Numerical aperture is a measure of a lens's ability to gather light. It is a critical factor that affects the resolution of an image; the higher the numerical aperture, the better the resolution produced by the microscope.

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Resolution is defined as the ability to tell that two separate points or objects are distinct from one another. In microscopy, a low-resolution image appears fuzzy, whereas a high-resolution image appears sharp. Resolution is primarily affected by two factors: the wavelength of the light or electron beam used (with shorter wavelengths yielding higher resolution) and the numerical aperture of the lens. Additionally, resolution can be increased by using stains that enhance the contrast of the specimen.

Resolution

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Numerical Aperture

Shorter wavelengths are able to resolve smaller objects. For example, an electron microscope has a much higher resolution than a light microscope because it uses an electron beam with a very short wavelength, as opposed to the long-wavelength visible light used by a light microscope.

Electron Microscope Resolution Example

At very high magnifications, the small amount of air between a specimen and the lens can scatter light rays before they are focused, compromising resolution due to the large difference in refractive indices between air and glass. To prevent this, a drop of immersion oil is applied to fill the space between the specimen and a specialized oil immersion lens. Because the oil has a refractive index very similar to that of glass, it increases the maximum angle at which light can enter the lens, thereby capturing more light and improving the resolution of the image.

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