To determine the wavelength of sodium light using Fresnel biprism by measuring the fringe width of the interference pattern.

Fresnel's biprism is an optical device that produces interference fringes by splitting light from a single source into two coherent beams. The biprism consists of two acute-angled prisms joined at their bases, effectively acting as two prisms with very small apex angles.

When monochromatic light from a narrow slit falls on the biprism, it splits into two coherent beams that appear to come from two virtual coherent sources. These two beams interfere in the region of overlap, producing bright and dark fringes.

The distance between consecutive bright (or dark) fringes is called fringe width (β). The wavelength of light can be calculated using the relationship between fringe width, distance between coherent sources, and the distance from biprism to the screen.

1. Set up the optical bench and place the sodium lamp at one end.
2. Place an adjustable slit near the sodium lamp and adjust it to get a narrow vertical slit.
3. Mount the Fresnel biprism on the optical bench with its edge parallel to the slit.
4. Place the eyepiece at the other end of the optical bench.
5. Switch on the sodium lamp and allow it to warm up for proper monochromatic light.
6. Adjust the positions until clear interference fringes are visible through the eyepiece.
7. Focus the eyepiece to get sharp fringes and align the crosswire perpendicular to the fringes.
8. Count and measure the distance covered by 10-20 fringes to calculate fringe width.
9. For finding distance between coherent sources, use displacement method with a convex lens.
10. Place the lens between biprism and eyepiece and find two positions where sharp images of the slit are formed.
11. Measure the displacement distance and calculate the separation between coherent sources.
12. Record all measurements and repeat the observations for accuracy.

Table 1: Fringe Width Measurement

Table 2: Distance Measurements

• Ensure the sodium lamp is warmed up properly for stable monochromatic light output.
• Keep the slit as narrow as possible while maintaining sufficient light intensity for clear fringes.
• The biprism edge should be exactly parallel to the slit for uniform fringes.
• Avoid vibrations of the optical bench during measurements as they affect fringe stability.
• Take multiple readings for fringe width measurement to minimize random errors.
• Ensure proper alignment of all optical components on the bench.
• Keep the room as dark as possible to observe fringes clearly.
• Handle optical components carefully to avoid scratches or damage.
• Check that the eyepiece crosswire is perpendicular to the fringes.
• Record distances accurately using proper measuring instruments.

• What is the principle behind Fresnel's biprism?
• Why is sodium light preferred for this experiment?
• What are coherent sources and how does biprism create them?
• Explain the formation of interference fringes in Fresnel biprism.
• What is the difference between Fresnel and Fraunhofer diffraction?
• How does the fringe width depend on wavelength and distance between sources?
• What happens to fringe width if we increase the distance between biprism and screen?
• Why can't we use white light for this experiment?
• What is the displacement method and why is it used?
• How do you distinguish between bright and dark fringes?
• What are the sources of error in this experiment?
• Compare the advantages of biprism over Young's double slit.
• What is the condition for constructive and destructive interference?
• How does the refractive index of biprism material affect the experiment?
• What happens if the slit width is too large or too small?