What you need to know
- 3.8(i)Core Practical 5: observations and labelled drawings of suitable animal cells.
- 3.8(ii)Graticule measurements and scale.
- 3.7(ii)Staining to increase contrast.
Preparation for supervised school practical work. Use the school’s approved specimen, staining, hygiene and instrument procedures; this page does not request biological samples or student data.
By the end, you should be able to…
- Focus safely at low and high power.
- Calibrate for each objective using a known stage scale.
- Produce evidence-based drawings and measurements with units.
Build your understanding
Choose and prepare a suitable specimen
Use a teacher-approved prepared animal-cell slide or a thin stained preparation supplied under the school’s protocol. Prepared slides allow the same observation and measurement skills without requiring students to provide biological samples. No personal measurements or images are submitted to Scholar Lab.
For an approved wet mount, a thin specimen in a small liquid drop is covered carefully with a coverslip lowered at an angle to reduce trapped air. Use the stain and quantities specified by the school. Staining increases contrast; avoid confusing stain deposits or bubbles with organelleA structure inside a cell with a particular function, such as a mitochondrion or ribosome..
Focus without damaging the slide
Place the slide on the stage and begin with the lowest-power objective. Check the instrument’s movement from the side so the lens cannot strike the coverslip. Use coarse focus at low power to find the specimen, then fine focus and illumination adjustments to sharpen it.
Centre the cell before switching to a higher objective, because the field of view becomes smaller. Use fine focus at high power and re-adjust illumination as needed. Do not force knobs or allow the objective to touch the slide. Record objective and eyepiece powers; their product gives total optical magnificationHow many times larger an image is than the real object: magnification = image size ÷ actual size., not automatically drawing magnification.
Calibrate the eyepiece scale
An eyepiece graticuleA scale in the microscope eyepiece. It has no units until it is calibrated with a stage micrometer. has arbitrary divisions. A stage micrometerA microscope slide with a scale of known divisions, used to calibrate the eyepiece graticule. has a known scale. Check the engraved or supplied scale: do not assume every micrometer uses the same division size. Replace the specimen with the stage micrometer and focus it using the objective to be calibrated.
Align the two scales at one point, then find a second alignment as far away as is practical. Divide the known stage distance by the corresponding number of eyepiece divisions. For example, if 100 µm spans 40 eyepiece divisions, one division represents 2.5 µm at that objective.
Repeat and record calibrationChecking or setting a measuring instrument against a known standard, such as finding what one eyepiece division is worth. for each objective. Changing objective changes the real distance represented by one eyepiece division. Keep the same optical configuration when transferring the calibration to the specimen.
Measure cells and describe variation
Replace the micrometer with the animal-cell slide. Measure the chosen dimension in graticule divisions and multiply by the calibration for that objective. At 2.5 µm per division, a cell spanning 18 divisions measures 45 µm. State whether you measured length, width or another defined dimension.
Measure several clearly bounded cells across different fields rather than selecting only the largest. Use a consistent dimension and sampling rule. Record individual values, units, mean and spread; cell variation is biological as well as measurement-related. Estimates should reflect the scale’s resolutionThe smallest distance between two points that can still be seen as separate points. and the clarity of boundaries.
Make a biological drawing
Draw a large, proportionate outline with single clear pencil lines and no shading. Label only structures that are visible, using straight ruled lines that meet the structure without arrowheads or crossing. Include a descriptive title, the specimen and a calibrated scale bar.
A light-microscope drawing may show a cell boundary, cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles. and nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores.. Do not add mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place., ribosomeA small organelle made of rRNA and protein, where translation takes place. or Golgi merely because a textbook diagram has them. If the drawing is 90 mm wide and the measured cell 45 µm, drawing magnification is 90,000/45 = ×2,000, regardless of the microscope’s optical magnification.
Risk controls and evaluation
Handle slides and coverslips by the edges and follow the school’s procedure for broken glass. Avoid skin and eye contact with stains; use the protective equipment specified locally. Never direct sunlight into a microscope mirror. Use lens tissue for optics and keep liquid away from electrical parts.
Any fresh biological material requires the teacher’s hygiene and disposal protocol; do not share sampling equipment. Prepared slides avoid that collection step. Recalibration, clear boundary criteria and repeated measurements improve confidence, but cannot make a blurred boundary exact.
NOT: the numbers on the eyepiece graticule are not micrometres until you calibrate them.
- Calibrate at the magnification you will use
- Remove the stage micrometer before measuring the slide
- Stain can make boundaries easier to see
- Quote the unit with the cell size
Two traps this diagram blocks
- Do not treat graticule divisions as micrometres without a calibration.
- Do not keep the stage micrometer in place and call it the specimen.
From the diagram alone
Question
Explain how an eyepiece graticule and a stage micrometer are used to measure a cell. [4]
Show a model answer
Line up the eyepiece scale with the stage micrometer at the magnification you will use, and calculate the real length of one eyepiece division. Remove the micrometer, measure the cell in divisions, and multiply by that length. Recalibrate if the objective is changed.
The microscope measurement lab
Focus safely, calibrate the eyepiece graticule against a stage micrometer at each objective, then measure fictional animal cells.
Simulation only. The slide and cells are fictional. Handle real slides by the edges, avoid contact with stains and never direct sunlight into a microscope mirror.
Method 6 steps
- Prepare a temporary mount of animal cells (for example a cheek-cell model or prepared slide), add a drop of stain such as methylene blue, and lower a coverslip at an angle to avoid air bubbles.
- Put a stage micrometer on the stage and focus at low power: coarse focus first, watching from the side, then fine focus.
