Presentation on theme: "Polarization of Light Waves"— Presentation transcript:
1Polarization of Light Waves Chapter 5Polarization of Light Waves
2Polarization of Light Waves Each atom produces a wave with its own orientation ofAll directions of the electric field vector are equally possible and lie in a plane perpendicular to the direction of propagationThis is an unpolarized wave
3A wave is said to be linearly polarized if the resultant electric field vibrates in the same direction at all times at a particular pointPolarization can be obtained from an unpolarized beam byselective absorptionreflectionscattering
4Polarization by Selective Absorption The most common technique for polarizing lightUses a material that transmits waves whose electric field vectors in the plane are parallel to a certain direction and absorbs waves whose electric field vectors are perpendicular to that direction
5E. H. Land discovered a material that polarizes light through selective absorption He called the material PolaroidThe molecules readily absorb light whose electric field vector is parallel to their lengths and transmit light whose electric field vector is perpendicular to their lengths
6The intensity of the polarized beam transmitted through the second polarizing sheet (the analyzer) varies asI = Io cos2 θIo is the intensity of the polarized wave incident on the analyzerThis is known as Malus’ Law and applies to any two polarizing materials whose transmission axes are at an angle of θ to each other
7The intensity of light transmitted through two polarizers depends on the relative orientation of their transmission axes. (a) The transmitted light has maximum intensity when the transmission axes are aligned with each other. (b) The transmitted light intensity diminishes when the transmission axes are at an angle of 450 with each other. (c) The transmitted light intensity is a minimum when the transmission axes are at right angles to each other.
8Polarization by Reflection When an unpolarized light beam is reflected from a surface, the reflected light isCompletely polarizedPartially polarizedUnpolarizedIt depends on the angle of incidenceIf the angle is 0° or 90°, the reflected beam is unpolarizedFor angles between this, there is some degree of polarizationFor one particular angle, the beam is completely polarized
9The angle of incidence for which the reflected beam is completely polarized is called the polarizing angle, θp
10θp + 90o + θp = 180°, so that θ = 90° - θp . Using Snell’s law and taking n1 = 1.00 and n2 = n, we haveBecause sin θ2 = sin(90° - θp) = cos θp , the expression for n can be written asθp may also be called Brewster’s Angle
11Polarization by Double Refraction When light travels through an amorphous material, such as glass, it travels with a speed that is the same in all directions.That is, glass has a single index of refraction. In certain crystalline materials, however, such as calcite and quartz, the speed of light is not the same in all directions. Such materials are characterized by two indices of refraction. Hence, they are often referred to as double-refracting or birefringent materials.
12When unpolarized light enters a calcite crystal, it splits into two plane-polarized rays that travel with different speeds, corresponding to two angles of refraction
13One ray, called the ordinary (O) ray, is characterized by an index of refraction, nO that is the same in all directions.The second plane-polarized ray, called the extraordinary (E) ray, travels with different speeds in different directions and have an index of refraction, nE, that varies with the direction of propagation.
14there is one direction, called the optic axis, along which the ordinary and extraordinary rays have the same speed, corresponding to the direction for whichnO = nE .The difference in speed for the two rays is a maximum in the direction perpendicular to the optic axis.
15Polarization by Scattering When light is incident on a system of particles, the electrons in the medium can absorb and reradiate part of the lightThis process is called scatteringAn example of scattering is the sunlight reaching an observer on the earth becoming polarized
16The horizontal part of the electric field vector in the incident wave causes the charges to vibrate horizontallyThe vertical part of the vector simultaneously causes them to vibrate verticallyHorizontally and vertically polarized waves are emitted
17Optical ActivityCertain materials display the property of optical activityA substance is optically active if it rotates the plane of polarization of transmitted lightOptical activity occurs in a material because of an asymmetry in the shape of its constituent materials
18The angle through which the light is rotated by a specific material depends on the length of the sample and on the concentration if the substance is in solution.One optically active material is a solution of common sugar dextrose.A standard method for determining the concentration sugar solutions is to measure the rotation produced by a fixed length of the solution.
19photoelastistitymaterials, such as glass and plastic, become optically active when stressedIf the plastic is stressed, regions of greatest stress rotate the polarized light through the largest angles.a series of bright and dark bands is observed in the transmitted light;