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Physics · 2025 · 3 marks
CBSE 2025 · Region 4 · Set 3 · Q26
(a) Write the conditions under which two light waves originating from two coherent sources can interfere each other (i) constructively, and (ii)…
(a)
Write the conditions under which two light waves originating from two coherent sources can interfere each other (i) constructively, and (ii) destructively, in terms of wavelength. Can these be applied for two lights originating from two sodium lamps ? Give reason.
(b)
Monochromatic light of green colour is used in Young's double slit experiment and an interference pattern is observed on a screen. If the green light is replaced by red monochromatic light of the same intensity, how will the fringe width of interference pattern be affected ? Justify your answer.
Marking-scheme solution
(a)(i)
Conditions for constructive interference:
$\displaystyle \Delta=n \lambda$
(ii)
Conditions for destructive interference:
$\displaystyle \Delta=\left(n+\frac{1}{2}\right) \lambda$
$\displaystyle n=0,1,2 \ldots$
No.
Two independent monochromatic sources of light cannot be coherent.
(b)
Fringe width $\displaystyle \beta=\frac{\lambda D}{d}$
$\displaystyle \lambda_{\text {red }}>\lambda_{\text {green }}$
$\displaystyle \beta_{\text {red }}>\beta_{\text {green }}$
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CBSE Class 12 Physics past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.