IB Physics Polarisation flippingphysics by Nothingnerdy


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Slideshow on Polarisation for IB HL Waves Phenomena; also SL option on Sight and Waves. Flippingphysics by Nothingnerdy

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  • IB Physics Polarisation flippingphysics by Nothingnerdy

    1. 1. presentsa production Polarisation 1
    2. 2. EM waveAn electromagnetic wave consists of sinusoidally varying electric and magnetic fields whichoscillate at right angles to each other. The wave energy travels perpendicularly to these fields. Electric field Wave energy Magnetic field
    3. 3. Plane polarisation Light from most sources eg the Sun has its electric and magnetic fields in all showing only E-field directions perpendicular to the direction of travel. If the electric field onlyvibrates in one direction (with Polarised the magnetic field at right angles to it), the wave is polarised.
    4. 4. Polaroid A plastic which polarises light as it passes through it.If the plane of the If the plane of the If the plane of thepolaroid is parallel polaroid is polaroid is at an to the electric perpendicular to angle to the field, all of the the electric field, electric field, only energy of a none of the wave some of the wave polarised wave energy passes energy passes passes through. through. through.
    5. 5. Reflected light When light is reflected from a surface it is reflected and refracted and both waves are partly polarised Unpolarised light Incident Reflected part Plane-polarised light unpolarised polarisedLight polarised in other plane Refracted part polarised
    6. 6. Polaroid
    7. 7. Brewster angle Light reflected from a surface is partly polarised. At a certain angle, known as the Brewster angle, when the refracted and reflected rays are at 90º, the reflected ray is totally polarised. eg Brewster angle of water (n = 1.33),where ø is the Brewster angle ø = tan-1(1.33) = 53.1º and n is the refractive index
    8. 8. Polarisation filters The unpolarised light Polarising passes through a polarising filter filter and the intensity is reduced to 50% whatever the angle of the filter. Light direction 2 parallel filters If the second filter is aligned with the first one, all of the light polarised by Light the first filter is transmitted direction by the second one.Images: lectureonline
    9. 9. Filters at an angle 2 perpendicular 2 angled filters filters Light Light direction direction If the second filter is at 90º If the two filters are at an to the first one, none of the angle to each other, only light polarised by the first some of the light polarised by filter is transmitted by the the first filter is transmitted second one. by the second one.Images: lectureonline
    10. 10. Malus’ law Calculates intensity of polarised light incident on a filterI = intensity of emerging light; I0 = intensity of light incident on second filter; ϑ = angle between filters If 2 filters are at 45º to each other, and I0 = 400 Wm-2, I = 400*cos245 =Image: fsu 400*0.7071 2 = 200 Wm-2 First filter is called polariser; second filter is analyser
    11. 11. Optical activityOptically active polariser analysersubstances (eg strong no lightsugar solution) rotate the unpolarised vertically polarisedplane of polarisation. This Perpendicularcan be demonstrated by filters block all lightplacing sugar solutionbetween a polariser andan analyser which will Light with rotated planechange the intensity of of polarisation emergesthe emerging light. unpolarised vertically polarised rotated plane some light emerges sugar solution
    12. 12. Stress analysis Some plastics are optically active. When they experience forces, they rotate the plane ofpolarisation of different colours by different amounts. We can analyse the forces from the colours.
    13. 13. Liquid Crystal A liquid crystal display such as the one in a laptop screen, rotates the plane of polarisation by 90º. When it is under a voltage,however, it does not. The screen contains polarisers at right angles.No voltage: backlight transmits throughfirst polariser, liquidcrystal (turns light), and perpendicular polariser. Voltage: no rotation Image: bestgamingmonitorat crystal, so the light does not pass the By applying voltages across different second polariser. pixels, dark areas construct the images.
    14. 14. Practice problems What is the Brewster angle of a diamond with a refractive index of 2.42? [67.5º] Two polarisers are oriented at 50º to each other. What proportion of the light intensity is transmitted? [0.41] Two polaroids are aligned so the light passing though them is a maximum. At what angle should one be placed so that the intensity is reduced to a) half? b) 10%? [a)45º;b)72º]
    15. 15. a production MUCH MORE AThttp://nothingnerdy.wikispaces.com http://nothingnerdy.wikispaces.com/LINK 15