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The Microscope

The Microscope. Honors Forensic Science. I. The Microscope. A. An optical instrument that uses a lens or combination of lenses to magnify and resolve the fine details of an object. II. Images. A. Virtual image i . Can only be seen by looking through a lens and cannot be viewed directly

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The Microscope

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  1. The Microscope Honors Forensic Science

  2. I. The Microscope • A. An optical instrument that uses a lens or combination of lenses to magnify and resolve the fine details of an object

  3. II. Images • A. Virtual image • i. Can only be seen by looking through a lens and cannot be viewed directly • B. Real image • Can be viewed directly

  4. III. Types of Microscopes • A. Magnifying glass • i. Magnification of 5-10 times • Ii. Single lens

  5. B. Compound Light Microscope • i. Magnification up to 1500 times • Ii. Multiple lenses • Iii. Objective lens • 1. lower lens of a microscope that is positioned directly over the specimen • Iv. Eyepiece lens • 1. lens of a microscope into which the viewer looks, same as ocular lens

  6. v. Mechanical system • 1. base – support on which instrument rests • 2. arm – C-shaped /handle/support • 3. stage – horizontal plate upon which specimens are placed for study

  7. v. Cont. • 4. body tube – hollow tube on which objective and eyepiece are mounted at opposite ends • 5. coarse adjustment – focuses lenses • 6. fine focus – focuses but on smaller magnitude

  8. c. Optical system • i. Illuminator • 1. transmitted illumination – light that passes up from condenser and through specimen • 2. vertical or reflected illumination – illumination of a specimen from above

  9. Ii. Condenser – collects light rays from illuminator and concentrates them onto specimen • Iii – objective lens – lens closest to specimen • 1. most microscopes are parfocal – when an image is focused with one objective in position, the other objective can be rotated into place and then the field will remain in focus

  10. Iv. Eyepiece or ocular lens • 1. monocular – one eyepiece • 2. binocular – two eyepieces

  11. D. Magnifying power • 1. total power = power of objective lens times power of eyepiece lens

  12. e. Numerical aperture • i. The ability of an objective lens to resolve details into separate images instead of one blurred image is directly proportional to the numerical aperture • Ii. Ex. A lens with a NA of 1.30 can separate details that are twice as close as compared to lens with NA of 0.65

  13. f. Other considerations • i. Field of view • 1. the area of the specimen that can be seen after it is magnified • 2. as magnifying power increases, field of view decreases

  14. Ii. Depth of focus • 1. the thickness of a specimen entirely in focus under a microscope • 2. decreases as magnifying power increases

  15. IV. Comparison Microscope • A. Two compound microscopes combined into one unit • B. Uses a bridge incorporating a series of mirrors and lenses to join them • Very useful in forensic science when side-by-side comparisons are necessary

  16. D. Vertical or reflected illumination • i. Used when comparing bullets, cartridges, other opaque objects • E. transmitted illumination • i. Compare hairs or fibers

  17. V. Stereoscopic Microscope • A. Power of 10x to 125x • B. Can present a 3-D image of object • C. Formation of right side up image • D. Very frequently used in crime lab • E. Wide field of view and great depth of focus • F. Used often for examination of paint, soil, gunpowder residues, marijuana, etc.

  18. VI. Polarizing Microscope • A. When a beam of light passes through certain types of substances, it emerges vibrating in only one plane • B. Plane-polarized light • i. Light confined to a single plane of vibration

  19. C. Polarizer • i. Device that permits passage of light waves vibrating in only one plane

  20. D. Second polarizing crystal – analyzer • E. Can modify stereo or compound microscopes so can detect polarized light • F. Application – study materials that polarize light ex. Birefringent substances

  21. VII. Microspectrophotometer • A. An instrument that links microscope to a spectrophotometer • B. Spectrophotometer = an instrument used to measure and record the absorption spectrum of a chemical substance • C. Allows better comparison of substances

  22. VIII. Scanning Electron Microscope (SEM) • A. Image is produced by aiming a beam of electrons onto the specimen and studying electron emissions on a closed TV circuit or computer • B. High magnification • C. High resolution

  23. D. High depth of focus • E. Used as tool for determining whether or not a suspect has recently fired a gun • F. X-ray analyzer

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