What’s the resolution of a microscope? Is this related to numerical aperture?
November 24, 2008
A microscope’s resolution is its ability to form separate images of lines or dots. It’s defined in terms of the actual distance between details on the object. The wave nature of light puts a practical and theoretical limit on the resolution of a white-light microscope as about half a wavelength of visible light, about 250 nanometers or a quarter-micron. This also limits the highest useful magnifications to the range of 1000x-2000x. Resolution depends solely on the objective lens, as the eyepiece just magnifies light from the objective.
The numerical aperture of a microscope lens is a measure of the increase in resolution due to the lens being able to gather a wider cone of light from the specimen. This depends on how close the lens is to the specimen and also if oil or another fluid is used between the lens and the specimen. Both increase the numerical aperture and the microscope’s resolving power.
Numerical aperture is higher for higher-power objectives because they are used very close to the specimen. Its value for the objective is usually engraved on the barrel and runs between 0.04 for very low-power objectives and 1.4 for very high-power objectives. A typical 40x objective that will be used at 400x with a typical 10x eyepiece has a numerical aperture of 0.65.
A condenser’s numerical aperture should equal or exceed that of the objective lens to effectively illuminate the specimen. Higher powers will need condensers with higher numerical apertures, meaning the condenser lens is closer to the specimen. Condensers with numerical apertures of 1.2 are typical for objectives delivering magnifications of 400x.