Statement A — TRUE ✓
In interference and diffraction, energy is not created or destroyed. It is merely redistributed — less energy in dark fringes, more in bright fringes. Average intensity over the whole pattern = 2I₀ = sum of individual intensities. This is perfectly consistent with the law of conservation of energy.
Statement B — FALSE ✗
Diffraction and interference are NOT exclusive to light. They are general wave phenomena exhibited by all waves — sound, water, radio, X-rays, and even matter waves (electrons). The Davisson-Germer experiment proved electron diffraction. Statement B is incorrect.
Conclusion: A is true, B is false → Option 3
A common misconception is that energy is destroyed in dark fringes of an interference pattern. This is incorrect. The total energy falling on the screen remains constant — it is simply redistributed. Dark fringes (zero intensity) are compensated by brighter bright fringes (intensity 4I₀ instead of 2I₀). Mathematically: average intensity = (I_max + I_min)/2 = (4I₀ + 0)/2 = 2I₀ = sum of individual intensities (I₀ + I₀). Energy is conserved.
Diffraction and interference are properties of all wave phenomena. Sound waves diffract around buildings and interfere to create loud and quiet spots. Water waves from two sources create interference patterns visible on the water surface. Radio waves diffract around mountains. X-rays diffract off crystal lattice planes (X-ray crystallography). Electrons diffract off nickel crystals (Davisson-Germer). Neutrons are diffracted in nuclear reactors. Claiming these phenomena are exclusive to light is a fundamental error.
📌 Interference: Superposition of waves from a finite number of discrete coherent sources. All fringes have equal width and nearly equal intensity (except for diffraction envelope).
📌 Diffraction: Superposition of waves from infinite coherent sources spread across an aperture. Central maximum is twice as wide as secondary maxima. Intensity decreases with order.
📌 Common: Both are superposition phenomena, both require wave nature, both conserve energy.
In single slit diffraction, minima occur at asinθ = nλ (n = ±1, ±2...) where a is slit width. The central maximum has double the angular width of secondary maxima. Width of central maximum = 2λD/a. As slit width decreases, diffraction becomes more prominent and the central maximum widens. For a = λ, light spreads in almost all directions.
Diffraction is significant when the size of the obstacle or aperture is comparable to the wavelength of the wave. For light (λ ~ 500 nm), a slit must be of micron width to show clear diffraction. Sound (λ ~ 0.1–10 m) diffracts easily around everyday objects. This is why we can hear sound around corners but cannot see around them — light's wavelength is too small relative to everyday obstacles.
Diffraction limits the resolving power of optical instruments. Two closely spaced objects can be resolved only if their diffraction patterns are sufficiently separated (Rayleigh's criterion: minimum resolvable angle θ = 1.22λ/D, where D is aperture diameter). Larger telescope mirrors (larger D) resolve finer details — this is why astronomers build large telescopes.