Resultant Amplitude and Intensity – Complete Notes, Revision, Important Questions & Downloads
Resultant Amplitude and Intensity turns the superposition idea into the two formulas NEET actually tests: one for amplitude and one for intensity. This topic is built around Amplitude and Intensity of Superposed Waves, where the fixed phase difference phi controls whether the point on the screen brightens, dims, or vanishes completely. The working pair is A = sqrt(a1^2 + a2^2 + 2a1a2 cos phi) and I = I1 + I2 + 2sqrt(I1I2) cos phi, with the identical-source shortcut I = 4I0 cos^2(phi/2). Once these are secure, the maxima-minima conditions in interference stop feeling like separate formulas.
NEET Weightage & Exam Pattern
Wave Optics| NEET Year | Questions from this Topic | Bar | Marks |
|---|---|---|---|
| 2024 | 1 | 4 | |
| 2023 | 1 | 4 | |
| 2022 | 1 | 4 | |
| 2021 | 0 | 0 | |
| 2020 | 1 | 4 | |
| Topic Weightage | 4 | 16 |
Identical sources reduce the algebra sharply: once I1 = I2 = I0, the entire result becomes I = 4I0 cos^2(phi/2).
The common trap is to use amplitude addition directly for intensity without squaring or without checking whether the sources are identical.
Preparation Strategy
Separate Amplitude Formula From Intensity Formula Write the two formulas on different lines during practice: first A in terms of a1, a2, and cos phi, then I in terms of I1, I2, and cos phi. This prevents mixing amplitude symbols with intensity symbols mid-solution.
Lock the Three Special Cases For phi = 0, result is maximum; for phi = pi, result is minimum; for identical sources use I = 4I0 cos^2(phi/2). These three cases answer most NEET objective questions without full expansion.
Watch the Phase Unit Before substituting, check whether the question gives phi in degrees, radians, or through path difference. Many wrong answers come from using cos 180 instead of cos pi, or from forgetting that Delta = lambda/2 means phi = pi.
Use the Shortcut Only After the Equality Check Do not jump to 4I0 cos^2(phi/2) unless the two sources have equal intensity. If I1 and I2 are unequal, stay with I = I1 + I2 + 2sqrt(I1I2) cos phi.
Download Topic Notes
PDF · Cheat Sheet · MCQ Set · PYQTopic Coverage
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Quick Revision
Concept → Trap → Example1) Amplitude and Intensity of Superposed Waves
Core FormulaFor two waves of the same frequency with fixed phase difference phi, A = sqrt(a1^2 + a2^2 + 2a1a2 cos phi) and I = I1 + I2 + 2sqrt(I1I2) cos phi.
- Use the amplitude form when the problem gives a1 and a2, and use the intensity form when the problem gives I1 and I2 directly.
- For identical sources I1 = I2 = I0, the result simplifies to I = 4I0 cos^2(phi/2), which makes maxima and minima almost instant to read.
- Trap: applying the identical-source shortcut when I1 and I2 are unequal, or forgetting that minimum intensity is zero only when the sources are equal.
US Curriculum Gaps
Note for NRI/OCI students studying abroad.Symbol Switching Is Tested Aggressively in NEET
Students are often comfortable with superposition qualitatively but less comfortable when the exam switches from amplitudes to intensities in the same question.
- a1 and a2 cannot be substituted into the intensity formula directly
- equal-source shortcut must be justified before use
Phase-Based Numericals Appear in Short Form
NEET likes quick calculations with phi = 0, pi/2, pi, or with path difference converted into phi first, so speed matters more than long derivations here.
- Delta = lambda/2 implies phi = pi
- maximum and minimum should be recognized without full expansion
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Physics Revision Checklist Revision Checklist
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Frequently Asked Questions
Notes · Downloads · Revision · Important QuestionsWhy do I need both amplitude and intensity formulas?
When can I use I = 4I0 cos^2(phi/2)?
Why is the intensity not always zero in destructive interference?
What is the quickest way to detect maximum intensity?
How do I handle degrees and radians safely here?
What happens to the formula when the waves are not coherent?
Why is this topic so important for later interference formulas?
What is the most common mistake in this topic?
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