FRESNEL BIPRISM
Fresnel Biprism is a device which is used to produce coherent sources and thus show the
phenomenon of interference. In practice it consist of a thin glass plate with one of its face
ground and polished till a prism is formed with one of its angle of about 179 0 and two
side angles of ½ o each.
When a light is incident on an ordinary prism, the ray bent through an angle called angle
of deviation. As a result the ray emerging out of the prism appears to have emanated from
a source S’ located at a small distance above the real source. If we arrange two prisms
placed base to base the bending of light takes place in such a way that light appears to
come from two virtual sources S1 and S2. The refraction of light in biprism also produces
two virtual sources S1 an S2.
FRESEL BIPRISM
Two ordinary prisms placed base to base forming two virtual sources Sand S2.
A monochromatic light source illuminated the vertical slit which acts as a narrow linear
monochromatic light source. The biprism is placed in such a way that the refracting edge
is parallel to the length of the slit. A single cylindrical wave front impinges on the
biprism. The top portion of the wave front is refracted downward and appears to have
emanated from the virtual source S1. The lower segment falling on the lower part of the
biprism is refracted upward and appears to have emanated from the virtual source S 2. The
virtual source S1 and S2 are images of the same source S produced by the refraction due
to biprism and hence are coherent. Hence the light source is in a position to interfere and
,produce fringes in a region beyond the biprism. Thus biprism creates two virtual images
of the source (similar to the combination of two prisms placed base to base)
The theory of interference and formation of fringe in case of Fresnel biprism is the same
as described for Young’s Double Slit Experiment. At point O a central bright fringe is
formed having alternate bright and dark fringes on either side of O. The fringe width i.e.
the spacing between consecutive bright or dark fringes is given by:
, 𝜆𝐷
𝛽 =
𝑑
Where-
D is the distance between source or slit and screen/eye-piece
d is the distance between two virtual light sources (i.e. S1 and S2)
λ is the wavelength of the monochromatic source
Application of Fresnel Biprism
Determination of wavelength of monochromatic light using Fresnel
Biprism
𝛽𝑑
The wavelength of light can be determined using the equation𝜆 = . In order to use this
𝐷
relation the values of
β are to be measured experimentally
D
D
Experimental Set-Up
Fresnel Biprism is a device which is used to produce coherent sources and thus show the
phenomenon of interference. In practice it consist of a thin glass plate with one of its face
ground and polished till a prism is formed with one of its angle of about 179 0 and two
side angles of ½ o each.
When a light is incident on an ordinary prism, the ray bent through an angle called angle
of deviation. As a result the ray emerging out of the prism appears to have emanated from
a source S’ located at a small distance above the real source. If we arrange two prisms
placed base to base the bending of light takes place in such a way that light appears to
come from two virtual sources S1 and S2. The refraction of light in biprism also produces
two virtual sources S1 an S2.
FRESEL BIPRISM
Two ordinary prisms placed base to base forming two virtual sources Sand S2.
A monochromatic light source illuminated the vertical slit which acts as a narrow linear
monochromatic light source. The biprism is placed in such a way that the refracting edge
is parallel to the length of the slit. A single cylindrical wave front impinges on the
biprism. The top portion of the wave front is refracted downward and appears to have
emanated from the virtual source S1. The lower segment falling on the lower part of the
biprism is refracted upward and appears to have emanated from the virtual source S 2. The
virtual source S1 and S2 are images of the same source S produced by the refraction due
to biprism and hence are coherent. Hence the light source is in a position to interfere and
,produce fringes in a region beyond the biprism. Thus biprism creates two virtual images
of the source (similar to the combination of two prisms placed base to base)
The theory of interference and formation of fringe in case of Fresnel biprism is the same
as described for Young’s Double Slit Experiment. At point O a central bright fringe is
formed having alternate bright and dark fringes on either side of O. The fringe width i.e.
the spacing between consecutive bright or dark fringes is given by:
, 𝜆𝐷
𝛽 =
𝑑
Where-
D is the distance between source or slit and screen/eye-piece
d is the distance between two virtual light sources (i.e. S1 and S2)
λ is the wavelength of the monochromatic source
Application of Fresnel Biprism
Determination of wavelength of monochromatic light using Fresnel
Biprism
𝛽𝑑
The wavelength of light can be determined using the equation𝜆 = . In order to use this
𝐷
relation the values of
β are to be measured experimentally
D
D
Experimental Set-Up
