Which part of are classified by their particle size?

Soil particles vary greatly in size, and soil scientists classify soil particles into sand, silt, and clay. Starting with the finest, clay particles are smaller than 0.002 mm in diameter. Some clay particles are so small that ordinary microscopes do not show them. Silt particles are from 0.002 to 0.05 mm in diameter.

Concave mirror is used in headlights of vehicles, because when the bulb of headlight is placed at focus of concave mirror, it allows the light to spread out to infinity (longer distances).