Use a scale bar when an image has been resized
Keep the specimen and scale bar on the same copy.
Section B2.2 · Size of specimens
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall the distinction between plant, animal and bacterial cells. Read the supplied evidence before drawing a conclusion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
An image at ×400 is doubled in every length. What is its new magnification?
What you will learn
- Explain why resizing changes image magnification.
- Calculate actual length using a scale-bar ratio.
A copied picture has a new scale
If an image is enlarged by a factor of two in each length, its magnification doubles while the actual specimen size stays unchanged. A printed ×400 label copied without adjustment may therefore no longer describe the resized display. Length scaling is not area scaling.
A scale bar printed within the image changes length with the specimen image when both are resized together. Its labelled actual length remains the same. This permits a ratio on the current copy, provided the image has not been stretched non-uniformly.
Compare corresponding lengths
On a particular copy, a scale bar labelled 0.05 mm actual length measures 10 mm, and a specimen measures 30 mm along the same direction. The specimen is three bar lengths, so actual specimen length = 30 ÷ 10 × 0.05 = 0.15 mm.
Both measurements must be made on the same copy and in the same direction for the scale to apply. If the image alone is resized or stretched while the bar is unchanged, the ratio is invalid. The microscope photographs are for observation; the following hypothetical measurement exercise supplies its own values.
Sources for this lesson
Pause and explain
Why can an embedded scale bar help after proportional resizing?
Worked example
A 10 mm drawn bar represents 0.05 mm actual length. A specimen on the same copy is 30 mm long. Find actual length.
Show the worked solution
- The specimen is 30 ÷ 10 = 3 bar lengths.
- Each represents 0.05 mm.
- 3 × 0.05 = 0.15 mm.
Answer 0.15 mm.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A 12 mm bar represents 0.03 mm; a specimen is 48 mm on that copy. Find actual length.
Give me a hint
Count bar lengths.
Compare my reasoning
- 48 ÷ 12 = 4.
- 4 × 0.03 = 0.12 mm.
0.12 mm.
Look for these in your work
- Correct ratio.
- Actual-length unit.
An image at ×300 is reduced to half its former length. Give new magnification and state whether actual size changed.
Give me a hint
Scale the length ratio.
Compare my reasoning
- 300 × 0.5 = 150.
- The actual specimen is unchanged.
×150; actual size unchanged.
Look for these in your work
- Scale ratio correctly.
- Separate actual size.
A specimen drawing is stretched horizontally by two, but its scale bar is not. Can the old bar give a valid horizontal length? Explain a remedy.
Give me a hint
The representation and bar no longer share the scale.
Compare my reasoning
- Their ratio has changed falsely.
- Restore proportional scaling of both or use the correct horizontal scale.
No; use a correctly scaled same-copy bar or restore proportional resizing.
Look for these in your work
- Identify mismatched scaling.
- Give a remedy.
Common mistakes
- Use the same copy for both measurements.
- A scale-bar label is actual length; measured bar length is image length.
What can you explain now?
What remains unchanged when a specimen image is resized?
Compare with the explanation
The actual specimen size.
Only the representation changes.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain why a bar ratio can survive resizing and when it cannot.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.