POLARIZATION

EM Waves Light is an electromagnetic wave. EM waves are transverse. Thus, the electrical field can vibrate in any direction perpendicular to the direction of propagation. Most light sources (candles, incandescent light bulbs etc..) emit light that is unpolarized – the electric field has all possible directions of vibrations

POLARITY In the case of transverse wave, a number of directions a particle can execute a periodic motion perpendicular to the direction of propagation. So two similar waves may differ from each other because of their different directions of vibration. THE DEPENDANCE OF THE PROPERTIES OF A WAVE WITH THE DIRECTION OF PROPAGATION is called POLARITY.

Polarization

Polarization Wave having a characteristic of polarity is called POLARIZED WAVE. Transforming unpolarized light into polarized light. Restricting electric field vector E in a particular plane so that the vibration occur in a single plane. Plane of vibration: Plane containing the direction of propagation and the plane of electric field vibration. Plane of polarization: Plane perpendicular to the plane of vibration.

Polarization

Types of polarization

Types of polarization Linear polarization: Electric field vector oscillate along straight line in one plane. Circular polarization: Circularly polarized light consists of two perpendicular electromagnetic plane waves of equal amplitude and 90° difference in phase. Elliptically polarized: Elliptically polarized light consists of two perpendicular waves of unequal amplitude which differ in phase by 90°. 

Methods of achieving polarization Reflection Scattering Dichroism Birefringence

Reflection Unpolarized light can be undergo polarization by reflection of non metallic surfaces like snow, glass. The reflected light is partially polarized and the degree of polarization depends on the angle of incidence. When angle of incidence equal to the angle of polarization, the degree of polarization is high. Incident angle is such that the angle between the reflected and refracted ray is 90 degree. Such an incident angle is polarized angle or Brewster's angle. Reflected ray is linearly polarized parallel to the reflecting surface.

MALUS LAW According to Malus, when completely plane polarized light is incident on the analyzer, the intensity I of the light transmitted by the analyzer is directly proportional to the square of the cosine of angle between the transmission axes of the analyzer and the polarizer.

MALUS LAW A Amplitude of incident radiation Θ Angle between analyzer and polarizer The component of amplitude II to the optic axis = A cosθ Transmitted Intensity I = ? When θ = 0 or 180, I=? When θ = 90, I =?

Brewster’s law

Brewster’s law Brewster's found that there is a relation between the angle of incidence and the refractive index. When light is incident at polarizing angle: Tan (i) = Refractive index of the material Then i = ? If the angle of incidence is not exactly the Brewster's angle then the reflected ray will be only partially polarized.

Brewster’s law- Proof According to Brewster's law, µ = tan i Tan i = (sin i / cos i) = µ Snells law, µ = ? i+ r = 90

Scattering When light strikes the atoms of a material, it will often set the electrons of those atoms into vibration. The vibrating electrons then produce their own electromagnetic wave that is radiated outward in all directions.  This newly generated wave strikes neighboring atoms, forcing their electrons into vibrations at the same original frequency. 

Scattering This absorption and reemission of light waves causes the light to be scattered about the medium. (This process of scattering contributes to the blueness of our skies) This scattered light is partially polarized

Dichroism Polarization by selective absorption Eg: Tourmaline crystal

Birefringence / Double Refraction

Birefringence Polarization due to double refraction Crystals like calcite If an object is viewed through a double refracting crystal two images are seen The ray corresponds two both images are polarized.

Calcite crystal

Calcite crystal Doubly refracting crystal Rhombohedron – 6 faces. 2 opposite corners A & H blunt corners ( 3obtuse angles meet) The direction of line passing through this corners are called optic axis. Ray incident in the direction of optic axis there is no double refraction.

Huygens's theory of double refraction A point source of monochromatic light in a doubly refracting crystal is origin of two wavefronts. O-Ray travel with same velocity in all direction so the corresponding wavefront is spherical. E-Ray travel with different velocity in different direction so the corresponding wavefront is elliptical. Along the direction of the optic axis both rays travel with same speed.

