Calculating Cell Size and Magnification
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of microscope use, standard form, and unit conversions. You can test your knowledge on these below.
What is the equation for magnification?
Magnification = Image size ÷ Actual size
Covert 5mm to μm?
5 x 1000 = 5000μm
Convert 5000μm into standard form?
5000 = 5x10³μm
Topic Explainer Video
Check out this @JoeDoesBiology video that explains calculating cell size and magnification or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Calculating Cell Size and Magnification
In microscopy, understanding how to calculate cell size and magnification is essential for interpreting biological images.
Magnification Equation
Magnification = Image size / Actual size
- Magnification has no units.
- Image size and actual size must be in the same units before calculation (e.g., convert mm to µm where 1 mm = 1000 µm).
- Rearrange the equation depending on what is being asked:
- Actual size = Image size ÷ Magnification
- Image size = Actual size × Magnification
The Formula Triangle May Help You Here!
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Using A Stage Micrometer And An Eyepiece Graticule
When using a microscope, we need to calibrate the eyepiece graticule using a stage micrometer to measure the size of cells.
What Are They?
- Eyepiece Graticule: A ruler inside the eyepiece with an arbitrary scale (no fixed units).
- Stage Micrometer: A slide with a scale of known length (e.g., 1 division = 10 µm).
Calibrating The Eyepiece Graticule
Place the stage micrometer on the stage and focus the microscope.
Align the scales of the stage micrometer and the eyepiece graticule.
Count how many divisions of the eyepiece graticule fit into one division of the stage micrometer.
Calculate the value of one graticule division.
Size of one eyepiece unit = length of micrometer scale / number of eyepiece units
Worked Example
- Stage micrometer (shown in red): 1 division = 10 µm
- 10 graticule divisions (shown in blue) align with 5 stage micrometer divisions (50 µm).
- 1 graticule division = 50 µm / 10 = 5 µm
- Now, if a cell is 5 graticule divisions long, its actual size is: 5 × 5 µm = 25 µm
Key Terms
- Magnification: How many times larger an image appears compared to the actual object.
- Resolution: The ability to distinguish between two separate points.
- Eyepiece Graticule: A measuring scale in the microscope eyepiece (arbitrary units).
- Stage Micrometer: A slide with a known scale used for calibration.
- Calibration: Determining the actual size of eyepiece divisions using a stage micrometer.
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Exam Tips
Always convert units before calculating so all values are in the same unit!
- 1 mm = 1000 µm. This is the most common conversion in Biology.
To convert from mm to µm you x by 1000
-
1 µm = 1000 nm
To convert from µm to nm you x by 1000
Magnification has no units, but actual size and image size should be in the same unit and these should be written in your answer.
Always take your measurements in mm, not cm, as this makes it much easier to convert your answer into µm.
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A student observes a cell under a microscope. The image of the cell is 50 mm long, and the magnification is ×500. Calculate the actual size of the cell in µm. (2 marks)
- Convert units: 50 mm = 50,000 µm
- Calculate: 50,000 / 500 = 100 µm
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!