit.
Call a first aider in the event of an emergency.
2 2 4
This experiment contains mains electrical parts, see separate risk assessment.
This experiment is sometimes run in a darkroom, see separate risk assessment.
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Images
Publicity photo: 
Experiment photos: