Fraunhoffer Diffraction
Fraunhoffer diffraction occurs when both the source of light and the screen where the diffraction pattern is observed are effectively at infinite distances from the diffracting obstacle. This condition is achieved in practice by using converging lenses to make the light rays parallel before and after passing through the obstacle. The resulting diffraction patterns are sharp and well-defined.
Single Slit Diffraction
When plane wavefronts of monochromatic light pass through a single narrow slit, the light spreads out and forms a diffraction pattern on a distant screen. This pattern consists of a bright central maximum, flanked by alternating dark and progressively fainter bright fringes.
Condition for Fraunhoffer Diffraction: The incident wavefront is plane, and the diffracted light is observed at infinity (or in the focal plane of a converging lens).
- Principle: According to Huygens' principle, every point on the wavefront within the slit acts as a source of secondary wavelets. These wavelets interfere to produce the observed pattern.
- Minima: Dark fringes (minima) occur when the path difference between waves from the two edges of the slit is an integral multiple of the wavelength. The condition for minima is given by:
a sinθ = nλ
where 'a' is the width of the slit, 'θ' is the angle of diffraction, 'n' is an integer (±1, ±2, ...) representing the order of the minimum, and 'λ' is the wavelength of light. - Maxima: Bright fringes (maxima) occur approximately at angles where the path difference is an odd multiple of λ/2. The central maximum (n=0) is the brightest and broadest, with its width being twice that of the secondary maxima.
Double Slit Diffraction
When light passes through two narrow, parallel slits, the observed pattern is a combination of interference and diffraction. It consists of interference fringes whose intensity is modulated by the single-slit diffraction pattern of each individual slit.
- Principle: Each slit produces its own diffraction pattern. Additionally, the light waves from the two slits interfere with each other, creating an interference pattern. The resultant intensity is the product of the single-slit diffraction intensity and the two-slit interference intensity.
- Intensity Distribution: The envelope of the intensity pattern is determined by single-slit diffraction, while the finer fringes within this envelope are due to double-slit interference. Missing Orders: If a principal maximum of the interference pattern coincides with a minimum of the diffraction pattern, that interference maximum will be absent or