Q.25 Which one of the following remains unchanged when light waves enter water from air?
(A) Wavelength (B) Wavenumber (C) Frequency (D) Intensity

Frequency remains unchanged when light waves enter water from air. This is a fundamental property of electromagnetic waves during refraction.

Option Analysis

Wavelength decreases in water because the speed of light reduces (v = c/n, where n ≈ 1.33 for water), and since frequency stays constant, λ = v/f shortens.​
Wavenumber (k = 2π/λ or 1/λ) increases as wavelength decreases.​
Frequency (f) is invariant because it depends on the source, not the medium; wave crests per second remain the same.​
Intensity generally decreases due to reflection and absorption at the interface, though some transmits.​

Correct Answer

(C) Frequency.​

Light waves entering water from air trigger refraction, bending the path due to speed change. The key phrase “light waves enter water from air” highlights a core optics concept: frequency remains unchanged amid shifts in other properties. This principle underpins phenomena like rainbows and lenses, vital for competitive exams like CSIR NET Life Sciences.

Refraction Basics

Light slows in water (refractive index n=1.33), following Snell’s law: n1 sinθ1 = n2 sinθ2. Speed drops from c (3×10^8 m/s in air) to c/n, but the wave equation v = fλ holds, with f fixed.​

Property Breakdown

• Wavelength (A): Shortens as λ_water = λ_air / n; e.g., 600 nm in air becomes ~450 nm in water.​​

• Wavenumber (B): Inversely proportional to λ, so it rises (¯k = 1/λ).​

• Frequency (C): Constant, as photons maintain energy E = hf; medium doesn’t alter source oscillation.​

• Intensity (D): Reduces via partial reflection (Fresnel equations, ~2% for normal incidence) and scattering.​