Diffraction & Gratings
Single slit, gratings, CD. Builds on Double-Slit Interference.
You shine a laser through a single narrow slit onto a screen. You then make the slit NARROWER, expecting the bright spot to get narrower too.
What actually happens to the pattern when you make the slit narrower?
The central bright band gets WIDER! This is single-slit diffraction, and it’s beautifully counterintuitive. When you squeeze light through a narrower opening, the wave spreads out MORE, not less. The angular width of the central maximum is 2λ/a — inversely proportional to slit width. Make the slit half as wide, and the pattern doubles in size!
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Adjust the slit width and watch the diffraction pattern change. Make the slit narrower — the central maximum widens. Make it wider — the pattern tightens toward geometric shadow.
a sinθ = mλ (dark fringes)
Narrower slit (↓a) → larger angles (↑θ) → wider pattern. This is the wave equivalent of the uncertainty principle: confine position, and direction spreads out.
The tracks on a CD act as a diffraction grating. White light splits into a rainbow because each color diffracts at a different angle. The track spacing (~1.6 μm for CD) is comparable to visible light wavelengths.
Every telescope has a circular aperture that diffracts light. The Airy disk pattern sets the fundamental resolution limit: θ ≈ 1.22λ/D. Bigger mirror → sharper images.
X-rays diffract off crystal lattices (acting as 3D gratings). Watson, Crick, and Franklin used X-ray diffraction to discover DNA’s double helix structure.
The colorful rings sometimes seen around the Sun or Moon through thin clouds are caused by diffraction of light by tiny water droplets.
“Squeeze a wave through a smaller gap and it spreads MORE, not less. This single idea limits every microscope, explains CD rainbows, and revealed the structure of DNA.”
Diffraction Lab
Compare single-slit diffraction with multi-slit (grating) patterns. Adjust slit width, number of slits, and wavelength. See how gratings produce sharp spectral peaks from white light.
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Start with a single slit and red light. Narrow the slit — watch the pattern widen. Now switch to a grating with 2, 5, then 50 slits. See how the peaks sharpen dramatically with more slits! Try white light through a grating to see the full spectrum emerge.
Single slit: diffraction creates a broad envelope. Double slit: interference creates fringes within that envelope. Grating (many slits): fringes become razor-sharp peaks. More slits = sharper peaks = better spectral resolution.