CBSE Class 12 Physics Diffraction and Polarisation of light Theory

CBSE Class 12 Physics Diffraction and Polarisation of light Theory. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

 1. DIFFRACTION

1.1 Meaning of Diffraction
It is the spreading of waves round the corners of an obstacle, of the order of wave length.

1.2 Defination of Diffraction
The phenomenon of bending of light waves around the sharp edges of opaque obstacles or aperture and their encroachment in the geometrical shadow of obstacle or aperture is defined as diffraction of light.

1.3 Necessary Conditions of Diffraction of Waves
The size of the obstacle (a) must be of the order of the wavelength of the waves (λ).
a/λ > 1
Note:Greater the wave length of wave higher will be its degree of diffraction. This is the reason that diffraction of sound & radio waves is easily observed but for diffraction of light,
additional arrangement have to be arrange. λsound > λlight Wave length of sound is nearly equal to size of obstacle. If size of obstactle is a & wavelength is λ then,
S.No. a V/S λ Diffraction
(1) a << λ Not possible
(2) a >> λ Not possible
(3) a ≈ λ Possible

1.4 Interpretation of Diffraction
As a result of diffraction, maxima & minima of light intensities are found which has unequal intensities. Diffraction is the result of superposing of waves from infinite number of coherent sources on the same wavefront after the wavefront has been distorted by the obstacle.

1.5. Example of Diffraction
(a) When an intense source of light is viewed with the paritally opened eye, colours are observed in the light.

(b) Sound produced in one room can be heard in the nearby room.

(c) Appearance of a shining circle around the section of sun just before sun rise.
(d) Colored spectrum is observed if a light source at far distant is seen through a thin cloth

1.6 Two type of diffraction
Fresnel Diffraction : Fresenel diffraction which involves non-plane (spherical) wavefronts, so that the source s and the point p (where diffraction effect is to be observed )
are to be at a finite distance from the diffracting obstacle

Fraunhofer Diffraction: Fraunhofer diffraction deals with wavefronts that are plane on arrival and an effective viewing distance of infinity. If follows that fraunhofer diffraction is
an important special case of fresnel diffraction. In youngs double slit experiment, we assume the screen to be relatively distant, that we have fraunhofer conditions.

1.7 DIFFERENCE BETWEEN INTERFERENCE & DIFFRACTION OF LIGHT

1.8 RESOLVING POWER (R.P)
A large number of images are formed as a consequence of light diffraction from a source. If two sources are separated such that their central maxima do not overlap, their images
can be distinguished and are said to be resolved R.P. of an optical instrument is its ability to distinguish two neighbouring points. Linear R.P. = d/λD
Angular R.P. = d/λ
D = Observed distance
d = Distance between two points

(1) For Diffraction Maxima : a sin θ = (2n – 1) λ/2
(2) For Diffraction Minima : a sin θ = nλ
(3) The maxima or minima is observed due to the superposition of waves emerging from infinite secondary sources between A & B points of slit.
(4) Fringe width : The distance between two secondary minima formed on two sides of central maximum is known as the width of central maximum

Fraunhofer diffraction for single slit

Ex.1 When wave of wavelength 0.2 cm is made inciddent normally on a slit of width 0.004m, then the semi-angular width of central maximum of diffraction pattern will be-
(1) 60⁰ (2) 30⁰ (3) 90⁰ (4) 0⁰

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