Waves, Electrons & Photons

• Diffraction is waves spreading out around obstacles or through gaps — it's most obvious when the gap size ≈ wavelength.
• A diffraction grating (loads of tiny parallel slits) splits light into sharp bright fringes, governed by d sin θ = nλ.
• Electrons diffract too — firing them through graphite produces rings, proving matter has wave properties.
• The de Broglie equation λ = h/mv gives every moving object a "matter wavelength" — but it's only noticeable for tiny particles.
• Waves hitting a boundary between two materials are partly reflected and partly transmitted, depending on how different the two materials' densities are.
• Pulse-echo techniques (ultrasound, sonar) use reflected wave pulses and timing to measure distances and build images.
• Wave-particle duality: light behaves as a wave (diffraction/interference) AND as a particle — photons (photoelectric effect).
• A photon's energy is fixed by its frequency: E = hf = hc/λ.

What is diffraction?

Diffraction is the spreading out of waves when they pass through a gap or around an obstacle. It happens to every type of wave — light, sound, water, radio, even electrons (yes, really — see Section 4!).

Key fact students often miss: when a wave diffracts, its wavelength and frequency stay exactly the same. The only thing that changes is its amplitude — and it usually gets smaller, because some of the wave's energy spreads out sideways instead of continuing forward (some energy is "dissipated").

What controls how much a wave diffracts?

The amount of spreading depends on how the gap size (or obstacle size) compares to the wavelength:

• Gap ≈ wavelength → strong diffraction, waves spread out a lot (almost like a semicircle).
• Gap >> wavelength → weak diffraction, the wave barely bends and just carries on mostly in a straight line.

Huygens' Construction — explaining WHY diffraction happens

Huygens proposed that every point on a wavefront acts like its own tiny point source, sending out its own circular "secondary wavelet." The new wavefront a moment later is simply the curve that touches (is tangential to) all of these tiny wavelets.

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