IB Physics Polarisation flippingphysics by Nothingnerdy
presentsa production Polarisation 1
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
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.
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.
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
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
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
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
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
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
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.
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.
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]
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