Final answer:
The largest NA for a lens designed to be used in air is 1, but immersion lenses can achieve an NA greater than 1 by utilizing a medium with a refractive index higher than air.
Step-by-step explanation:
The largest theoretically possible Numerical Aperture (NA) for an objective lens designed for use in air is NA = n sin α, where n is the refractive index of the medium between the lens and the specimen, and α is half the angle of acceptance. As the angle α increases, the NA becomes larger, enabling more light to be gathered from a smaller focal region, which in turn provides higher resolution. For a lens operating in air (where n = 1), the maximum value of sin α cannot exceed 1, meaning that the largest NA achievable is 1. However, it is important to note that immersion lenses using a medium such as oil or water can have an NA greater than 1. This is because immersion media have a refractive index greater than air, thus allowing for a higher NA and improved resolution.