Electron Microscopy TEM and SEM: Principles and Uses

Electron microscopy uses a beam of electrons instead of light to image specimens at much higher magnification and resolution. Transmission electron microscopy (TEM) passes electrons through an ultra-thin specimen, while scanning electron microscopy (SEM) scans the surface to reveal three-dimensional detail.

It matters because many structures such as viruses, ribosomes and internal organelle membranes are far below the resolution of light microscopes. Electron microscopy made modern cell biology and nanotechnology possible.

Key Concepts

• Electron gun: heated filament producing electrons accelerated by high voltage, typically 40 to 300 kilovolts.
• Electromagnetic lenses: magnetic coils focusing the electron beam instead of glass lenses.
• TEM: forms two-dimensional internal image; resolution up to about 0.1 nanometres.
• SEM: detects secondary electrons from surface; gives three-dimensional view with resolution about 1 to 10 nanometres.
• Vacuum: essential because electrons scatter in air; specimens must be fixed, dehydrated and often metal-coated.

Specimens for electron microscopy cannot be alive due to preparation and vacuum requirements.

How It Works

1. Electrons are emitted and accelerated towards the specimen in a vacuum column.
2. In TEM, electrons passing through dense parts scatter more, creating contrast on a fluorescent screen or camera.
3. In SEM, a focused beam scans point by point and detectors build an image from emitted secondary electrons.
4. Magnetic lenses adjust focus and magnification electronically.

Heavy metal stains such as uranyl acetate increase contrast in TEM by scattering electrons strongly.

Worked Example

Suppose a researcher wants to view a virus of diameter 100 nanometres. A light microscope with resolution 200 nanometres cannot separate its features. In TEM at 100,000× magnification, the virus image size becomes 100 nanometres × 100,000 = 10,000,000 nanometres or 10 millimetres on the screen, easily measurable. In SEM, the same virus coated with gold appears as a distinct spherical particle with surface texture. This calculation shows why electron microscopy is chosen when specimen size is below 200 nanometres and internal or surface detail is required.

Common Misconceptions

• Misconception: Electron microscopes use light focused by glass. Correction: they use electron beams and electromagnetic lenses in vacuum.
• Misconception: SEM shows internal structure like TEM. Correction: SEM reveals surface topography; TEM reveals internal ultrastructure.
• Misconception: Living cells can be watched in electron microscopes. Correction: preparation kills cells and vacuum prevents live observation.

Real-World Applications

TEM is used to study viruses, mitochondrial cristae and synaptic structures, while SEM examines insect surfaces, pollen grains and nanomaterials. In medicine, electron microscopy aids diagnosis of kidney diseases and viral infections. Semiconductor industry relies on SEM for chip inspection at nanometre scale.

To recap, TEM and SEM overcome the light resolution barrier by using electrons. TEM excels at internal detail and SEM at surface three-dimensional imaging, together completing our view of the microscopic world.