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Polarisation

Introduction
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

Packing Away: 

Put everthing 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.

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.

4. Uses of polarisation

Possible uses of polarisation - polaroid sunglasses, measuring stress on equipment, analysing substances, etc.

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.

Risk Assessment
Date risk assesment last checked: 
Thu, 29/12/2016
Risk assesment checked by: 
Andrei Ruskuc
Date risk assesment double checked: 
Wed, 01/02/2017
Risk assesment double-checked by: 
Mattlem
Risk Assessment: 
DESCRIPTION Using a light box with a crossed polars, polythene strips, and plastic rulers.
RISKS
  1. Cuts from broken objects (such as rulers)
  2. Trip hazard if placed on floor
  3. Electrical hazard
  4. Box may get hot
ACTION TO BE TAKEN TO MINIMISE RISKS
  1. Do not allow children to bend items to point where they are likely to break. Remove items which are broken.
  2. 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.
  3. See electrical parts RA
  4. Turn off box between demonstrations to prevent excessive heating
ACTION TO BE TAKEN IN THE EVENT OF AN ACCIDENT
  • Call first aider in case of injury
  • This experiment contains mains electrical parts, see separate risk assessment.
    This experiment is sometimes run in a darkroom, see separate risk assessment.
    Images
    Publicity photo: 
    Experiment photos: