Teaching focus
Use this simulation to prepare students for their first microscope practical and to connect the actions they perform on a microscope with changes in the image they observe. It is particularly useful for distinguishing focus, magnification and contrast, which students can easily confuse.
The simulation begins with prepared slides. It works well as a whole-class demonstration before practical work, followed by paired or independent practice.
Learning objectives
Students should be able to:
- Identify the main microscope parts and explain their functions.
- Follow a sensible sequence for positioning, securing and focusing a slide.
- Calculate total magnification from the eyepiece and objective magnifications.
- Explain how staining increases contrast and makes existing cell structures easier to see.
- Compare the visible features of onion epidermal cells and cheek cells.
Suggested teaching sequence
- Introduce the microscope
Open Use the microscope with the part labels displayed. Identify the eyepiece, objective lenses, stage, stage clips, light, coarse-focus knob and fine-focus knob.
Distinguish the role of the stage, which supports the slide, from the clips, which hold it in place. Hide the labels briefly and ask students to identify the parts before beginning.
- Rehearse the practical routine
Select Onion and Guided practical. Students complete each action before moving to the next step.
The sequence includes lowering the stage, placing the prepared slide over the light aperture, closing the clips, checking the ×4 objective and switching on the light. Students then watch from the side while bringing the stage closer to the objective.
Once looking through the eyepiece, they lower the stage slowly with coarse focus until cell outlines appear, then use fine focus to sharpen the image. Encourage small adjustments and careful observation rather than rapid movements from one end of a control to the other.
The slide and focus knobs can be dragged directly. Buttons and sliders provide alternative controls.
- Investigate magnification
After obtaining a sharp image, ask students to centre an area of interest and select the ×10 objective. They should use fine focus to sharpen the image again. The ×40 objective can then be explored.
The simulation uses a ×10 eyepiece throughout:
| Objective lens | Total magnification |
|---|
| ×4 | ×40 |
| ×10 | ×100 |
| ×40 | ×400 |
Total magnification = eyepiece magnification × objective magnification
Ask students to describe two changes as magnification increases: the cells appear larger, and a smaller area of the specimen is visible. This helps explain why starting with the lowest-power objective makes it easier to locate the specimen.
Use the slide arrows to demonstrate that moving the slide in one direction makes its image move in the opposite direction.
- Show why staining helps
Open Why add a stain? Start with the structure labels hidden and ask students to compare the unstained and stained views.
Choose Watch the stain appear and direct attention to the nucleus. Its position and size stay the same in the comparison, but it becomes easier to distinguish from its surroundings.
Repeat with both specimens:
- Onion cells: iodine produces an amber appearance and increases the contrast of visible structures.
- Cheek cells: methylene blue makes the nucleus stand out against the paler cytoplasm.
Use Defocus both views to demonstrate that a stained image can still be blurred. Ask students which control would solve the problem, then select Restore focus.
Reinforce the three separate ideas:
- Focus changes image sharpness.
- Magnification changes apparent image size.
- Stain increases contrast between structures and their surroundings.
- Identify and compare cells
Open Explore the cells to compare onion and cheek cells at the same magnification. Begin with labels hidden, then reveal them after students have suggested the structures they can identify.
Draw attention to the adjoining, elongated onion cells and their cell walls. Compare these with the flattened, irregular cheek cells, which have a cell membrane but no cell wall.
Both specimens show nuclei and cytoplasm. These onion bulb epidermal cells have no chloroplasts; students should not assume that every plant cell contains chloroplasts.
Useful discussion questions
- Why should we start with the lowest-power objective?
- How do you know whether an image needs better focus or greater contrast?
- Why should you centre the specimen before increasing magnification?
- Which focus control should you use at higher power?
- Was the nucleus present before the stain was added?
- What happens to the area you can see when magnification increases?
- Which visible features help you distinguish the onion cells from the cheek cells?
For a short assessment, ask students to explain the complete process of obtaining a clear microscope image, using the terms stage, clips, objective, coarse focus, fine focus and stain correctly.
Tips for classroom use
For an immediate demonstration, choose Free practice, then Load a focused sample. This lets you investigate magnification, staining or deliberate defocusing without repeating the setup.
Use View all steps to revisit the routine before students handle real microscopes. Full-screen mode is useful when projecting the simulation.
Practical guidance and model limitations
Real microscope designs vary. Follow the procedure demonstrated for your school’s microscopes, keep the objective clear of the slide and use fine focus at higher power. The simulation includes a safe travel limit and locks coarse focus when higher-power objectives are selected.
The cell views are illustrative models rather than photographs or calibrated measurements. They show selected structures useful for this lesson, rather than every organelle present in a cell.
Switching between stained and unstained views is a comparison feature. Real staining is not reversed by a switch, and Fade between views does not represent stain concentration. Slide preparation, cheek-cell sampling and stain handling should be taught separately using your school’s practical instructions.