The numerical aperture of a microscope objective is the measure of its ability to gather light and to resolve fine specimen detail while working at a fixed object (or specimen) distance. Resolution of an optical microscope is defined as the smallest distance between two points on a specimen that can still be distinguished as two separate entities.
Definition of Numerical aperture
Image-forming light waves pass through the specimen and enter the objective in an inverted cone as illustrated in the following Figure.
The numerical aperture or NA is calculated by the equation:
n is the refractive index of the immersion media between test specimen and objective lense; u or a is the one-half of the objective’s opening angle. The largest a is 90 degree; therefore, if in air the largest NA is 1, or 0.95 in practical. The immersion media can be water (RI=1.3), oil or glass(RI=1.51). And NA can be larger than 1, e.g. 1.4.
Relationship between NA and resolution
Resolution is related to the NA value by the following equations:
The larger NA, the better resolution the microscope can get. The second term in equation 3 is the NA value of the condenser in the configuration of figure 1 (right). Other factors also affect the resolution; thus the multiplications in equation 1 and 2 are different.
Figures and some information of this post are from Reference 1, Reference 2, Reference 3 and more details can be obtained there.