New topics in physics education: Polarizers, polarized light, liquid crystals and LCDs

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1 New topics in physics education: Polarizers, polarized light, liquid crystals and LCDs Mojca Čepič Faculty of Education, University of Ljubljana, Slovenia September 2015, Krakow

2 Outline Why new topics in education? Our experiences: Liquid crystals and polarization Two examples: How to teach LCD How to teach polarization Conclusions Advertisements

3 Novel scientific results in education - WHY? Fundamental research, what is it? Soft matter Astrophysics Particle physics Applications. What do we teach? What do students learn? Fundamental but old physics Lack of context Emphasis on abstract considerations

4 Novel scientific results in education - WHY? How do we often teach? Traditional? - Lecture, equations, proofs by experiments - Many idealizations: friction, mass, tension,. The context? - Is the studied topic relevant for every day life? - Do everyday experiences exist and can be recalled? Is a research in considered field still active? - Are research questions interesting for students? - Can students experience a bit of scientist s passion?

5 Is there any theory? Not with respect to this specific question: How to introduce new scientific results to education at various levels? Theories to start from How students learn Importance of preliminary knowledge Constructivism, providing experiences Teachers education.

6 Our experience: Liquid crystals The main motivation for the choice of topic: My personal attitude and knowledge. Questions to be aswered: - Do easily observed phenomena exist? - Are easy experiments possible? - Are liquid crystals relevant for every day life? - Aesthetic, beautiful - Are they explainable on the conceptual level? - Do students have enough preliminary knowledge?

7 The light motiff: What do we watch every day Liquid crystals in displays Mobile phones Tablet computers Portable computers Computer screens TV screens Digital watches Step counters...

8 The teaching module: How does a LCD operate? How colours on the screen are formed? How is the pixel constructed? Polarizers and anisotropic materials Liquid crystals and their properties Ordering of liquid crystals Influencing the transmitted light via LC Manipulation of LC structure

9 How colours on the screen are formed?

10 green yelow white a part of the picture

11 Additive colour mixing green + red = yelow green + blue = cyan blue + red = magenta green + blue + red = white

13 Which part of this picture is this?

14 Elementary unit of LCD pixel Pixel has three parts, one for each color; brightness of each part can be manipulated.

15 Structure of the screen Planinšič, Gojkošek, EJP 2011

16 Pixel s structure Polarizer Glass Electronics Liquid crystal Conductive layer Nonpolarized light Conductive layer Layer for orientation Layer for orientation Color filters Glass Polarizer Red 255 Green122 Blue 0

17 Pixel s structure Polarizer Glass Electronics Liquid crystal Conductive layer Glass Polarizer Nonpolarized light Conductive layer Layer for orientation Layer for orientation Color filters

18 Polarizers Crossed polarizers Parallel polarizers

19 But: The polarized light is difficult Students are familiar with linearly polarized light only.

20 Polarization states Non polarized light Partially polarized light Polarized light Linear polarization Circular polarization Elliptical polarization

21 Methods of light polarization Linearly polarized light Phenomenon: Linear dichroism One polarization is more absorbed Linear polarizers Sun glasses 3D glasses Outer layers of LCD

22 Examples Regular sunglasses Liquid crystal display

23 Optical properties of 3D glasses. What are their drawbacks?

24 Isotropic / anisotropic materials between crossed polarizers

25 Methods of light polarization Circularly and elliptically polarized light Phenomenon: Linear birefringence Propagation of one polarization is slower. Transparent anisotropic materials. Adhesive tapes. Transparencies. Liquid crystals.

26 How do round glasses differ from previous glasses? The two situations: - Incident light is polarized - Transmitted light is analyzed

27 Circular polarizers Why are they used?

28 Photographic circular polarizers

29 Optical activity Circular dichroism Light with left or right handed polarization is absorbed more than another. Circular birefringence Light with left or right handed polarization propagates faster than another. Many misunderstandings with this respect.

