What factor affects the resolving power of a light microscope?

The resolving power of a light microscope is fundamentally influenced by the numerical aperture of the lens. Numerical aperture (NA) is a measure of a lens's ability to gather light and resolve fine specimen detail at a fixed object distance. It incorporates the refractive index of the medium in which the lens is working (air, oil, etc.) and the sine of the maximum angle of the light cone that can enter or exit the lens.

A higher numerical aperture allows the microscope to collect more light and produce a clearer, more detailed image. This is essential for distinguishing between closely spaced objects, which is the core purpose of resolution in microscopy. Without an adequately high numerical aperture, the microscope would be unable to resolve smaller structures, leading to blurred images and poor detail.

The other options, while they may influence overall optical performance or usability, do not directly affect the fundamental resolving capability of the microscope as much as the numerical aperture does. The size of the eyepiece impacts magnification but not resolution. The brightness of the bulb can improve visibility but does not enhance the ability to resolve fine detail, and the type of illumination affects contrast rather than resolving power directly. Therefore, the numerical aperture is the critical factor when considering the resolving power of a light microscope.