2.
Which of the following best explains why the refractive index of water changes with
temperature?
a. Due to change in density
b. Due to change in surface tension
c. Due to change in viscosity
d. Due to evaporation

The refractive index of water changes with temperature primarily due to variations in its density, making option a the correct answer.

Option Analysis

a. Due to change in density: Water expands as temperature rises (except near 4°C), reducing its density. Lower density means fewer molecules per unit volume, allowing light to travel faster through the medium, which decreases the refractive index (n = c/v). This is the dominant effect, as confirmed by experimental data showing nearly linear correlation between refractive index and density.​

b. Due to change in surface tension: Surface tension affects liquid-air interfaces but has no direct impact on bulk refractive index, which measures light propagation through the liquid volume.​

c. Due to change in viscosity: Viscosity influences flow resistance and slows particles but does not alter the speed of light propagation or molecular polarizability significantly enough to change refractive index.​

d. Due to evaporation: Evaporation reduces volume over time but is irrelevant to instantaneous temperature effects on refractive index in a closed system.​

The refractive index of water changes with temperature due to density shifts, a key concept in optics and physics exams like CSIR NET. As temperature increases, water’s density decreases, directly lowering its refractive index from about 1.333 at 20°C to lower values at higher temperatures.​

Scientific Mechanism

Refractive index (n) is the ratio of light speed in vacuum (c) to the medium (v). In water, rising temperature causes thermal expansion, reducing molecular density and thus optical density. Studies show dn/dT ≈ -10^{-4}/°C, mainly from density (ρ), with minor contributions from polarizability. This inverse density-refractive index relation holds for most liquids.​

Practical Implications

• Optics Applications: Temperature corrections are vital in refractometers (e.g., subtract 0.00045 per °C).​

• Exam Relevance: Questions test density’s primacy over viscosity or surface tension.

• Exceptions: Water’s anomalous expansion below 4°C reverses the trend briefly.​