# 24.7 ELECTRON MICROSCOPE To see objects too small for a conventional light microscope, scientists use the **electron microscope**. This instrument employs a beam of electrons rather than light. Since electrons have a wavelength thousands of times shorter than visible light, electron microscopes can reveal structures at the nano-scale with exceptional clarity. This section explains the basic principles of electron microscopy, including the use of magnetic fields as lenses to focus electron beams. ## Definition and Principle The *electron microscope* is a device based on the **wave nature of electrons**. It uses accelerated electrons as an illumination source to observe objects in detail, achieving remarkably high resolution at the nano-scale. This is a direct application of . ## Mechanism of Operation * **Electron Source**: Tungsten metal and a high potential source are used to excite electrons via thermionic emission, forming an electron stream (beam). * **Focusing**: Magnetic coils (**magnetic lenses**) are used to focus the electron beam onto the target substance. These lenses utilize the Lorentz force to deflect and converge the charged electron particles. * **Control**: Increasing the applied potential increases the velocity of electrons, which decreases their de Broglie wavelength. The strength of the magnetic lenses allows for precise focusing. ## Wave Nature Advantage The electron microscope demonstrates the practical application of the wave nature of electrons. The resolving power of a microscope is limited by the wavelength of the radiation used. Because electrons can be accelerated to high speeds, their de Broglie wavelength is thousands of times shorter than visible light, enabling the distinction of details not visible with optical microscopes. ## Wavelength Comparison | Illumination Source | Wavelength | | :--- | :--- | | Visible Light | $4000\ \mathrm{\AA}$ to $7000\ \mathrm{\AA}$ | | Electrons (accelerated to $10{,}000\ \mathrm{eV}$) | About $0.12\ \mathrm{\AA}$ | ## Sample Preparation The sample must be carefully prepared (often sliced extremely thin) and placed in a **vacuum chamber**. A vacuum is essential because electrons are easily scattered or absorbed by air molecules. ## Components The electron microscope comprises the following key parts in order from electron source to image detection: 1. **Electron gun**: Generates the electron beam. 2. **Condenser lens**: Focuses the electron beam onto the sample. 3. **Sample stage**: Holds the sample. 4. **Objective lens**: Forms the initial magnified image. 5. **Intermediate lens**: Further magnifies the image. 6. **Projector lens**: Projects the final image onto the detector. 7. **Detector or screen**: Displays the final image (often a fluorescent screen). 8. **Vacuum system**: Maintains a vacuum to prevent electron scattering. The image may be captured as a photograph, called an *electron micrograph*. ## Resolution The detail detection capability of any microscope is comparable to the wavelength of the radiation used. Electrons can be accelerated to very high kinetic energies, resulting in very short wavelengths (about 100 to 1000 times shorter than visible light). Consequently, modern electron microscopes can resolve details as small as $0.2\ \mathrm{nm}$. ## Applications The introduction of the electron microscope significantly eased the study of: * **Biology**: Visualizing viruses, organelles, and DNA. * **Material Science**: Examining the atomic structure of metals and crystals. * **Nanotechnology**: Designing and inspecting circuits at the nano-scale.