(MIC) Microscopes Module Overview

Microscopes Module Overview

Microscopes provide a direct image of a small object of interest that often would not be seen by the naked eye. There are numerous types of microscopes. Each type offers the forensic scientist a unique opportunity to view and analyze different types of forensic evidence. Whether the Forensic Scientist is analyzing paint chips or examining for gun shot residue, microscopes make the seemingly ordinary minute evidence come alive in very detailed images. Today, all modern crime labs are equipped with multiple microscopes due to their importance in evidence analysis.

Essential Questions

  1. What are the parts of a microscope?
  2. Are some types of microscopes better than others for certain types of evidence analysis?
  3. When is a microspectrophotometer used in Forensic Science?
  4. Why are microscopes important to Forensic Science?

Key Terms

  1. Ocular Lens - The lens that you look through at the top of the microscope.
  2. Objective Lenses - The magnifying lenses that typically range from 4X to 100X on a compound microscope.
  3. Coarse Focus - This is the rough focus knob on the microscope. You use it to move the objective lenses toward or away from the specimen.
  4. Fine Focus - This is the knob used to fine tune the focus on the specimen. It is also used to focus on various parts of the specimen. Generally one uses the coarse focus first to get close then moves to the fine focus knob for fine tuning.
  5. Stage - The flat plate where the slides are placed for observation.
  6. Stage Clips - Clips on the stage used to hold the slide in place.
  7. Aperture - The hole in the stage through which the base (transmitted) light reaches the stage.
  8. Illuminator - A light source mounted under the stage. Three types of light are commonly used- Tungsten, Fluorescent and Halogen. Tungsten is the least expensive and most common. Fluorescent is bright, white and runs cool and Halogen is very bright and white but gives off heat like tungsten.
  9. Condenser Lens - A lens mounted in or below the stage whose purpose is to focus or condense the light onto the specimen. The higher power objective lenses have very tiny diameters and require concentrated light to work properly. By using a condenser lens you will increase the Illumination and resolution. Condenser lenses are not required on low power microscopes.
  10. Diaphragm - Generally a five hole disc placed under the stage on a high power microscope. Each hole is of a different diameter. By turning it, you can vary the amount of light passing through the stage opening. This will help to properly illuminate the specimen and increase contrast and resolution. The diaphragm is most useful at the higher powers.
  11. Compound Microscope - A compound microscope uses two lenses to magnify smaller particles such as insect larvae found on a decomposing body or tree pollen collected from a victim's clothing. Compound microscopes require thin evidence samples that are placed on a glass slide for examination.
  12. Comparison Microscope - A comparison microscope is a version of the compound microscope that magnifies two different samples at the same time, side by side, and can even overlay one image over another. This is useful for comparing two fibers, for example, to see if they are microscopically similar and came from the same source.
  13. Stereoscopic Microscope - A stereoscopic microscope views particles of evidence in three dimensions and allows the investigator to turn over a tiny object such as a seed or a hair beneath the microscope. This is useful for evidence that is not thin enough to see well with a standard compound microscope.
  14. Scanning Electron Microscope - A scanning electron microscope uses beams of electrons rather than light to look at elements of tiny specimens such as gunpowder residue from bullets or ink from a ransom note.
  15. Polarizing Microscope - Similar to compound light microscopes, however it has a "polarizer" in the condenser which can be rotated to analyze the sample. When a ray of light is decomposed into two rays such as this, it is called birefringence. Polarizing microscopes are used to study birefringent materials such as crystalline solids, some illicit drugs, and certain fibers.
  16. Microspectrophotometer - A hybrid tool which combines a microscope with a spectrophotometer. A microspectrophotometer allows the scientist to acquire spectra of extremely small sample areas non-destructively. Measurements can be made while light is transmitted through the sample, reflected from it or even when the sample is made to emit light. It is often used in Forensics for trace evidence analysis such as comparison of dyes in fibers.
  17. Spectra - A continuum of color formed when a beam of white light is dispersed (as by passage through a prism) so that its component wavelengths are arranged in order
  18. Magnification - The act or process of enlarging the physical appearance or image of something.
  19. Resolution - The shortest distance between two separate points in a microscope's field of view that can still be distinguished as distinct entities. It directly relates to the clarity of the image when viewed.

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