Types of Crystals Depending on the refractive index of O-Ray and E-Ray, crystals are classified into two. POSITIVE CRYSTAL & NEGATIVE CRYSTAL

Positive crystal O-Ray lies outside the surface of E-Ray Velocity of O-Ray is constant. At optic axis velocities are same μ ordinary < μ Extraordinary VE-Ray Varies, maximum along optic axis VE-Ray = VO-Ray. VE-Ray Minimum perpendicular to the direction f optic axis. Eg: quartz,ice

Negative Crystal E-Ray lies outside the surface of O-Ray Velocity of O-Ray is constant. At optic axis velocities are same μ ordinary > μ Extraordinary VE-Ray Varies, minimum along optic axis VE-Ray = VO-Ray. VE-Ray Maximum perpendicular to the direction f optic axis. Eg : tourmaline

NICOL PRISM

NICOL PRISM

NICOL PRISM Invented by William Nicol 1828. It is a device to produce and analyze plane polarized light. Calcite crystal whose length is 3 times that of its breadth is used. By grinding and polishing the angles of small faces are 680 and 1120 . This crystal is then cut into 2 pieces , then these two pieces are made optically flat by polishing and joined together using a transparent material called CANADA BALSAM.

TOTAL INTERNAL REFLECTION When a ray travelling fro denser medium to rarer medium at an angle of incidence greater than the critical angle total internal reflection takes place in the boundary between two media.

NICOL PRISM The refractive index of canada balsam is μ= 1.55 When the rays split into two then, μ o-ray = 1.658 μ e-ray = 1.486 Ordinary ray strikes on the canada balsam layer with an angle greater than the critical angle and then O-Ray undergo total internal reflection. The reflected rays absorbed by the black paint coated on the sides of the crystal.

NICOL PRISM E-Ray strikes on the canada balsam layer at an angle less than the critical angle and it will be coming out through the other end of the crystal .

Quarter wave plate Quarterwave plate is a device which produces path difference of λ/4 and a phase difference of π/2 between two rays. Q.wave plate is used to produce circularly polarized and elliptically polarized light.

Half wave plate Doubly refracting uniaxial crystal The thickness of the half wave plate is such that it introduces a path difference of λ/2 and phase difference of π between two rays. Eg: mica, quartz

Production of polarized light Unpolarised light Nicol prism Plane polarized light Plane polarized light falls normally on a quarter wave plate, such that the vibration in the incident light making an angle 45 degree with the optic axis of QWP. Circularly polarized light CIRCULARLY POLARISED LIGHT

ELLIPTICALLY POLARISED LIGHT Unpolarised light Nicol prism Plane polarized light vibration in the incident light making an angle,   0, 45, 90 degree with the optic axis of QWP. elliptically polarized light

DETECTION OF POLARISED LIGHT plane polarized light Plane polarized light produced by a Nicol prism allowed to fall on another Nicol. When the analyzer is rotated once w.r.t polarizer ?

DETECTION OF POLARISED LIGHT circularly polarized light

DETECTION OF POLARISED LIGHT elliptically polarized light

Polaroids It is an optical device based on selective absorption to produce plane polarized light for commercial purpose. When unpolarized light is transmitted through a Polaroid, it emerges with one-half the intensity and with vibrations in a single plane; it emerges as polarized light. Parallel position intensity transmitted and perpendicular position absorbed.

Uses of polaroids Polaroids are used in the laboratory to produce and analyze plane polarized light. Polaroids are widely used as polarizing sun glasses. They are used to eliminate the head light glare in motor cars. They are used to improve colour contrasts in old oil paintings. polaroid films are used to produce 3-D moving pictures. They are used as glass windows in trains and aeroplanes to control the intensity of light. In aeroplane one polaroid is fixed outside the window while the other is fitted inside which can be rotated. The intensity of light can be adjusted by rotating the inner polaroid. In calculators and watches, letters and numbers are formed by liquid crystal display(LCD) through polarization of light. Polarization is also used to study size and shape of molecules.  

Applications of polarized light 3-D movies: Polarization is used for 3-D movies, in which the images intended for each eye are projected by using two different projectors. Polarized 3-D glasses with suitable polarized filters ensure that each receives the intended image.

Applications of polarized light Plane polarized light reduces glare. In foreign countries the wind screen in front of the driver is polarizer. Hence it reduces glare and thus accidents can be minimized.