NEET Physics Notes Magnetostatics EMI and AC, EM waves-Wave Optics-Diffraction
Diffraction
Diffraction
Diffraction of light is the phenomenon of bending of light around the edges of an aperture or obstacle and entry of light even in the region of geometrical shadow, when size of aperture *.or obstacle is comparable to wavelength of light used.
Diffraction is characteristic of all types of waves. Greater the wavelength, more pronounced is the diffraction effect. It is due to this reason that diffraction effect is very commonly observed in sound.
Fresnel Diffraction
If either source or screen or both are at finite distance from the diffracting device (obstacle or aperture) the diffraction is called Fresnel type common examples, are diffraction at a straight edge, narrow wire or small opaque disc etc.
Diffraction due to Single Slit Width of Central Maximum
Fraunhoffer’s arrangement for studying diffraction at a single narrow slit (width = a) is shown in adjoining figure. Lenses L1 and L2 are used to render incident light beam parallel and to focus parallel light beam.
As a result of diffraction, we obtain a broad, bright maxima at symmetrical centre point O and on either side of it, we get secondary diffraction maxima of successively falling intensity and poor contrast, as shown in figure.
Condition of diffraction minima is given by
where, n = 1,2, 3, 4….
But the condition of secondary diffraction maxima is
where, n =1, 2, 3, 4….
Angular position of nth secondary minima is given by
Polarisation of Light
Polarisation of light is the phenomenon of restricting the vibrations of light (electric vector) in a particular direction, perpendicular to the direction of propagation of light wave. Such light is called plane polarised. Unpolarised light and plane polarised light are generally represented as shown in figures (a), (b) and (c).
Light waves are transverse waves, in which vibrations of electric field vector are taking place perpendicular to the direction of propagation of light.
Ordinary light, emitted by sun or an electric lamp etc, propagating in a given direction consists of many independent waves whose vibrations are randomly oriented in a plane which is perpendicular to the direction of light propagation.
Such light is said’ to be unpolarised light. The above discussion tells how the light being polarised or unpolarised.
By double refraction in certain crystals like calcite, quartz and tourmaline etc., incident unpolarised light splits up into two light beams of equal intensities with polarisation. One of the ray is ordinary ray (O-ray), it obeys Snell’s law. Another ray extraordinary ray (2?-ray), it does not obeys Snell’s law.
Intensity of plane polarised light received from a tourmaline crystal or polaroid is half of the incident unpolarised light.
Nicol prism is another good device to produce plane polarised light. It is based on the property of double refraction.
When plane polarised light obtained by passing unpolarised light through a polariser, is allowed to be incident on an analyser (another polarising crystal/nicol/polaroid) and it is rotated w.r.t. the polariser, intensity of emergent light varies depending upon then- relative orientation.
Law of Malus
According to Malus law, the intensity of emergent light out of an analyser varies as 
is the angle between the planes of transmission of the polariser and the analyser.
Brewster’s Law
When a beam of unpolarised light is reflected from the surface (unpolished) of a transparent medium of refractive index n at the polarising angle ip, the reflected light is completely plane polarised. According to Brewster’s law,
Moreover, in such an event, the reflected and transmitted rays are mutually perpendicular. Thus, angle of refraction 
Optical Rotation
Optical rotation is the property of rotating the plane polarised light by passing the same through an optically active substance.
Such substances are of two types
- Leavo Rotatory
These are substances which are left handed and rotate the plane of
polarisation in an anti-clockwise direction, e.g. oxalic acid – L. - Dextro Rotatory
These are substances which are right handed and rotate the plane of
polarisation clockwise direction. The angle of rotation depends on the length of the material density/concentration of the material, wavelength of light used and temperature, which the experiment is carried out. e.g. oxalic acid-d.
Polaroids
Polaroids polarise light. A number of needle shaped crystals quinine and iodosulphate with their axes parallel to one another are packed between two sheets of plastic. This arrangement serves as the polaroids.
The important uses are
- These reduce excess glare and hence, sun glasses are fitted with polaroid sheets.
- These are also used to reduce headlight glare of cars.
- They are used to improve colour contrast in old oil paintings.
- In wind shields of automobiles.
- In window panes.
- In three dimensional motion pictures.