NEET Physics Notes Magnetostatics EMI and AC, EM waves-Wave Optics-Coherent Sources
Two light sources are said to be coherent, if they emit light of exactly same frequency (or wavelength), such that the originating phase difference between the waves emitted by them is either, zero or remains constant.
For sustained interference pattern the interfering light sources must be coherent one.
There are two possible techniques for obtaining coherent light sources.
- In division of wavefront technique, we divide the wavefront emitted by a narrow source in two parts by reflection, refraction or diffraction.
- In division of amplitude technique, a single extended light beam of large amplitude is splitted into two or more waves by making use of partial reflection or refraction.
Interference in Thin Films
In white light thin films, whose thickness is comparable to wavelength of light, show various colours due to interference of light waves reflected from the two surfaces of thin film.
For interference in reflected light condition of constructive interference (maximum intensity),
Shift in Interference Pattern
- If a transparent thin sheet of thickness t and refractive index nm is placed in the path of one of the superimposing waves (say infront of slit S2 of Young’s double slit experiment), then it causes an additional path difference due to which interference pattern shift.
- If due to presence of thin film the fringe pattern shifts by n fringes, then
Shift is independent of the order of fringe and wavelength.
It is a device to produce coherent sources by division of wavefront.
Fresnel’s biprism consists of two right angled prisms, each of refracting angle nearly 30′ kept base to base. The biprism is shown in figure.
As shown, if a is small angle of biprism and a is separation of biprism from coherent source S, then
The distance between the coherent sources and screen, then
The fringe width is given by