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Sound bends around corners — you can hear someone in the next room without a direct line of sight. Radio waves reach into valleys blocked by hills. But visible light casts sharp shadows. If all three are waves, why does light seem to travel in straight lines while sound and radio waves bend so easily?
Sound bends around corners — you can hear someone in the next room without a direct line of sight. Radio waves reach into valleys blocked by hills. But visible light casts sharp shadows. If all three are waves, why does light seem to travel in straight lines while sound and radio waves bend so easily?
Diffraction is the spreading of waves around obstacles and through gaps. The key factor is the ratio of wavelength to gap size. When this ratio is large, dramatic spreading occurs. When it is small, the wave barely bends — which is why light appears to travel in straight lines in everyday life.
Diffraction is the bending and spreading of waves when they pass through an opening or around an obstacle. It is most significant when the wavelength is comparable to the size of the gap. The condition for destructive interference (dark fringes) in single-slit diffraction defines the pattern.
Huygens' principle explains diffraction: every point in a wavefront acts as a new wave source. When waves pass through a slit, the secondary wavelets from different parts of the slit interfere. At certain angles, wavelets from the top and middle of the slit cancel exactly — creating dark fringes. The central maximum (m = 0) contains most of the energy. Its full angular width is 2θ₁, where the first minimum sits at sin θ₁ = λ/a; when the angle is small this is just 2λ/a. For light (λ ≈ 500 nm) through a 1 mm slit, the central maximum is only 0.06° wide — essentially a straight line. Through a 0.1 mm slit it widens to 0.6°. Through a 1 μm slit the small-angle shortcut breaks down: sin θ₁ = 0.5, so θ₁ = 30° and the central band spans a full 60° — the wave fans out across the whole screen. This is exactly why the simulation must use micron-scale slits to make diffraction visible. Diffraction explains why optical instruments have resolution limits (telescopes cannot distinguish stars too close together), why X-rays reveal crystal structures (their wavelength matches atomic spacing), and why AM radio (long wavelength) diffracts around hills better than FM (shorter wavelength).