30 Pixel s structure Polarizer Glass Electronics Liquid crystal Conductive layer Glass Polarizer Nonpolarized light Conductive layer Layer for orientation Layer for orientation Color filters

31 What is a liquid crystal? usual materials crystal liquid liquid crystals crystal liquid crystals liquid

32 Phase transitions liquid crystal liquid liquid crystal crystal

33 The meaning of the name Properties of liquids they flow Properties of a crystal they are microscopically ordered they have anisotropic properties

34 What is called a liquid crystal? One phase has to be a liquid crystalline phase.

35 A sandwich of a liquid crystal between is transparent. The liquid crystalline phase Liquid crystals in the liquid crystalline phase are anisotropic.

36 Photo: Maja Pečar

37 heating The anisotropic liquid crystalline phase The isotropic phase

38 Anisotropy Structure with an orientation leads to anisotropic properties.

39 Molecular properties... Elongated molecules 5CB cholesterol OOCBP DOBAMC bananas

40 Properties of liquid crystals One additional LC phase (at least) Anisotropic Two refraction indexes The ordering is needed Influence the polarization of light Are easily manipulated by external fields

41 Where does one get liquid crystals for experiments? Friends in research laboratories Proffesional providers: Merck, By the synthesis in the school lab

42 Synthesis in the school laboratory O O cholesteryl benzoat N O CH 3 Synthesis of cholesteryl benzoat High school Vič, Ljubljana (A. Mozer) MBBA

43 Synthesis O Cl + N - HCl O HO O holesteril benzoat cholesteryl benzoat O CH 3 NH 2 + O H C O CH 3 - H 2 O N MBBA oz. N-(4-metoxibenzylidylen)-4-butylanylin

approx 100 000 molecules Along the short axes

44 Why does LC look milky? 3 nm 0.5 nm Cluster μm Along the long axes approx molecules Along the short axes approx molecules Schematic cartoon of the nematic phase

long molecular axe - Clean surface is rubbed by velvet - Long

detergent - Hydrophobic heads stick to the glass - Long molecules

45 LC have to be ordered. How? Planar alignment of long molecular axes Homeotropic alignment of long molecular axe - Clean surface is rubbed by velvet - Long molecules order along the scratches - Clean surface is dipped in detergent - Hydrophobic heads stick to the glass - Long molecules order along the hydrophylic tails of detergent molecules Ordered liquid crystal are very anisotropic: Δn 0.3

46 How to observe anisotropic properties of liquid crystals It is necessary to make a cell thinner than the cluster size. What do you need? - An object slide - A cover slide - An adhesive tape - A liquid crystal For planar alignment - Some velvet for rubbing For homeotropic alignmet - Some detergent Object glass Rubbing direction Cover glass Tape

47 Rubbing direction Making the cell Object glass Cover glass Tape a) Rubbing determine the direction of long molecular axes

48 Rubbing direction Making the cell Object glass Cover glass Tape b) Tapes determine the distance between glasses the spacers

49 Object glass Rubbing direction Cover glass Tape Making the cell c) Adding a small amount of LC c) A small amount of liquid crystal is added

50 Object glass Rubbing direction Cover glass Tape Making the cell c) Adding a small amount of LC d) A cover slide is placed on the LC forming a bridge between the spacers

51 e) The cell is inserted between crossed polarizers and is observed under the microscope

52 1 cm 1.5 cm Professional experimental LC cell Due to birefringence the colour appears in thick cells.

$Birefringence another phenomenon: Double refraction The light splits into two beams polarized perpendicularly to each Other$

53 Birefringence another phenomenon: Double refraction The light splits into two beams polarized perpendicularly to each Other with different speeds of light and refraction indexes. Refraction indices in liquid crystals differ significantly ( ).

54 Preparation of the wedge LC cell layer of foil microscope slide Check the polarization of the beams. direction of rubbing cover glass Find the area on the cell where the laser beam splits into 2 beams which can be observed on the remoted screen. What happens if we heat the cell? 54

55 Pixel s structure Polarizer Glass Electronics Liquid crystal Conductive layer Glass Polarizer Nonpolarized light Conductive layer Layer for orientation Layer for orientation Color filters

56 How does it work? polarizer analyzer

57 Display Polarizer Analizer Electric field orients elongated molecules along the electric field.

58 Model of a pixel

59 Model of a pixel

60 Do we understand how is the pixel manipulated?

61 Our questions - Explainable on conceptual level - Enough preliminary knowledge - Easily observed phenomena - Possible experiments - Relevant for every day life - Aestetic, nice, if possible

