03-11-2012, 10:47 AM
Fresnel Biprism
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Aim of the experiment
To measure the wavelength of Sodium Light
Apparatus required
A sodium Lamp
A optical bench
A slit
A bi-prism
An eye-piece
Theory
The Fresnel biprism is a prism which has one of its angles slightly less than two
right angles and two equal small base angles. It acts like two very thin prisms placed
base to base.
When rays from a slit, S, illuminated by a monochromatic light, such as sodium
light are made to be incident on the plane face of the biprism (PQR), the emergent
rays from the two halves of the biprism appear to diverge from two coherent virtual
sources, S1 and S2 (Fig. 1). If a screen (AB) is placed with its plane perpendicular to
the plane containing the slit and the common base of the biprism, the emergent beams
of light overlap on the screen producing alternate dark and bright fringes.
Procedure
1. Mount the gadgets on the optical bench.
2. Study all the movements on each stand.
3. Ensure that all the pieces are aligned at roughly the same height
4. Remove the stand with the convex lens from the optical bench.
5. Bring the eyepiece close to the biprism.
6. Looking through the eyepiece you will see a bright vertical patch of light.
slight rotation of the biprism in its own plane will break up this patch into
vertical equidistance fringes.
7. Adjust the slit width to get the best compromise between brightness and
sharpness of the fringe pattern.
8. Move the eyepiece slowly away from the biprism along the optical bench to a
distance of about 100 cms. Keeping the fringe pattern all the time in the field
of view.
9. Keeping eyepiece at a distance of 100 cm from the biprism, measure the fringe
width by measuring the distance traversed by the eyepiece in crossing about
10 fringes using the main and circular scales on the eye-piece.