Question:

When a ray of light is refracted from one medium to another, then the wavelength changes from \(6000\text{ \AA}\) to \(4000\text{ \AA}\). The critical angle for the interface will be

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Key: $\lambda \propto \frac{1}{n}$
Updated On: May 8, 2026
  • \(\cos^{-1} \left( \frac{3}{2} \right)\)
  • \(\sin^{-1} \left( \frac{2}{\sqrt{3}} \right)\)
  • \(\sin^{-1} \left( \frac{2}{3} \right)\)
  • \(\cos^{-1} \left( \frac{2}{\sqrt{3}} \right)\)
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The Correct Option is C

Solution and Explanation


Concept: Refractive index relation: \[ \frac{n_2}{n_1} = \frac{\lambda_1}{\lambda_2} \]

Step 1:
Substitute values. \[ \frac{n_2}{n_1} = \frac{6000}{4000} = \frac{3}{2} \]

Step 2:
Critical angle formula. \[ \sin C = \frac{n_1}{n_2} \] \[ \sin C = \frac{2}{3} \]

Step 3:
Conclusion.
\[ C = \sin^{-1}\left(\frac{2}{3}\right) \] Final Answer: Option (C)
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