Computer screens and sunglasses

Transcription

1 Nowadays, lots of computer display screens are of a type called an LCD screen. LCD stands for liquid crystal display. If you ve used a calculator, MP3 player, mobile phone, laptop or flat screen TV today, you almost certainly would have been using an LCD screen. The secret behind all of these displays is something called a liquid crystal. Liquid crystals are a special kind of chemical that is halfway between a normal liquid and a solid crystal. But liquid crystals are also able to change something special about light passing through them. To understand this, we ll show you how to try a really simple experiment at school or at home. Then we ll look into how these marvellous LCD screens actually work. LIGHT FANTASTIQUE Light is made up of waves that vibrate up and down at right angles to the direction they are travelling in, just like the picture below. But a light wave can be twisted round so that it vibrates up and down vertically, left and right horizontally or diagonally at any angle in between. A polarising filter is a special piece of glass that only lets through light waves that are vibrating in one direction. Light that only vibrates in one direction is called polarised light. So you can use a polarising filter to let through only light that is polarised in a vertical direction. Or you can twist the filter around by 90 degrees so it only lets through light that is horizontally polarised. Let s try a simple experiment to explore polarisation. 1

2 You will need: sunglasses with polarising lenses Blu-Tack a torch (or you could just use the bright light coming in through a window) a glass of water sugar. Ease out the lenses from an old pair of polarising sunglasses (ask your mum before you start wrecking her favourite pair of Guccis!). Look through one of the lenses at the torch and you ll see that the light looks dimmer (that is the point of sunglasses after all). You might find it easier if you lay the torch down on a table and stick the polarising lens upright in a blob of Blu-Tack. Hold the second sunglass lens up the same way as the first lens. Look through both the lenses at the torch light. You ll still be able to see the torch light it won t look much dimmer than with just the one polarising filter. Now start slowly twisting the sunglass lens in your hand. Notice how the torch light coming through the two lenses becomes dimmer? It will be completely black when you have twisted by 90 degrees (a right angle). As you keep twisting round, the light coming through the two lenses gets brighter again. What on earth is going on? The first sunglass lens put in front of the torch acts to polarise all the light coming through it in one direction. If you hold the second filter in the same way then it will let this light straight through (because it has already been polarised in the right direction). As you twist the second filter round, it blocks more and more of the light from getting through. Eventually, when the two filters are at 90 degrees to each other, they block light polarised in any direction so no torch light gets through. Make sure the second polarising filter is turned at 90 degrees to the first so that they don t let any torch light through, and then stand it in another blob of Blu-Tack. Leave a space between the two lenses wide enough to stand the glass of water. Now for the sugar! Try to dissolve as much sugar as you can in the water. Stir it until all of the sugar grains have dissolved and stop adding sugar when you notice that it isn t dissolving any more. You ve made a saturated sugar solution. 2

3 Place this glass of sugar solution between the two sunglass lenses (polarising filters). Look again through the second sunglass filter, the glass of sugar solution and the first sunglass filter at the torch light. What do you see? If this has worked properly, you should see a little torch light coming all the way through, even though the two polarising filters are turned 90 degrees to each other. Keep looking through to the torch and carefully take the glass of sugar solution away again. The polarising filters go back to looking completely black. Replace the glass of sugar solution and some light gets through again. Can you guess what s happening? What is the sugar doing to the polarisation of the torch light? The first sunglass lens (filter) is only letting through light of a particular polarisation, let s say vertical. The second filter is turned at 90 degrees horizontally, so it cuts out all the light. When we put the glass in between the sugar actually twists the vertically polarised light round so that some of it gets through the second filter. 3

4 LIQUID CRYSTALS Liquid crystals work in exactly the same way they twist the direction in which light is polarised. LCD screens have a layer of liquid crystals placed between two polarising filters turned at right angles to each other. And, if you pass electricity through them, liquid crystals swivel around, which changes the way they twist the polarisation of the light. But the liquid crystals don t twist the light coming through the LCD screen by the right amount, so it is blocked by the second polarising filter, which makes that part of the screen look dark. By wiring up the LCD cleverly you can create different shapes on the screen (like letters and numbers). You can also turn the shapes light and dark by switching the electricity off and on to different parts of the liquid crystal layer. This is how the numbers are displayed on a calculator screen. Nifty! LCD computer screens and TVs are made slightly differently. They also use a layer of liquid crystals to twist the polarisation of light passing through the screen and electrical wires to switch different sections between light and dark. But LCD TV screens are wired up in such a way that the liquid crystal layer is divided up into a fine grid of squares. Each tiny liquid crystal square is called a picture element, or pixel. The TV can turn any of these pixels white or black to build up a detailed image in shades of grey. The PDF In the Picture has more about pixels and computer images. To show colour on an LCD screen each pixel is divided into red, green and blue stripes. A tiny colour filter is placed in front of each of the stripes, and by varying the brightness of all three the pixel can be turned into any colour you like. The zoomed-in view below shows the three colour stripes that build up a vibrant picture on a TV or computer screen. 4

5 Peter Halasz So there you go now you can tell your friends all about how the brightly coloured screen on their mobile phone is just like a pair of sunglasses and a glass of sugar water! 5