NEB Class 12 Physics Polarization of Light Notes in PDF Complete Handwritten. Physics Notes 2082: All Chapters | New Curriculum | Class 12 Physics Notes download.
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NEB Class 12 Physics Polarization of Light Note Handwritten in PDF
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What is Polarization?
An ordinary (unpolarized) light beam has electric field vibrations in all planes perpendicular to the direction of propagation.
Polarized light has vibrations confined to only one plane.
Why polarization proves light is transverse (not longitudinal):
- Only transverse waves can be polarized the vibrations are perpendicular to propagation
- Longitudinal waves (like sound) cannot be polarized vibrations are along propagation
- The fact that light can be polarized proves its electric field vibrations are transverse
Methods of Polarization
1. By reflection (Brewster’s method):
- At a specific angle of incidence called Brewster’s angle i_p: reflected light is completely plane polarized
- Brewster’s Law: tan i_p = n (n = refractive index of medium)
- At Brewster’s angle: reflected ray and refracted ray are perpendicular (90Β°)
- For glass (n = 1.5): i_p = arctan(1.5) β 56.3Β°
2. By refraction:
- Light refracted through a glass slab at Brewster’s angle is partially polarized
- Passing through many glass plates (pile of plates) increases polarization
3. By scattering:
- Scattered light (e.g., blue sky light) is partially polarized
- Light scattered at 90Β° to incident beam is completely polarized
4. By Polaroid:
- Most practical method
- Special synthetic material that only transmits vibrations in one direction
Brewster’s Law Derivation
At Brewster’s angle i_p:
- The reflected and refracted rays are perpendicular
- So: i_p + r = 90Β° β r = 90Β° β i_p
- By Snell’s law: n = sin i_p / sin r = sin i_p / sin(90Β° β i_p) = sin i_p / cos i_p
Therefore: n = tan i_p (Brewster’s Law)
This derivation is a standard 4-mark NEB exam question.
Malus’s Law
When plane polarized light of intensity Iβ passes through a polaroid analyser at angle ΞΈ to its transmission axis:
I = Iβ cosΒ²ΞΈ
- ΞΈ = 0Β°: I = Iβ (maximum β transmission axis parallel to polarization)
- ΞΈ = 90Β°: I = 0 (complete extinction β transmission axis perpendicular)
- ΞΈ = 45Β°: I = Iβ/2
Application: Two Polaroids with their axes at 90Β° transmit no light used in LCD screens, variable-density sunglasses, and polarimetry.
Polaroids Construction and Uses
A Polaroid is a sheet of material containing aligned iodoquinine sulphate crystals or similar molecules that absorb one component of light and transmit the perpendicular component.
Uses of Polaroids:
- Sunglasses: Polaroid lenses reduce glare from horizontal reflections (road surfaces, water) horizontal vibrations are absorbed
- LCD screens: All liquid crystal displays use polarizers every pixel works by rotating polarized light
- Camera filters: Polarizing filters reduce reflections and enhance colour saturation in photos
- 3D cinema glasses: Left and right lenses are polarized at 90Β° to each other each eye receives different image
- Stress analysis: Polarized light through glass shows stress patterns (photoelastic analysis)
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Frequently Asked Questions
What is polarization of light in Class 12 Physics?
Polarization is the restriction of light vibrations to a single plane. Ordinary light vibrates in all planes perpendicular to propagation. Polarized light vibrates in only one plane. Polarization proves light is a transverse wave only transverse waves can be polarized. Longitudinal waves like sound cannot be polarized.
What is Brewster’s law in Class 12 Physics?
Brewster’s law: tan i_p = n, where i_p is polarizing angle and n is refractive index of the medium. At Brewster’s angle, reflected light is completely plane polarized. The reflected and refracted rays are perpendicular (90Β° angle between them). For glass (n=1.5): i_p = arctan(1.5) β 56.3Β°.
How do you derive Brewster’s law Class 12?
At Brewster’s angle i_p: reflected and refracted rays are perpendicular, so i_p + r = 90Β°, meaning r = 90Β° β i_p. By Snell’s law: n = sin i_p / sin r = sin i_p / sin(90Β°βi_p) = sin i_p / cos i_p. Therefore n = tan i_p. This is Brewster’s law β a standard 4-mark NEB board exam derivation.
What is Malus’s law in Class 12 Physics?
Malus’s law: when polarized light of intensity Iβ passes through an analyser at angle ΞΈ to its axis, transmitted intensity I = Iβ cosΒ²ΞΈ. At ΞΈ=0Β°: I = Iβ (maximum). At ΞΈ=90Β°: I = 0 (complete extinction). At ΞΈ=45Β°: I = Iβ/2. Used in LCD screens, polarimeters, and variable-density optical filters.
How does polarization prove light is transverse?
If light were longitudinal (like sound), vibrations would be along propagation direction rotating a polarizer would have no effect. But rotating a polarizer does affect light transmission complete extinction occurs at 90Β°. This can only happen if light vibrations are perpendicular to propagation, proving light is transverse.
What are uses of Polaroid in Class 12 Physics?
Polaroid uses: sunglasses reduce glare from horizontal reflections (road surfaces, water) by absorbing horizontal vibrations. LCD screens use polarizers in every pixel. Camera polarizing filters reduce reflections and deepen sky colour. 3D cinema glasses use perpendicular polarizers for left and right eyes to see separate images.
What are the methods of polarization in Class 12?
Four methods: by reflection (at Brewster’s angle reflected light is completely polarized), by refraction (pile of glass plates at Brewster’s angle), by scattering (sky light at 90Β° to sun is polarized), and by Polaroid (most practical β absorbs one component, transmits perpendicular component selectively).
Which polarization topics are most important for Exam?
Most important: Brewster’s law derivation (tan ip = n) (4 marks), Malus’s law statement and application (2 marks), how polarization proves transverse nature of light (2 marks), uses of Polaroid (2 marks). Brewster’s law derivation is asked almost every alternate year in NEB Class 12 Physics board exam.
