Newton's Rings Setup:
* Point Source: A point source of light, usually a monochromatic light source like a sodium lamp, is used.
* Lens: A plano-convex lens is placed on a flat glass plate.
* Reflected Light: Light from the point source is reflected by both the lens surface and the glass plate.
Why a Point Source is Essential:
1. Interference: Newton's Rings are formed due to interference between the reflected light waves. For constructive interference to occur, the light waves must have a constant phase difference.
2. Path Difference: The path difference between the two reflected waves depends on the thickness of the air film between the lens and the plate. This thickness varies across the surface, leading to different path differences and thus different interference patterns.
3. Extended Source: If an extended source is used, light from different points on the source would reach the lens at different angles. This would lead to varying path differences and a blurred interference pattern, making it impossible to observe distinct rings.
In Conclusion:
Using a point source of light ensures that the path difference between the reflected waves remains constant, leading to a clear and distinct interference pattern of Newton's Rings. An extended source would cause the pattern to become blurry and unusable for analysis.