62 LC Module For the first year of primary school teachers min lectures (the concepts, ) - 90 min chemistry lab (synthesis of LC) - 90 min physics lab (transition temperatures, polarization and polarizers, cell fabrication, double refraction). For the first year of future physics teachers 2 x 90 minutes lectures, 2 x 90 minutes physics lab

63 Preliminary knowledge Have students heard anything about LC before? Two groups were tested: - Students in the first year at the Faculty of Education (N=448) Average: 1.5 points out of 8 possible. LC screens and optical properties. - Students in the first year at representative Faculties at the University of Ljubljana (N=1121) - Science and technology - Social studies and humanistic sciences - Law - Medicine

64 Results Pre-service 1st year primary school teachers: N=90 Methods: pre-test, classroom observations, worksheets, tests and semi-structured interviews. Pre-test: 28 short questions (15 20 minutes) 7 Q general data, 19 Q LC Test 1 (immediately after): 17 short questions Test2 (a part of final exam 4 weeks later): 17 Q

65 Percentage of students Comparison of the three tests Percentage of achieved points Pre test Post test Exam

66 Why, What, Whom and How WHY Motivation WHAT Learning objectives WHOM Audience HOW Methods EVALUATION

67 Why new topics in education? Our experience: The liquid crystals A suggestion of theoretical framework Metastudy Conclusions

68 Examples Our experiences and considerations Other reported examples Metastudy Where to look (journals?) What to choose (which reports?) Criteria

69 Paper I Paper II

70 Paper III American Journal of Physics Paper IV Physics Education 2006

71 Paper V American Journal of Physics Paper VI American Journal of Physics Paper VII American Journal of Physics

72 Papers VIII The Physics Teacher (2015) Light-Emitting Diodes: - A Hidden Treasure - Exploration of Underlying Physics - Learning New Physics - Solving Complex problems

73 Criteria Does the topic consider ongoing research? Which level (kindergarten. graduate) What should students learn Necessary preliminary knowledge Necessary teacher s knowledge Methods Development of experiments Was it tested in the classroom? Evaluation

74 Topic Liq. Cryst. Level Aims Preknow Semi- Cond. On going yes High Sc. Int.Phys yes yes Stand Pract yes High Sc. yes yes Stand Pract Friction yes High Sc. Int.Phys Superc onduct. Optics Lab. Single foton Laser physics yes Teacher s yes no Stand Pract Method Exp Test Eval yes yes yes quanti activ yes yes both yes no no general no Descr Descr no no yes Uni general no Lab yes no no older Uni general no Lab yes no no older Uni general no Lab yes no no LED yes High Sc. Int.Phys yes no Pract yes no no

75 The 4th W: WHO Who should do the job? The researcher with a sharing desire Help The PER people and educators The teachers

76 Tasks Choose the topic Determine the level Define the goals Test the preliminary knowledge Verify the curriculum Decide for the methods Design experiments Design the teaching module Prepare the materials Test and evaluate the module Design the teachers education and materials Make experiments available

77 Conclusions Current research in the school Our experiences: Polarized light and LC Theoretical considerations Other examples Report on the metastudy

Liquid crystal through experiments by Mojca Čepič Publisher: CONCISE PHYSICS, Morgan & Claypool and Institute of Physics joint publishing Electronic version in December 2014 Paper

78 Liquid crystal through experiments by Mojca Čepič Publisher: CONCISE PHYSICS, Morgan & Claypool and Institute of Physics joint publishing Electronic version in December 2014 Paper version in April 2015 Short introduction to liquid crystals Experiments with liquid crystals Understanding a liquid crystal display Accompanying experiments The teaching module Further reading

79 Acknowledgement Financial support of projects (Agency of research and development Slovenia) is warmly acknowledged: J5-4002: Introduction of modern interdisciplinary topic into education Liquid crystals ( ) J5-0365: Teaching and learning more demanding interdisciplinary topics related to physics ( )

80 Ljubljana team Dr. Katarina Saša Dr. Maja Jerneja Dr.

MP5: Soft Matter: Physics of Liquid Crystals

MP5: Soft Matter: Physics of Liquid Crystals 1 Objective In this experiment a liquid crystal display (LCD) is built and its functionality is tested. The light transmission as function of the applied voltage