# Wave Optics IV

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Different Parts of the Wave Front at the Slit act as Secondary Sources:

a) Diffraction pattern is the result of interference of waves from these sources.

b) The intensity plot looks as follows, with there being a bright central maximum, followed by smaller intensity secondary maxima, with there being points of zero intensity in between, whenever

Emission, Absorption and Scattering:
a) These are the three processes by which matter interacts with radiation. In emission, an accelerated charge radiates and loses energy.

b) In absorption, the charge gains energy at the expense of the electromagnetic wave.

c) In scattering, the charge accelerated by incident electromagnetic wave radiates in all direction.

Polarization:
a) It specifies the manner in which electric field E oscillates in the plane transverse to the direction of propagation of light. If E oscillates back and forth in a straight line, the wave is said to be linearly polarized. If the direction of E changes irregularly the wave is unpolarized.

b) When light passes through a single polaroid P1 light intensity is reduced to half, independent of the orientation of P1. When a second Polaroid P2 is also included, at one specific orientation w.r.t P1, the net transmitted intensity is reduced to zero but is transmitted fully when P1 is turned 900 from that orientation. This happens because the transmitted polarization by a polaroid is the component of E parallel to its axis.

c) Unpolarized sunlight scattered by the atmosphere or reflected from a medium gets (partially) polarized.

Optical Activity:

Linearly polarized light passing through some substances like sugar solution undergoes a rotation of its direction of polarization, proportional to the length of the medium traversed and the concentration to the substance. This effect is known as optical activity.

Intensity of the Light due to Polarization:

Where I is the intensity of light after polarization Io is the original intensity, q  is the angle between the axis of the analyzer & the polarizer

Brewster’s Law:
When an incident light is incident at the polarizing angle, the reflected & the refracted rays are perpendicular to each other. The polarizing angle, also called as Brewster’s angle, is
Polarization by Scattering:
a) Light is scattered when it meets a particle of similar size to its own wavelength. The scattering of sunlight by dust particles is an example of polarization by scattering.

b) Rayleigh showed that the scattering of light is proportional to the fourth power of the frequency of the light or varies as 1/ l4 where l is the wavelength of light incident on the air molecules of size ‘d’ where d << l . Hence blue light is scattered more than red. This explains the blue colour of the sky.

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