Water quality optics: The resistance of water to the passage of light—used to indicate the presence of finely dispersed suspended matter—is a measure of which routine laboratory parameter?

Introduction / Context:
Environmental engineers routinely assess how particulate matter affects water clarity. Instruments like nephelometers quantify how suspended solids scatter light. This question distinguishes turbidity from related but different optical or chemical measures such as true color or dissolved constituents.

Given Data / Assumptions:

• “Resistance to the passage of light” implies scattering by fine particles.
• Measurement is done using standardized methods (e.g., NTU with nephelometric technique).
• Samples may contain clay, silt, algae, or colloids.

Concept / Approach:
Turbidity reflects light scattering due to suspended matter; greater turbidity means poorer clarity. True color, by contrast, is due to dissolved chromophoric substances after filtration. Hardness relates to dissolved calcium and magnesium ions and is unrelated to optical clarity, while dissolved gases are invisible contributors to chemistry, not light scattering.

Step-by-Step Solution:
Map definition: light scattering by particles → turbidity.Differentiate from color: color remains after filtration; turbidity typically drops after filtration.Confirm that hardness/dissolved gases do not directly control light transmission.

Verification / Alternative check:
Nephelometric turbidity units (NTU) are widely used in drinking water standards because turbidity correlates with pathogen shielding potential and filter performance.

Why Other Options Are Wrong:
Color: measures dissolved pigments; not primarily scattering.Hardness: chemical property due to Ca2+/Mg2+; no direct optical effect.Dissolved gases: affect chemistry, not clarity.Alkalinity: acid-neutralizing capacity, not an optical measure.

Common Pitfalls:
Equating visible discoloration with turbidity; a sample can be highly colored yet low in turbidity after filtration, and vice versa.

Final Answer:
Turbidity