- Line up the eyepiece graticule with the stage micrometer and work out µm per eyepiece division for that objective. Repeat for each objective you will use.
- Replace the micrometer with the cell slide and focus on the cells (only fine focus at high power).
- Measure several cells in eyepiece divisions, choosing cells by a fixed rule, and convert to µm using the calibration.
- Make a labelled drawing of a cell, with clear continuous lines and no shading, and work out its magnification.
Aligned with the order of the specification practical. Simulation only: a teacher sets the real volumes, concentrations and risk controls; this is not a laboratory protocol.
Variables
- Independent
- Not an experiment with an independent variable: you are measuring cell size.
- Dependent
- Width (or diameter) of each cell in µm.
- Controlled
- The same objective and calibration for every measurement
- Cells chosen by a fixed rule from different fields
- Sharp focus before reading the scale
Safety
- Handle slides and coverslips by their edges; they break easily and the edges are sharp.
- Stains such as methylene blue stain skin and can irritate. Wear gloves and eye protection and avoid contact.
- Never point a microscope mirror at direct sunlight. Follow your school’s rules for any human cell samples.
Follow your school’s risk assessment; these notes are not a substitute for it.
Interpreting
- Actual size = number of divisions × µm per division for that objective.
- Magnification = image size ÷ actual size, with both in the same units (1 mm = 1000 µm).
- Report a mean and range of cell sizes. The spread comes from biological variation and from how sharp the boundaries are.
- A light microscope’s resolution limits how precisely you can place an edge, whatever the magnification.
Guided method · step 1 of 9
In Slides under the simulator, drag Stage micrometer onto the stage, or press it.
A stage micrometer is a slide with a scale of known length (here each small division is 10 µm). The eyepiece graticule has no units of its own, so it has to be calibrated against this scale.
1 · Calibrate the graticule
Line up the eyepiece scale with the stage micrometer at two points far apart. µm per division = stage distance ÷ eyepiece divisions. Repeat at every objective you measure with.
Saved: ×4 — · ×10 — · ×40 —
Slides
Objective and focus
Drag the stage micrometer onto the stage to begin calibrating.
2 · Measure cells
Drag a whole, clearly bounded cell along the scale. Read the boundaries in divisions, then convert with the calibration for this objective.
Choose cells by a rule (for example, the cell nearest the centre of each new field), not only the largest ones.
3 · Results and drawing magnification
Your fictional measurements appear here.
Evaluate the claim
Biological variation and unclear boundaries both add spread. A wider alignment interval reduces relative calibration uncertainty, but cannot make a blurred boundary exact. Light microscopes invert the image; this view moves with your drag for ease of use.
Make your reasoning visible
Calibrate at the objective used
A calibrationChecking or setting a measuring instrument against a known standard, such as finding what one eyepiece division is worth. from low power cannot simply be reused at high power.
Divide, then multiply
Known micrometer distance / graticule divisions gives µm per division; multiply by the cell’s division count.
Do not equate two magnifications
Optical magnificationHow many times larger an image is than the real object: magnification = image size ÷ actual size. is eyepiece × objective. Drawing magnification is drawing size / real size.
Key explanations
- The magnification of a drawing is image size divided by actual size. It is not the same number as the objective.
Avoid this misconception
The claim: One eyepiece division always represents one micrometre.
The correction: Its real size depends on calibration at the selected objective and optical configuration.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
A known 200 µm distance spans 50 graticule divisions. A cell spans 12 divisions at the same objective. Calculate cell length.
Show suggested answer
200/50 = 4 µm per division. Cell length = 12 × 4 = 48 µm.
Practice question
Retrieval and explanation · suggested answer
Why choose two widely separated alignment points when calibrating?
Show suggested answer
A fixed reading uncertainty is a smaller fraction of a larger measured interval, reducing relative calibration uncertainty.
Practice question
Retrieval and explanation · suggested answer
A ×10 eyepiece is used with a ×40 objective. A drawing of a 50 µm cell is 100 mm wide. Calculate optical and drawing magnifications.
Show suggested answer
Optical: 10 × 40 = ×400. Drawing: 100,000/50 = ×2,000.
Practice question
Retrieval and explanation · suggested answer
Why should you not draw ribosomes in a conventional light-microscope view of an animal cell?
Show suggested answer
Individual ribosomes are below the resolving ability of a conventional school light microscope. A biological drawing records observed features, not assumed ultrastructure.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
At one objective, 80 µm spans 40 eyepiece divisions. At a higher-power objective, the same 80 µm spans 100 divisions. A cell spans 30 divisions at high power. Calculate its length and explain the common error.
Show a hint
Use the calibration for the high-power objective.
Explore the reasoning
High-power calibration = 80/100 = 0.8 µm per division; cell length = 30 × 0.8 = 24 µm. Reusing the low-power value of 2 µm per division would wrongly give 60 µm.
Recall this lesson
Lesson recall questions
Question 1 of 5: Write the equation linking magnification, image size and actual size.
Magnification, resolution and staining
Practice question
Retrieval and explanation · suggested answer
Write the equation linking magnification, image size and actual size.
Try this from memory first.
Beyond the classroomBeyond the conventional light microscopeResearch · imaging · Enrichment, not required recall
The 2014 Chemistry Nobel Prize recognised super-resolved fluorescence microscopy. Specialised techniques can reveal detail beyond conventional light-microscopy limits.
Back to the spec: Specification 3.7: distinguish the performance of a particular imaging method from simply enlarging its output.
Nobel Prize in Chemistry 2014Question
Why does this not mean that digitally enlarging a school microscope image gives the same result?
Explore a possible response
The research methods collect or localise information differently. Zooming a finished image does not acquire new structural information.