## Wave Optics

# Polarisation

- Polarisation is the property of obstructing the vibration of particle.
- Polarisation Establishes the transverse nature of Light waves
- If the Electric vector vibrates in all directions in a plane perpendicular to the direction of propagation is called unpolarised light.
- If the Electric field vector is contained to a plane passing through the direction of propagation it is called plane polarised light.
- Polariser is the crystal used to polarise the unpolarised light.
- Analyser is the crystal used to analyse the polarised light.
- Malus law is given by I = I
_{0}cos^{2}θ - Amplitude from malus law a = a
_{0}cos θ - When three polaroids are present with consecutive angles θ
_{1}and θ_{2}Then \tt I^{1}=\frac{I}{2}\cos^{2}\theta_{1}\cos^{2}\theta_{2} - polarising angle or Brewster's angle is the angle of incidence at which reflected light is polarised.
- Brewster law States that tangent of polarising angle is equal to refractive index μ = Tan θp
- \tt \tan\ \theta_{p} =\frac{\mu}{1};\sin\ \theta_{p} =\frac{\mu}{\sqrt{\mu^{2}+1}};\cos\ \theta_{p} =\frac{1}{\sqrt{\mu^{2}+1}}
- Ordinary ray in double refraction is that ray which obeys laws of refraction.
- Extraordinary ray in double refraction is that ray which does not obeys law of refraction.
- For ordinary ray \tt \mu_{0}=\frac{\sin i}{\sin r}
- Optic axis the axis along which ordinary and Extraordinary rays have same speed.
- Dichroism is the property of unequal absorption of ordinary and Extraordinary rays by some crystals.
- man made polarising materials are called polaroids
- Polarisation is used to study the helical structure of nucleic acids
- Polarisation is used in polaroid sun goggles.
- Optical activity of a substance is measured with the help of polarimeter
- Speed of light wave is same it is called isotropic media
- Speed of light wave is not same it is called anisotropic media
- Diffraction pattern is due to interference of light from secondary waves of the same wave front
- Sound waves cannot be polarised
- Intensity of fraunhoffer diffraction at single slit \tt I_{0}=I_{0}\left(\frac{\sin\alpha}{\alpha}\right)^{2}\ where\ \alpha=\frac{\pi a}{\lambda}\ \sin\theta
- Youngs double slit Experiment

### View the Topic in this video From 00:14 To 3:54

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