Step 1: Understanding the Concept:
The question asks for the particle size range that optical microscopy is used to measure.
Step 2: Key Formula or Approach:
An optical microscope uses visible light to form an image, so its resolving power is tied to the wavelength of that light, roughly 0.4 to 0.7 \(\mu\). It cannot distinguish particles much smaller than this wavelength, and it is not the practical choice for particles large enough to see or sieve directly.
Step 3: Detailed Explanation:
Between these two limits sits the working range for optical microscopy, fine powders and suspended particles in the low micron range. Particles far below this, in the nanometer range, scatter light too little to be resolved and need electron microscopy instead. Particles far above this, in the hundreds of microns or millimeters, are easily handled with sieves or simple visual inspection, so a microscope is not the practical tool for them. This is why optical microscopy is described using a fine, low-micron range such as 0.5 to 1.50 \(\mu\) rather than the very small nanometer range or the coarse range used for larger granules.
Step 4: Final Answer:
Optical microscopy is used for particles in the 0.5 to 1.50 \(\mu\) range.