Wave properties — reflection, refraction, diffraction, interference comparison

Question

Compare reflection, refraction, diffraction, and interference. When does each phenomenon occur, and what happens to the wave’s speed, wavelength, and frequency in each case?

(CBSE 11, JEE Main & NEET theory question)

Solution — Step by Step

Phenomenon	What happens	When it occurs
Reflection	Wave bounces back from a boundary	Wave hits a surface/barrier
Refraction	Wave changes direction at a boundary	Wave enters a different medium
Diffraction	Wave bends around obstacles/through slits	Obstacle size is comparable to wavelength
Interference	Two waves overlap and combine	Two coherent waves meet at a point

Property	Reflection	Refraction	Diffraction	Interference
Speed	Unchanged	Changes	Unchanged	Unchanged
Wavelength	Unchanged	Changes	Unchanged	Unchanged
Frequency	Unchanged	Unchanged	Unchanged	Unchanged
Direction	Reverses	Bends	Spreads	Creates pattern
Amplitude	May change	May change	Decreases	Varies (constructive/destructive)

Reflection: Angle of incidence = angle of reflection
Refraction: $n_1 \sin\theta_1 = n_2 \sin\theta_2$ (Snell’s law)
Diffraction: Most prominent when slit width $\approx$ wavelength
Interference: Path difference determines constructive ( $n\lambda$ ) or destructive ( $(n+\frac{1}{2})\lambda$ )

Why This Works

All four phenomena arise from the wave nature of light/sound. Frequency never changes in any of them because frequency is set by the source — the medium doesn’t create new oscillations.

graph TD
    A["Wave hits boundary/obstacle"] --> B{"What's the situation?"}
    B -->|"Bounces back"| C["Reflection<br/>θᵢ = θᵣ"]
    B -->|"Enters new medium"| D["Refraction<br/>Speed & λ change"]
    B -->|"Obstacle ~ wavelength"| E["Diffraction<br/>Wave spreads"]
    B -->|"Two waves overlap"| F["Interference<br/>Constructive or Destructive"]
    C --> G["v, λ, f unchanged"]
    D --> H["v, λ change; f unchanged"]
    E --> I["v, f unchanged; spreads"]
    F --> J["Amplitude varies by<br/>path difference"]

The key insight: frequency is always conserved across all wave phenomena. Speed changes only when the medium changes (refraction). Wavelength changes only when speed changes ( $v = f\lambda$ , and since $f$ is constant, $\lambda$ must adjust with $v$ ).

Alternative Method — Identify by the Physical Setup

For MCQs, identify the phenomenon from the setup description:

“Bounces off a mirror/wall” → reflection
“Passes from air to glass” → refraction
“Bends around a corner/passes through a narrow slit” → diffraction
“Two slits produce bright and dark bands” → interference

For JEE: diffraction and interference are often tested together (single slit diffraction pattern vs double slit interference pattern). The key difference: interference fringes are equally spaced, while diffraction has a bright central maximum that is twice as wide as the other maxima.

Common Mistake

Students often say “frequency changes during refraction.” It does NOT. When light goes from air to glass, speed decreases and wavelength decreases, but frequency stays the same. If frequency changed, the wave would pile up or thin out at the boundary — which doesn’t happen in steady state. This is one of the most common wrong answers in NEET.

Question

Solution — Step by Step

Why This Works

Alternative Method — Identify by the Physical Setup

Common Mistake

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