Optical depth

Optical depth is a measure of how much a medium, like the atmosphere or a cloud, absorbs or blocks light as it passes through. If the optical depth is small (less than 1), most light makes it through; if it's large (greater than 1), the medium is thick and absorbs much of the light. Think of it like a fog: in light fog, you can see through easily (low optical depth), but in dense fog, visibility drops significantly (high optical depth). It helps scientists understand how transparent or opaque a material is to radiation or light.