Microscopy: Light, Electron, and Beyond

The Resolution Barrier

To see an object clearly, the instrument must distinguish two points that are very close together. This ability is called resolution. In light microscopy, resolution is limited by the diffraction of light. When light passes through a small aperture, it spreads out. If two points are closer than this spread, their images overlap and appear as a single blob. This is not a flaw in the lens, but a fundamental property of waves. The limit is determined by the wavelength of the light used and the numerical aperture of the lens. Visible light has wavelengths between approximately 400 and 700 nanometres. Consequently, standard light microscopes cannot resolve details smaller than about 200 nanometres. This means that while we can see the outline of a cell and large organelles, we cannot see the internal structure of ribosomes or the double layer of the cell membrane using standard white light. To see smaller structures, we must use a "probe" with a much shorter wavelength, such as electrons, or use clever optical tricks to bypass the diffraction limit.

Light Microscopy and the Diffraction Limit

The theoretical limit of resolution for a light microscope is described by the Rayleigh criterion. The minimum distance \( d \) between two distinguishable points is given by:

where \( \lambda \) is the wavelength of light and \( \mathrm{NA} \) (Numerical Aperture) is a measure of the light-gathering ability of the objective lens. The \( \mathrm{NA} \) is defined as \( n \sin \theta \), where \( n \) is the refractive index of the medium between the lens and the specimen, and \( \theta \) is the half-angle of the maximum cone of light that can enter the lens. Using blue light (\( \lambda \approx 450\ \mathrm{nm} \)) and a high-quality oil immersion lens (\( \mathrm{NA} \approx 1.4 \)), the theoretical limit is about 196 nm (the worked example below). In practice, due to aberrations and contrast issues, the effective limit is often cited as 200 nm. This limit explains why we use electron microscopy for sub-cellular structures: electrons have de Broglie wavelengths that are thousands of times shorter than visible light, so in principle they resolve far below a nanometre; for biological specimens, preparation and radiation damage usually limit TEM to about 1–2 nm.

Electron Microscopy: Transmission and Scanning

Electron microscopes use beams of electrons instead of photons. Because electrons have a much shorter wavelength, they can resolve much smaller details. There are two main types: Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). In TEM, a beam of electrons is transmitted through an ultra-thin section of the specimen (typically 50–100 nm thick). The electrons interact with the atoms in the specimen, and the resulting pattern is magnified by electromagnetic lenses to form a two-dimensional image. TEM provides high-resolution views of internal cellular structures, such as the cristae of mitochondria or the double membrane of the nucleus. In SEM, a focused beam of electrons is scanned across the surface of the specimen. The interaction of the beam with the surface generates secondary electrons, which are detected to build up an image of the surface topography with a strong three-dimensional appearance. SEM is excellent for observing the external shape of cells, pollen grains, or insect wings, but it does not provide internal structural detail.

Fluorescence and Super-Resolution

Fluorescence microscopy uses specific dyes or proteins (fluorophores) that absorb light at one wavelength and emit it at a longer wavelength. This allows specific structures to be highlighted against a dark background, providing high contrast. However, standard fluorescence microscopy is still limited by the diffraction barrier. Super-resolution techniques, such as STED (Stimulated Emission Depletion) and PALM (Photoactivated Localisation Microscopy), bypass this limit. STED uses a second laser to deplete fluorescence in a ring around the focal spot, effectively shrinking the area of emission to below the diffraction limit. PALM uses stochastic activation of sparse subsets of fluorophores, allowing their precise localisation to be determined with high accuracy. These techniques have resolved structures in the 20–50 nm range, revealing the detailed organisation of the cytoskeleton and membrane proteins that were previously invisible.