A-Level Biology: Microscopes Lecture Notes

Overview

• Focus on microscopes, magnification calculation, conversion of units, and calibration of the eyepiece graticule.
• Part of Topic 2: Methods of Studying Cells.
• This session covers types of microscopes, magnification, and eyepiece graticule calibration.

Types of Microscopes

Optical (Light) Microscopes

• Common in schools and use a beam of light to create images.
• Characteristics:
  • Lower resolution due to longer light wavelength.
  • Lower magnification compared to electron microscopes.
  • Produces color images and can use living samples.

Electron Microscopes

• Types: Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM).
• Use a beam of electrons, condensed by electromagnets, to create images.
• Characteristics:
  • Higher resolution due to shorter electron wavelength.
  • Higher magnification.
  • Only produce black and white images.
  • Samples must be in a vacuum and cannot be living.

Transmission Electron Microscope (TEM)

• Electrons pass through the specimen.
• Produces 2D images showing detailed internal structures.

Scanning Electron Microscope (SEM)

• Electrons scatter and reflect off the specimen's surface.
• Produces 3D images detailing texture and depth.

Key Concepts

Definitions

• Magnification: How many times larger an image is compared to the actual object size.
• Resolution (Resolving Power): Minimum distance between two distinguishable objects.

Calculations

• Magnification Formula:
  • Image Size = Actual Size × Magnification
  • Useful to rearrange to find unknown values.
• Conversion of Units:
  • Millimeters to Micrometers: Multiply by 1000.
  • Micrometers to Millimeters: Divide by 1000.

Calibration of Eyepiece Graticule

Eyepiece Graticule

• Glass disk with a scale within the eyepiece.
• Used to measure the actual size of objects viewed under the microscope.
• Requires calibration for different magnifications.

Calibration Process

1. Place a stage micrometer on the microscope stage.
2. Align the eyepiece graticule with the stage micrometer scale.
3. Count how many divisions on the eyepiece fit into one division on the micrometer.
4. Calculate the value of one division on the eyepiece graticule at the current magnification.

Worked Example

• Calculate magnification using a scale bar:
  • Measure the image size of the scale bar and convert units if necessary.
  • Use the formula Image Size / Actual Size = Magnification.

Summary

• Differences between optical and electron microscopes in terms of resolving power and magnification.
• Use of eyepiece graticule and its calibration with a stage micrometer.
• Important skills include magnification calculations and unit conversions.

Linked Topics

• Microbiology math skills, eukaryotic, and prokaryotic cell structures.
• Additional videos available for related content and skills, including rearranging formulas.

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