Which instrument has a fixed light source and provides a qualitative measurement but cannot detect particles that do not scatter light?

Which instrument has a fixed light source and provides a qualitative measurement but cannot detect particles that do not scatter light?

• A. Particle Counters
• B. Turbidity Meters
• C. Flow Cytometer
• D. Spectrophotometer

Answer: (B)

Turbidity is about how cloudy water looks, which is detected by how much light is scattered by particles in the sample. A turbidity meter uses a fixed light source and a detector arranged in a fixed geometry to measure that scattered light. Because the measurement comes from scattering, the instrument can indicate cloudiness but doesn’t identify or count all particles. If particles don’t scatter light, they won’t affect the reading, so these meters give a qualitative sense of turbidity rather than a complete particle census or characterization. Other instruments work differently: particle counters or flow cytometers rely on light scattering (and often fluorescence) to detect and count particles, providing detailed quantitative data. A spectrophotometer measures how much light a sample absorbs at specific wavelengths, which relates to color or dissolved substances, not suspended solids via scattering.

Turbidity is about how cloudy water looks, which is detected by how much light is scattered by particles in the sample. A turbidity meter uses a fixed light source and a detector arranged in a fixed geometry to measure that scattered light. Because the measurement comes from scattering, the instrument can indicate cloudiness but doesn’t identify or count all particles. If particles don’t scatter light, they won’t affect the reading, so these meters give a qualitative sense of turbidity rather than a complete particle census or characterization.

Other instruments work differently: particle counters or flow cytometers rely on light scattering (and often fluorescence) to detect and count particles, providing detailed quantitative data. A spectrophotometer measures how much light a sample absorbs at specific wavelengths, which relates to color or dissolved substances, not suspended solids via scattering.