Lesson focus: Microscopes reveal cells only when focus, magnificationHow many times bigger an image is than the real object. and scale are handled correctly.
Use a light microscope safely and calculate actual size from an image scale.
Build on earlier learning
Laboratory equipmentRevisit scale reading, units and resolution before measuring microscopic images.
Practical
Using a light microscope
- Place the slide on the stage and start with the lowest-power objective.
- Use the coarse focus to bring the stage close, then focus by moving it away while looking down the eyepiece.
- Switch to a higher-power objective and use only the fine focus.
- Draw what you see with clear single lines and no shading.
Record: A labelled drawing with the magnification (eyepiece × objective).
Safety: Never point a mirror microscope at the Sun.
You will be successful when you can…
- Label key light-microscope parts.
- Sequence safe focusing from low to high power.
- Distinguish magnification from resolution.
- Calculate actual size using a scale bar and convert units.
Words that carry the science
- magnification
- Image size divided by actual size.
- resolution
- Ability to distinguish two nearby points as separate.
- objective lens
- Lens close to the specimen that provides primary magnification.
- field of view
- The circular area visible through the microscope.
- micrometre
- A unit equal to one thousandth of a millimetre (µm).
Turn a magnified image into reliable evidence
01Focus safely from low power
Place the slide on the stage and select the lowest-power objective lensThe lens closest to the slide on a microscope, which you change to get different magnifications.. Looking from the side, raise the stage close to—but not touching—the lens, then look through the eyepiece and focus away safely.
Centre the specimen before changing objective. At higher power use fine focus; the working distance is smaller.
02Magnification is not resolution
magnificationHow many times bigger an image is than the real object. tells how many times larger the image is. resolutionHow well a microscope can show two things that are very close together as separate. tells whether close details can be separated. Enlarging a blurred image produces a larger blur.
03Use scale, not apparent size
Use a scale bar or calibrated field of viewThe circle you can see when you look down a microscope.. Convert units consistently: 1 mm = 1000 µm.
Actual size can be found from image size ÷ magnification, provided both sizes use compatible units.
Using a light microscope
Start on the lowest power, focus, then turn to a higher objective and refocus with the fine focus. Higher magnification shows fewer cells, each bigger, because the field of view shrinks.
Controls
Onion cells are about 0.25 mm long. Field of view is about 18 mm ÷ objective power for a ×10 eyepiece (typical).
Measure from a scale bar
On a printed image, a 50 µm scale bar measures 20 mm and a cell measures 32 mm.
- Form the ratio: cell image ÷ scale-bar image = 32 ÷ 20 = 1.6.
- Multiply by the scale-bar value: 1.6 × 50 µm.
- Calculate 80 µm.
- Report the actual cell size with the unit: approximately 80 µm.
How to make the reasoning visible: A scale-bar ratio remains valid if the whole image is enlarged or reduced together.
Calibrated image measurements
Treat these as illustrative image measurements.
| Image feature | Printed length | Scale information | Actual size |
|---|---|---|---|
| Cell A | 24 mm | 12 mm represents 50 µm | 100 µm |
| Cell B | 15 mm | 30 mm represents 200 µm | 100 µm |
| Nucleus C | 8 mm | 20 mm represents 25 µm | 10 µm |
| Cell D | 36 mm | 18 mm represents 40 µm | 80 µm |
Why can Cell A and Cell B have the same actual size but different printed sizes?
Convert 80 µm to millimetres.
For Cell A, 12 mm on the picture represents 50 µm. Show why Cell A is 100 µm across.
Check your understanding
Answer from memory, then check against the suggested answer.
Which objective should be used first?
What is resolution?
How many micrometres are in 1 mm?
What is magnification?
Where do you put the slide on a microscope?
An image is enlarged twofold on a screen. What happens to its scale bar?
Why is “this microscope has ×1000 magnification” incomplete evidence of image quality?
Why should you look from the side when you move the stage up towards the lens?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
On a micrograph, a 100 µm scale bar measures 25 mm. A cell on the same image measures 40 mm long.
Calculate the actual length of the cell. Show your working.
(4)Total for this question: 4 marks
Command word: For calculateA command word: work out a number answer, showing your working and the unit., write the ratio, substitute values and finish with a unit.