What is numerical aperture
The numerical aperture (NA or n.A.) of an objective or another optical element describes its light intensity and resolution. The larger the NA, the better a lens can resolve details in the specimen (see Abbe theory)
The value for the numerical aperture results from half the opening angle of the objective and the medium between the front lens and cover glass.
NA = n * sin α
n = refractive index of the medium (air, oil, ...)
α = half the opening angle of the lens
Is located air as a medium between lens and cover glass, the NA maximum 1 be. Otherwise the opening angle is too flat and the light is totally reflected on the cover glass.
To a higher NA and thus also to achieve a larger angle of incidence of light, total reflection at the phase boundary between the cover glass and the medium must be prevented. For this purpose, a medium is used which has roughly the same refractive index as glass. This is most of the time Immersion oil.
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