Study material 12th Physics - Wave Theory of Light Part II
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- The document discusses polarization of light through reflection and refraction. It explains Brewster's law which states that the tangent of the polarizing angle of incidence is equal to the refractive index of the medium. - Examples of how polaroid materials are used to absorb one polarized ray and transmit the other ray are provided. Common uses of polaroid materials include reducing glare in car headlights, 3D movie cameras, analyzing polarized light, and LCD screens.
Study material 12th Physics - Wave Theory of Light Part II
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- 4. o PA'N i angle of incidence and NA'D r angle of reflection In triangles A'BC and A'DC, side A'C is common. m A'BC m A'DC 90 ABC ADC From the V V geometry of the figure, it can be proved that, PA'N BA'C and NA'D DCA' BA'C DCA' i.e. NA'D PAN i.e. r i
- 6. PA'N i angle of incidence and, N'A'D r angle of refraction From the geometry of figure, it can be proved that, PA'N BA'C i.e. i BA'C Also, N'A'D DCA' i.e. r DCA'
- 7. In right angled triangle, ABC, sin i =BC/AC In right angled triangle, ADC, sin r =AD/AC According to Snell’s law, the ratio of sine of angle of incidence to sine of angle of refraction is always constant. This constant is called refractive index (n) of the given pair of media. 1 2 1 2 Sin i BC AC n Sin r AC AD CBC n AD C CSin i n Constant Sin r C
- 8. According to wave theory of light, light is a transverse electro-magnetic wave. Electro- magnetic waves are generated by varying electric and magnetic fields. These fields are mutually perpendicular and also perpendicular to the direction of propagation of the wave. Out of these two vectors, electric vector decides the optical characteristics of light, hence, called optical vector or light vector. When the variations in the electric field are taking place in one plane only, the light is called plane polarized or linearly polarized.
- 10. The propagation vector and electric vector from a plane. The plane in which the electric vector varies, is called plane of vibration and the plane perpendicular to ii is called plane of polarization. An unpolarized light can be polarized in different ways. Scientist Malus developed
- 11. The simplest method to polarize light by simultaneous reflection and refraction. When a beam of unpolarized light is incident on a plane glass surface, part of the light is reflected and the remaining part is refracted, i.e. transmitted through the glass. The reflected light is partially polarized. At a certain angle of incidence, the reflected light is completely polarized.
- 12. In 1892, Sir David Brewster discovered that, when light is incident on a transparent medium at a polarizing angle qP, the reflected light is plane polarized, in the plane of incidence. In this situation, the reflected and refracted rays are perpendicular to each other. He also proved that, the tangent of angle of incidence at which complete polarization takes place by reflection, is numerically equal to the refractive index of the refracting medium.
- 13. If qP is the polarizing angle, ‘n’ is the refractive index of the refracting medium and r is the angle of refraction, from the figure, r = 90 - qP . n= Sin qP /Sin r = Sin qP /Sin (90- qP) = Sin qP /Cos qP = Tan qP This is known as Brewster’s law. As the refractive index of a medium depends on wavelength of light, polarizing angle also depends on the wavelength.
- 14. Certain films or crystals have property to absorb one of the refracted rays and transmit the other. This property is called dichroism and the material is called dichroic. A sheet of such material is known as polaroid.
- 16. Uses of Polaroid 1. In motor car head lights to remove headlight glare. 2. In three dimensional movie cameras. 3. To produce and analyze polarized light. 4. It is used as filter in photographic cameras. 5. In window of aeroplanes to control amount of light. 6. In polarizing sunglasses (goggles) to protect the eyes from glare of sunlight . 7. They are used to improve colour contrast in old oil paintings. 8. They are used in calculators , watches, monitors of laptops which have LCD screens.
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