Lower size limit for optical microscopy

The theoretical limit of resolution of an optical microscope is expressed by the fundamental formula  

For X - 0.6 jum the resolving power is a maximum with NA = 0.95 (diy) and NA = 1.40 (wet) giving lower size limits, = 0.38 pm and 0.26 pm respectively. The images of particles having a separation of less than these limits merge to form a single image. 

The resolution of the human eye is around 0.3 mm, therefore the maximum effective magnification with white light is  

Rowe [24] showed that wide differences in particle sizing can occur between operators because of this effect. 

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