JEE Main Wave Optics: Superposition Amplitude Formula 💡

 

❓ Concept

🎬 Wave Optics – Resultant Amplitude in 60 Sec

Do light waves mil rahi hain…
par amplitude seedha add hoga ya cancel?

👉 Answer hamesha phase difference batata hai! 🌊✨

Wave optics mein calculation kam,
vector understanding zyada important hota hai 🔥

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🖼️ Concept Image

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✍️ Short Explanation

Superposition ka matlab simple addition nahi hota.
Light waves → electric field vectors hote hain.

Isliye:
👉 Vector addition apply hota hai.

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🔹 Step 1 — Superposition Principle (FOUNDATION 💯)**

When two waves meet at a point:

$$E_{\text{resultant}}=E_1+E_2$$

But important:

⚠️ Vector sum, not simple arithmetic sum.

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🔹 Step 2 — Same Frequency, Same Direction

Given waves:

$$E_1 = E_0 \sin(\omega t)$$
$$E_2 = E_0 \sin(\omega t + \phi)$$

✔ Same $\omega$ ⇒ stable interference
✔ Same direction ⇒ direct vector addition

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🔹 Step 3 — Phase Difference Controls Everything

Different cases:

• $\phi = 0$
  → Maximum constructive
  → Amplitude doubles

• $\phi = \pi$
  → Complete destructive
  → Amplitude zero

• $0 < \phi < \pi$
  → Partial constructive

👉 So phase = deciding factor

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🔹 Step 4 — Resultant Amplitude Formula (MOST IMPORTANT 🔥)**

For two waves of equal amplitude $E_0$:

$$A = \sqrt{ E_0^2 + E_0^2 + 2E_0^2\cos\phi }$$$$A = \sqrt{ 2E_0^2(1+\cos\phi) }$$$$A = 2E_0\cos\left(\frac{\phi}{2}\right)$$

📌 Golden formula to remember:

$$\boxed{A = 2E_0\cos(\phi/2)}$$

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🔹 Step 5 — Concept Insight

Amplitude:

• Maximum when $\cos(\phi/2)=1$

• Zero when $\cos(\phi/2)=0$
  

🧠 Important:
Phase ≠ time shift
It is angular separation between waves

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⭐ Golden Summary Box

• Superposition ⇒ vector addition

• Same frequency ⇒ stable interference

• Amplitude formula:

$$A = \sqrt{E_1^2 + E_2^2 + 2E_1E_2\cos\phi}$$

• Equal amplitudes:

$$A = 2E_0\cos(\phi/2)$$

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✅ Final Takeaway

Whenever you see:

$$\sin(\omega t) \quad \text{and} \quad \sin(\omega t + \phi)$$

👉 Immediately think:

Same ω ⇒ interference
Phase φ ⇒ controls amplitude

Geometry > algebra 😎