In interference experiment the path difference between two interfering waves at a point A on the screen is λ/3, where λ is the wavelength of these waves, and at another point B the path difference is λ/6. The ratio of intensities at points A and B is ______. (a) 3 (b) 4 (c) 1/3 (d) 1/4

Answer: (c). I = Imaxcos ²φ/2, φ = (2π)/(λ)Δ x. At A: φ = 2π/3, IA = Imaxcos ² 60° = Imax4. At B: φ = π/3, IB = Imaxcos ² 30° = 3Imax4. (IA)/(IB) = 1/3. So the answer is (c) 1/3.

In interference experiment the path difference between two… | JEE Main 2026 (6 Apr, Shift 1)

Q16JEE Main 2026 · 6 APR · SHIFT 1Medium

Superposition, Coherence & Intensity

In interference experiment the path difference between two interfering waves at a point $A$ on the screen is $\lambda/3$ , where $\lambda$ is the wavelength of these waves, and at another point $B$ the path difference is $\lambda/6$ . The ratio of intensities at points $A$ and $B$ is ______.

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Answer: (c)

$I = I_{max}\cos^2\dfrac{\phi}{2}$ , $\phi = \dfrac{2\pi}{\lambda}\Delta x$ .
At $A$ : $\phi = 2\pi/3$ , $I_A = I_{max}\cos^2 60^\circ = \dfrac{I_{max}}{4}$ .
At $B$ : $\phi = \pi/3$ , $I_B = I_{max}\cos^2 30^\circ = \dfrac{3I_{max}}{4}$ .
$\dfrac{I_A}{I_B} = \dfrac{1}{3}$ . So the answer is (c) $1/3$ .

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