Editing Numerical aperture (section)

{{Short description|Characteristic of an optical system}}
[[Image:Numerical aperture.svg|thumb|The numerical aperture with respect to a point {{mvar|P}} depends on the half-angle, {{math|''θ''{{sub|1}}}}, of the maximum cone of light that can enter or exit the lens and the ambient index of refraction. As a [[Pencil (optics)|pencil]] of light goes through a flat plane of glass, its half-angle changes to {{math|''θ''{{sub|2}}}}. Due to [[Snell's law]], the numerical aperture remains the same: {{math|1= NA = ''n''{{sub|1}}{{tsp}}sin ''θ''{{sub|1}} = ''n''{{sub|2}}{{tsp}}sin ''θ''{{sub|2}}}}.]]
In [[optics]], the '''numerical aperture''' ('''NA''') of an optical system is a [[dimensionless number]] that characterizes the range of angles over which the system can accept or emit light. By incorporating [[index of refraction]] in its definition, {{abbr|NA|numerical aperture}} has the property that it is constant for a beam as it goes from one material to another, provided there is no [[refractive power]] at the interface (e.g., a flat interface). The exact definition of the term varies slightly between different areas of optics. Numerical aperture is commonly used in [[microscopy]] to describe the acceptance cone of an [[Objective (optics)|objective]] (and hence its light-gathering ability and [[Optical resolution| resolution]]), and in [[fiber optics]], in which it describes the range of angles within which light that is incident on the fiber will be transmitted along it.