Measuring cells with a microscope
Use an eyepiece graticule at different magnifications to measure the real length of plant and animal cells.
Objectives
- Use low and high power objectives.
- Calculate total magnification.
- Measure a cell with an eyepiece graticule and convert divisions to micrometres.
Theory
Total magnification = eyepiece magnification × objective magnification. This microscope has a ×10 eyepiece. The eyepiece graticule is a scale of 100 divisions seen on top of the specimen. Its divisions do not change size when you change objective, so it must be calibrated for each one. For this microscope one division represents: ×4 objective: 25 µm · ×10 objective: 10 µm · ×40 objective: 2.5 µm Actual length = number of divisions × value of one division. 1 mm = 1000 µm. Plant cells have a cell wall and a regular shape; animal cells have neither.
Total magnification = eyepiece × objective Actual length = divisions × µm per division
Procedure
- Measure an onion cell with the ×4 objective.
- Measure an onion cell again with the ×10 objective.
- Measure a cheek cell at ×40.
- Convert each reading to micrometres.
Precautions
- Focus at low power first.
- At high power use only the fine focus, so the lens does not hit the slide.
- Measure several cells and average.
- Subject
- Biology
- Grade
- 9–11
- Chapter
- Cell (Nepal National Curriculum (CDC), grade 9)
- Difficulty
- Medium
- Duration
- 20 min
- Safety level
- Low
- Version
- 1.0
Apparatus
- Light microscope (×10 eyepiece)
- Eyepiece graticule
- Prepared slides
- Stain
- Coverslips
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