reflection coefficient
1. (R) The ratio of the amplitude of a reflected ray (A₁) to that of the incoming ray (A₀), such that R = A₁A₀. In the case of a normally incident ray, R can be expressed in terms of the acoustic impedances of the two media above and below the reflector, Z₁ and Z₂, so that R = (Z₂ − Z₁)/(Z₂ + Z₁). The range of values for R lie between −1 and +1. If R is negative a phase reversal (π) in the wave occurs at the reflector. For water/air R has a typical value of −1; for rocks R has an average value of 0.2 or less. See also TRANSMISSION COEFFICIENT. The reflection coefficient can also be expressed in terms of energy (R′), when R′ = R₂.

2. (k) A ratio of true resistivities, such that k = (ρ₂ − ρ₁)/(ρ₂ + ρ₁), where ρ₁ and ρ₂ are the true resistivities above and below an interface.

reflectance (reflectivity)
1. The ratio of electromagnetic radiation reflected by a surface to that which is incident upon it.

2. In ore microscopy, the amount of incident polarized light which is reflected off the surface of an opaque mineral. The value of the percentage reflectance (R%) at a specified wavelength is a useful quantitative aid to mineral identification and is determined by: R% = (intensity of reflected light/intensity of incident light) × 100.