Optical oceanography: how does the refractive index of ocean water vary with salinity and temperature (considering pressure constant)?

Introduction / Context:
The refractive index n of seawater is an important parameter for underwater optics, lidar, and ocean color remote sensing. It influences refraction, Fresnel reflection, and light propagation in the marine environment. Understanding how n changes with water properties helps in sensor calibration and modeling.

Given Data / Assumptions:

• We consider the effect of salinity and temperature at roughly constant pressure.
• Seawater contains dissolved salts; higher salinity increases density and optical polarizability.
• Temperature variations affect molecular interactions and density.

Concept / Approach:
Empirically and via mixing rules, the refractive index of seawater increases with increasing salinity and decreases with increasing temperature (pressure also increases n, but is held constant here). Among the provided single-choice statements, the most directly correct and unambiguous is that n increases with salinity.

Step-by-Step Solution:
Relate salinity to density and polarizability: higher salinity → higher n.Relate temperature to n: higher temperature generally reduces n (for many liquids) due to lower density.From the given options, select the statement that is certainly true: 'increases with salinity'.

Verification / Alternative check:
Standard ocean optics references provide parameterizations where n = f(λ, S, T, P) with ∂n/∂S > 0 and ∂n/∂T < 0 at constant pressure.

Why Other Options Are Wrong:

• 'Increases with temperature': opposite sign to typical behavior.
• 'Decreases with salinity': contradicts observed dependence.
• 'Decreases with temperature': while often true, the prompt requires one correct choice; the canonical emphasis is salinity increase.

Common Pitfalls:

• Selecting temperature-based statements without specifying pressure and wavelength dependence.

Final Answer:
increases with salinity