Imaging Biological Samples with Optical Microscopy

Optical microscopes use light sources to illuminate samples through a series of lenses. The condenser focuses light onto the specimen, while objective and ocular lenses work together to create a magnified, inverted two-dimensional image. Maximum magnification reaches 1000 times by multiplying the magnifications of both lenses, allowing visualization of structures like prokaryotic cells and large organelles.

Numerical aperture determines a lens's ability to gather light, depending on the angle of the light cone and the refractive index of the medium—air, water, or oil. Higher numerical aperture improves resolution and light collection. Resolution is inversely proportional to wavelength and numerical aperture; smaller values yield higher resolution in magnified images.

Optical microscopes have a resolution limit of 0.2 micrometers, allowing them to resolve prokaryotic cells and large organelles within eukaryotic cells, which are approximately 0.5 micrometers in diameter. Bacteria become visible around 400× magnification, but viruses remain too small to observe with optical microscopy.

At very high magnifications, air between the specimen and lens scatters light because air and glass have significantly different refractive indices. Oil immersion lenses use immersion oil with a refractive index similar to glass, filling this gap. This increases the maximum angle at which light strikes the lens, improving light collection and resolution.

Chromophores are pigments that absorb and reflect particular wavelengths of light. Stains artificially add chromophores to specimens, increasing contrast and resolution. Different colors interact differently with chromophores in specimen structures, making them appear darker than the bright background and creating maximally sharp images.

The condenser lens, located below the stage, focuses all light rays from the illuminator onto the specimen to maximize illumination. A diaphragm between the condenser and specimen adjusts light levels without moving the condenser. The rheostat dimmer switch also controls illuminator intensity, ensuring proper brightness for detailed image visualization.

When images are magnified, they become dimmer because there is less light per unit area of the image. To compensate, brightfield microscopes require intense lighting from high-intensity bulbs. The condenser lens focuses available light onto the specimen to maximize illumination at high magnifications.