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Distinguish between interference and diffraction of light. A double slit arrangement produces interference fringes for sodium light of wavelength 589 nm, that are 0.20 degree apart. What is the angular fringe separation if the entire arrangement is i…

Q.29 Distinguish between interference and diffraction of light. A double slit arrangement produces interference fringes for sodium light of wavelength 589 nm, that are 0.20 degree apart. What is the angular fringe separation if the entire arrangement is immersed in water? (R.I. of water = 1.33)

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Answer: Angular fringe separation in water \(\approx 0.15^\circ\).

Interference vs diffraction (any four):

InterferenceDiffraction
It is the superposition of waves from two separate (coherent) sources.It is the superposition of secondary wavelets from different parts of the same wavefront.
All bright fringes have (nearly) the same intensity.The central bright band is most intense; intensity of the other bright bands decreases rapidly.
Fringe widths are (nearly) equal.Fringe widths are not equal; the central maximum is twice as wide as the others.
Dark fringes are perfectly dark (minimum intensity nearly zero).Dark bands are not perfectly dark.
The number of fringes is large.The number of bands is small.

Numerical: The angular fringe separation is \(\theta = \dfrac{\lambda}{d}\). In water the wavelength becomes \(\lambda_w = \dfrac{\lambda}{n}\), so

\[\theta_w = \frac{\lambda_w}{d} = \frac{\theta}{n} = \frac{0.20^\circ}{1.33} \approx 0.15^\circ\]
Doubtly AI · 28 Oct 2026 AI-verified · Oct 2026 Report
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