Objective lens numerical aperture

While this information may seem like arcane trivia it is actually very important in the proper utilization of a microscope because it determines how the lens is used. For example, the maximum value of the numerical aperture of air is equal to 1 since the refractive index for air is 1 and the maximum value for the sine function is 1. Thus, a different media with the refractive index that is greater than 1 must be employed to overcome this limitation. Oil which has a refractive index of 1,4 is commonly used to increase the maximum numerical aperture to 1,4 this shows how the magnification can be increased by changing the media used. The numerical (q>erture is often printed on the objective, thus, numerical apertures less than 1 suggest that it is designed to be used... 

The first two numbers on the lens are always the magnification and the numerical aperture (NA) of the lens. Numerical aperture is defined as (n sina), where n is the refractive index of the medium in front of the objective and a is half the angular range of light that the lens can accept. The resolution of the tens, if it is perfect, is proportional to 1/(NA), and the depth of... 

Laser trapping is a technique to manipulate small sized materials, which was developed by Ashkin in 1970 [20, 21]. In this experiment, a laser beam is tightly focused by an objective lens with high numerical aperture (NA), and a dielectric... 

Dark-field illumination is classified into three types. The first one is for a microscope equipped with low numerical aperture (NA) objective lenses (see Fig. 1). To cast a shadow at the objective lens, a ring-slit as shown in Fig. IB is inserted into the light path. The second is for highNA (>0.5) objective lenses. Special, ready-made dark-field condensers or lenses are used for dark-field illumination. The third is independent... 

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