Magnification is calculated by dividing image size by actual size. Rearrange this relationship to find actual size: actual size = image size ÷ magnification. This microscopy skill is required at both SL and HL in IB Biology A2.2 Cell structure.
The Method
Measure the image with a ruler, then ensure that image size and actual size use the same units before calculating.
| Quantity required | Calculation |
|---|---|
| Magnification | |
| Actual size | |
| Image size |
Magnification has no unit because it is a ratio. Write it using the multiplication sign, for example ×400.
Unit conversion is essential:
- 1 mm = 1,000 µm
- 1 µm = 1,000 nm
- 1 mm = 1,000,000 nm
For greater reliability, measure across the widest visible part specified in the question and record the ruler reading precisely. If the boundary is thick, measure consistently from the same edge on both sides. Repeat the measurement when possible to reduce random uncertainty.
Worked Example: Finding Actual Size
A cell measures 36 mm on a micrograph taken at ×600 magnification.
Convert the image size first: 36 mm = 36,000 µm.
Therefore, the cell’s actual size is 60 µm.
Using a Scale Bar
If a micrograph has been resized, use its scale bar rather than a stated magnification. Suppose a 20 µm scale bar measures 40 mm, while a cell measures 70 mm:
This proportional method remains valid when the whole image and scale bar are enlarged together.
A common misconception is that the labelled magnification remains correct after an image is resized. It does not; magnification depends on the displayed or printed image size, whereas a scale bar changes proportionally with the image.
Exam Technique
Show the formula, unit conversion, substitution, and final answer. For calculate, working may be necessary to demonstrate the method. Always include an appropriate unit when reporting actual size, and never attach a unit to magnification.