can be calculated from the refractive index and the incidence angle with the Fresnel equations, which for normal incidence reduces to For common glass in air, n1 = 1 and n2 = 1.5, and thus about 4% of the incident power is reflected. At other incidence angles the reflectivity will also depend on the polarization of the incoming light. At a certain angle called Brewster's angle, p-polarized light (light with the electric field in the plane of incidence) will be totally transmitted. Brewster's angle can be calculated from the two refractive indices of the interface as Lenses The focal length of a lens is determined by its refractive index n and the radii of curvature R1 and R2 of its surfaces. The power of a thin lens in air is given by the Lensmaker's formula: where f is the focal length of the lens. Microscope resolution The resolution of a good optical microscope is mainly determined by the numerical aperture (NA) of its objective lens. The numerical aperture in turn is determined by the refractive index n of the medium filling the space between the sample and the lens and the half collection angle of light θ according to For this reason oil immersion is commonly used to obtain high resolution in microscopy. In this technique the objective is dipped into a drop of high refractive index immersion oil on the sample under study. Relative permittivity and permeability The refractive index of electromagnetic radiation equals where εr is the material's relative permittivity, and μr is its relative permeability. The refractive index is used for optics in Fresnel equations and Snell's law; while the relative permittivity and permeability are used in Maxwell's equations and electronics. Most naturally occurring materials are non-magnetic at optical frequencies, that is μr is very close to 1, therefore n is approximately . In this particular case, the complex relative permittivity εr, with real and imaginary parts εr and ɛ̃r, and the complex refractive index n, with real and imaginary parts n and κ (the latter called the "extinction coefficient"), follow the relation and their components are related by: and: where is the complex modulus. Wave impedance The wave impedance of a plane electromagnetic wave in a non-conductive medium is given by where is the vacuum wave impedance, μ and ϵ are the absolute permeability and permittivity of the medium, εr is the material's relative permittivity, and