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When light travels from one medium to another, which property of the wave REMAINS UNCHANGED?
Athe wavelength
Bthe speed
Cthe frequency
Dthe refractive index
Answer & Solution
Correct answer: C. the frequency
1. NCERT §9.3 explains the boundary conditions for refraction.
2. FREQUENCY is a property of the SOURCE. When light crosses into a new medium, the atoms at the boundary oscillate at the same rate as the incident wave drives them — so frequency stays the same.
3. The SPEED changes (slower in denser media). Since $v = f\lambda$ and $f$ is fixed, the WAVELENGTH also changes inversely with the speed.
4. In a medium of refractive index $n$: $v_\text{med} = c/n$, $\lambda_\text{med} = \lambda_\text{vac}/n$, $f$ unchanged.
5. Option A is wrong (wavelength changes). Option B is wrong (speed changes). Option D is a property of the medium, not the wave.
_Source: NCERT Class 12 Physics Part 2, Ch 9, §9.3 (Refraction — frequency, wavelength, speed), p. 2._
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