26.4 Microscopes

Microscopes are powerful tools that allow us to see the invisible world. From simple compound microscopes to advanced electron microscopes, these instruments magnify specimens using various techniques and components.

Understanding microscope types, magnification calculations, and advanced techniques is crucial for scientific research. Whether studying cells or materials, microscopes reveal details that shape our understanding of the microscopic world.

Microscopes

Components of compound microscopes

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• Compound microscopes magnify specimens using a combination of lenses
  • Objective lenses located near the specimen provide initial magnification (4x, 10x, 40x, or 100x)
  • Eyepiece (ocular lens) at the top of the microscope further magnifies the image from the objective lens (usually 10x)
• Illumination system includes a light source (LED or halogen lamp) that illuminates the specimen and a condenser lens that focuses light onto the specimen
• Stage is a platform where the specimen is placed for observation and a mechanical stage allows for precise movement of the specimen
• Focus knobs include coarse focus for initial focusing and large adjustments and fine focus for small, precise adjustments to achieve a clear image
  • Fine focus adjustments are crucial for optimizing the depth of field

Calculation of microscope magnification

• Total magnification is calculated by multiplying the objective lens magnification and the eyepiece magnification
  • Formula: $Total magnification = Objective magnification × Eyepiece magnification$
• Example calculation with an objective lens magnification of 40x and an eyepiece magnification of 10x results in a total magnification of $40 × 10 = 400x$
• The numerical aperture of the objective lens affects the maximum useful magnification

Types of microscopes compared

• Optical microscopes use visible light and a series of lenses to magnify specimens
  • Compound microscopes (described above) are a type of optical microscope
  • Stereo microscopes (dissecting microscopes) use two separate optical paths to provide a three-dimensional view of the specimen with lower magnification (typically up to 100x)
• Electron microscopes use a beam of electrons instead of light to create an image with higher magnification and resolution compared to optical microscopes
  1. Transmission Electron Microscope (TEM) passes electrons through a thin specimen, creating a two-dimensional image with magnifications up to 1,000,000x
  2. Scanning Electron Microscope (SEM) scans the surface of a specimen with electrons, creating a three-dimensional image with magnifications up to 500,000x
• Scanning probe microscopes use a physical probe to scan the surface of a specimen
  • Atomic Force Microscope (AFM) uses a fine tip to measure the surface topography of a specimen with atomic resolution
  • Scanning Tunneling Microscope (STM) uses a conductive tip to measure the electronic structure of a specimen's surface with atomic resolution

Advanced microscopy techniques

• Phase contrast microscopy enhances the visibility of transparent specimens by converting phase shifts in light passing through the sample into brightness changes
• Fluorescence microscopy uses specific wavelengths of light to excite fluorescent molecules in a specimen, allowing for visualization of specific structures or molecules
• High-resolution microscopy techniques aim to overcome the diffraction limit and reduce aberrations to achieve better image quality and resolution