Interference is the phenomenon where two or more waves superimpose to form a resultant wave of greater, lower, or the same amplitude. To observe stable interference, light sources must be coherent (having a constant phase difference).
YDSE demonstrates the wave nature of light. A monochromatic light source illuminates two closely spaced slits (S1 and S2), producing an interference pattern of bright and dark fringes on a distant screen.
The distance between two consecutive bright or dark fringes is constant and given by:
Where:
λ = Wavelength of light
D = Distance between slits and screen
d = Distance between the two slits
When a plano-convex lens is placed on a flat glass plate, a thin film of air is formed between them. This film has a variable thickness. When monochromatic light is reflected from this film, circular interference fringes called Newton's Rings are observed.
Using the diameters of the nth and (n+p)th dark rings:
Where R is the radius of curvature of the plano-convex lens.
Diffraction is the bending of light around the corners of an obstacle or aperture into the region of geometrical shadow.
| Feature | Fresnel Diffraction | Fraunhofer Diffraction |
|---|---|---|
| Source/Screen distance | Finite distance | Infinite distance (using lenses) |
| Wavefronts | Spherical or Cylindrical | Plane wavefronts |
| Mathematical complexity | Highly complex | Relatively simple |
When a plane wavefront is incident on a narrow slit of width 'e', the light is diffracted. A lens focuses these rays on a screen.
Where n = 1, 2, 3... represents the order of the minima. The central maximum is twice as wide as the secondary maxima.
A diffraction grating is an optical component with a periodic structure (thousands of lines per cm) that splits light into several beams travelling in different directions.
Where (e + d) is the Grating Element, θ is the angle of diffraction, and n is the order of the spectrum.
The Resolving Power is the ability of an instrument to show two closely spaced objects as separate.
Wavefront Division: "You Feel Bright" (Young's, Fresnel Biprism).
Amplitude Division: "Newton Measures Amplitude" (Newton's Rings, Michelson).