Since its invention, the classical optical microscope and its successive variants have generally relied on one key element: the objective lens. The objective lens can be a single or compound lens that typically has a short focal length and large numerical aperture, NA = n sinθ, where n is the refractive index of the medium between the objective lens and the sample, and θ is the maximum acceptance angle of the lens. The short focal length facilitates the incorporation of the objective into an optical system with a large magnification so that microscopic objects can be observed by human eye or with a digital camera. The large numerical aperture of the objective enables a well-designed imaging system to resolve microscopic features down to the classical limit of approximately λ/2 NA for incoherent light, where λ is the wavelength of the light.
Microscopes have become much more advanced...
