Why do oil-immersion objectives provide better resolution than dry objectives?

Oil-immersion objectives provide superior resolution compared to dry objectives primarily by increasing the numerical aperture (N.A.) of the system.12

The resolution of a microscope is fundamentally limited by the objective's ability to collect diffracted light from an object. The N.A. is calculated as the product of the sine of the half-angle of the light cone accepted by the lens and the refractive index of the medium between the specimen and the lens.12

While dry objectives are limited by the refractive index of air, oil-immersion objectives utilize a specialized oil with a refractive index similar to that of glass. This medium serves two critical functions:2
* Increased Light Collection: By filling the space between the specimen and the objective, the oil allows for a higher N.A., enabling the lens to capture more diffracted waves, which is essential for reconstructing finer details.12
* Wavelength Reduction: The effective wavelength of light is shortened when it propagates through a denser medium, which further enhances the microscope's ability to resolve smaller structures.12

Consequently, while dry objectives are typically limited to an N.A. of approximately 0.95, oil-immersion objectives can achieve working N.A. values as high as 1.4.12