What's the role of numerical aperture in microscope resolution?

The numerical aperture of a microscope determines its resolving power, affecting the detail and clarity of the observed image.

The numerical aperture (NA) is a critical factor in the resolution of a microscope. It is a dimensionless number that characterises the range of angles over which the system can accept or emit light. In simpler terms, it is a measure of the microscope's ability to gather light and resolve fine specimen detail at a fixed object distance.

The resolution of a microscope, which is the smallest distance between two points that can still be distinguished as separate entities, is directly influenced by the numerical aperture. The higher the numerical aperture, the better the resolution. This is because a higher numerical aperture allows more light to enter the microscope, which in turn provides a brighter and more detailed image.

The numerical aperture is determined by the medium between the specimen and the lens (the immersion medium) and the half-angle of the maximum cone of light that can enter or exit the lens. The formula for numerical aperture is NA = n sin θ, where n is the refractive index of the medium and θ is the half-angle of the maximum cone of light.

The refractive index of the medium is also important. For example, oil immersion lenses use oil between the specimen and the lens to increase the refractive index and therefore the numerical aperture, resulting in a higher resolution.

In conclusion, the numerical aperture is a crucial factor in determining the resolution of a microscope. By understanding how it works, you can better understand the capabilities and limitations of different microscopes.