FRESNEL BIPRISM

FRESNEL’S BIPRISM INTERFERENCE AUGUSTIN-JEAN FRESNEL who contributed significantly to the establishment of the wave theory of light and optics. He gave a simple arrangement for the production of interference pattern. Fresnel rediscovered the interference of light with the help of two coherent sources produced using a biprism . With his concept , he eliminated the problem of bending of light at the corner of the slits. He used two virtual images of a source to produce the interference pattern.

BIPRISM Fresnel’s biprism consists of two acute angled prisms with their bases in contact . Actually , it is constructed as a single prism having obtuse angle 179˚ at B and acute angles of 30˚each at A and D. The source S is a narrow slit illuminated by the light obtained from a monochromatic source of light (sodium vapour lamp)

Align the slit and the vertex of the biprism such that they are coplanar. With these adjustments, the interference fringes can be observed through the eyepiece. Eye piece may be placed at a distance of appoximatly 75 cm from the biprism. Measure the distance (D) between the slit and the eyepiece Measure the slit width  with the help of verinier attached to the eye piece Insert biconvex lens between the biprism and the eyepiece without disturbing any other components The image of the two virtual slits on the eyepiece can be observed. Measure the lateral separation (d1) of the two images.

Move the lens to another position 2 as and again capture the image of the slits on the eyepiece. Measure the separation between the images (d2) again. (simple geometrical optics tells that the object and image distances can be interchanged in an imaging system). From the measured values of d1 and d2 the separation between the silts can be found by Estimate the wavelength of light using equation Β=ƛD/d Where  is the wavelength of the monochromatic light used

MEASUREMENT OF d For the measurement of d, we use displacement method. Place a convex lens of short focal length between biprism and eyepiece. The position of lens is adjusted so that a sharp pair of the slits is obtained in the field of view of the eyepiece. Measure the distance between the images S₁ and S₂ i.e. d₁.

MEASUREMENT OF d Monochromatic light from a narrow slit S falls on the biprism, the axis of which must be in line with the slit. The refracting angles of the biprism are very small, usually about 0.25°. This prism forms two virtual images of the slit S₁ and S₂ in the plane of S and these two virtual images act as the sources for two sets of waves which overlap and produce an interference pattern on the screen. The fringes are much brighter than those produced by young’s slits ,because of the very much greater amount of light that can pass through the prism compared with that passing through the double slit arrangement.

The formula used is the same as for young’s slits , the only problem being the measurement of the separation of the two virtual sources S₁ and S₂ . This can be done by placing a convex lens between the biprism and the screen or eyepiece and the measuring the separation (S) of the images of S₁ and S₂ produced by the lens . If the object and image distances (u and v) are found , the value of d can be calculated from d/s=u/v Using a two position method removes the need to measure u and v . If d₁ and d₂ are the separation of the two image slits in the two positions then : d=√d₁d₂