Final answer:
The subject's question concerns the numerical aperture of a microscope's objective lens, which is important for gathering light and affects resolution and image brightness.
NA is affected by magnification, lens aperture size, and illumination adjustments. Techniques like immersion can enhance NA, while sample thickness may limit light and resolution.
Step-by-step explanation:
The student's question pertains to the relationship between the numerical aperture (NA) of a microscope objective lens, the brightness of the image, and the thickness of the sample.
The NA is a critical parameter that indicates the light-gathering ability of the microscope objective lens, which directly affects the resolution and the brightness of the image observed through the microscope. It is calculated as NA = n sin α, where 'n' is the refractive index of the medium between the lens and the specimen, and 'α' is the half-angle of the maximum cone of light that can enter the lens.
High NA lenses typically provide better resolution by capturing more light, but as magnification increases, the lens aperture decreases, necessitating adjustments to the illumination intensity, condenser position, and iris diaphragm to maintain image quality.
Furthermore, immersion techniques such as placing oil, glycerine, or water between the objective lens and the cover slip can improve NA by reducing the refractive index mismatch, thus increasing resolution.
However, the actual light-gathering ability, and hence the achievable resolution of the microscope, may be limited by the thickness of the sample, which can restrict the amount of light that passes through and is collected by the objective lens.