Water Drops High-speed Photography

The term droplet is a diminutive form of 'drop' - and as a guide is typically used for liquid particles of less than 500 µm diameter. In spray application, droplets are usually described by their perceived size (i.e., diameter) whereas the dose (or number of infective particles in the case of biopesticides) is a function of their volume. This increases by a cubic function relative to diameter; thus a 50 µm droplet represents a dose in 65 pl and a 500 µm drop represents a dose in 65 nanolitres.
Optics
Due to the different refractive index of water and air, refraction and reflection occur on the surfaces of raindrops, leading to rainbow formation.