In electromagnetic absorption spectroscopy, for example, interaction coefficient (e.g. Q in cm) is variously called opacity, absorption coefficient, and attenuation coefficient. In nuclear physics, area cross-sections (e.g. σ in barns or units of 10 cm), density mean free path (e.g. τ in grams/cm), and its reciprocal the mass attenuation coefficient (e.g. in cm /gram) or area per nucleon are all popular, while in electron microscopy the inelastic mean free path (e.g. λ in nanometers) is often discussed instead.