The shape of the lesson
By the end of the lesson, learners can
- Light and electron microscopes: Click down the size ladder from a person to a carbon atom.
- Measuring with a graticule: Switch on the stage micrometer, then change the objective.
- Magnification and scale bars: Electron micrographs rarely state a magnification; they carry a scale bar.
- Slides and drawings: The practical side of 1.1: making a temporary preparation and turning what you see into a useful drawing.
- M = I ÷ A with I and A in the same unit.
- 1 mm = 1000 μm; 1 μm = 1000 nm.
- Graticule: calibrate against the stage micrometer for each objective.
- Resolution: light 200 nm, electrons about 0.5 nm.
One picture every idea returns to
Magnification is how many times larger the image is than the real object: magnification = image size ÷ actual size, with both in the same unit (1 mm = 1000 μm, 1 μm = 1000 nm). Resolution is the smallest distance between two points that can still be seen as separate. It is limited by wavelength: a light microscope resolves about 200 nm, an electron microscope about 0.5 nm, so only the electron microscope shows organelle detail. To measure with a light microscope, calibrate the eyepiece graticule against a stage micrometer for each objective.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Bigger is not the same as clearer.. A light microscope can magnify a cell 1500 times, yet it will never show you a ribosome. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 11 min | Teach and check: Light and electron microscopes | Click down the size ladder from a person to a carbon atom. Project tab 6 of the lab (Cell structure lab, size ladder) and let a student drive it while the class predicts. | Whole class, then pairs |
| 11 to 17 min | Teach and check: Measuring with a graticule | Switch on the stage micrometer, then change the objective. Project tab 4 of the lab (Cell structure lab, virtual light microscope) and let a student drive it while the class predicts. | Pairs |
| 17 to 23 min | Teach and check: Magnification and scale bars | Electron micrographs rarely state a magnification; they carry a scale bar. Project tab 5 of the lab (Cell structure lab, magnification and scale bars) and let a student drive it while the class predicts. | Pairs |
| 23 to 30 min | Teach and check: Slides and drawings | The practical side of 1.1: making a temporary preparation and turning what you see into a useful drawing. | Round Table |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Slides and drawings, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Round Table
One sheet travels round the team. Each student adds one line of working or one idea and passes it on, so the solution is built four hands at a time and every contribution is visible.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Increasing the magnification does not improve the resolution. | Beyond about ×1500 a light microscope just gives a larger blur, because two points closer than 200 nm still merge. Define both terms separately: magnification is about size, resolution is about detail. |
| Convert before you divide. | The most common slip is dividing a length in mm by a length in μm, which puts the answer out by a factor of 1000. Write both measurements in μm first, and check the answer is a sensible size for that structure. |
Microscopes, magnification and resolution: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Define magnification and resolution.
A chloroplast measures 42 mm across in an electron micrograph at ×8000. Calculate its actual diameter in μm.
A scale bar labelled 2 μm is 30 mm long. A bacterium in the same micrograph is 45 mm long. Calculate the magnification and the actual length of the bacterium.
A student lines up an eyepiece graticule with a stage micrometer (1 small division = 10 μm) using the ×10 objective. 50 graticule divisions match 90 micrometer divisions. A root hair cell is then 23 graticule divisions long. Calculate its length.
Explain why a transmission electron microscope can show the cristae of a mitochondrion but a light microscope cannot.
A student wants to add a 20 μm scale bar to a drawing of a cell made at ×600. Calculate how long the bar should be, and state one difference between a TEM and an SEM image.
Answer key and marking notes
- Magnification: the number of times larger an image is than the actual object (image size ÷ actual size).
- Resolution: the smallest distance between two points that can be distinguished as separate points.
- 42 mm = 42 000 μm.
- Actual size = 42 000 ÷ 8000 = 5.25 μm (5.3 μm).
- 30 mm = 30 000 μm; magnification = 30 000 ÷ 2 = ×15 000.
- Bacterium image = 45 000 μm.
- Actual length = 45 000 ÷ 15 000 = 3.0 μm.
- 90 micrometer divisions = 900 μm.
- 1 graticule division = 900 ÷ 50 = 18 μm.
- Length = 23 × 18 = 414 μm (0.414 mm).
- Cristae are separated by distances far less than 200 nm.
- Light has a wavelength of 400 to 700 nm, so the light microscope cannot resolve points closer than about 200 nm.
- Electrons have a much shorter wavelength, so the TEM resolves to about 0.5 nm and the cristae appear as separate structures.
- Bar length = 20 × 600 = 12 000 μm = 12 mm.
- A TEM image is a 2D section through the specimen; an SEM image shows the 3D surface (TEM also has the higher resolution).
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which statement about magnification and resolution is correct?
- AIncreasing the magnification beyond the limit of resolution makes the image larger but shows no extra detail.
- BIncreasing the magnification of a light microscope always increases its resolution in proportion.
- CResolution is the number of times larger the image is than the real size of the specimen.
- DAn electron microscope has a higher magnification but the same resolution as a light microscope.
A student's drawing of a cheek cell is 54 mm wide. The actual width of the cell is 60 μm. What is the magnification of the drawing?
- A×900
- B×0.9
- C×90
- D×1.1
The figure is a drawing of a chloroplast from an electron micrograph. On the printed micrograph the scale bar is 20 mm long and the chloroplast, from X to Y, is 64 mm long. What is the actual length of the chloroplast?
- A6.4 μm
- B3.2 μm
- C0.63 μm
- D128 μm
A good light microscope has a resolution of about 200 nm. Which structure could be seen as a distinct object with it?
- Aa mitochondrion, about 1 μm long
- Ba ribosome, about 25 nm in diameter
- Ca microtubule, about 25 nm wide
- Dthe phospholipid bilayer, about 7 nm thick
A student makes a temporary mount of onion epidermis in a drop of iodine solution. Why is the coverslip lowered slowly at an angle, supported by a mounted needle?
- Ato avoid trapping air bubbles under the coverslip
- Bto spread the iodine stain evenly into every cell
- Cto flatten the tissue until it is one cell thick
- Dto stop the epidermal cells from losing water by osmosis
With the ×10 objective, 40 eyepiece graticule divisions line up exactly with 0.36 mm on a stage micrometer. The student switches to the ×40 objective (same eyepiece) and a cell spans 12 graticule divisions. What is the length of the cell?
- A27 μm
- B108 μm
- C432 μm
- D2.7 μm
Two membranes in a cell are 50 nm apart. A student photographs them using a light microscope at ×1500, then enlarges the photograph 20 times, so the total magnification is ×30 000. What will the enlarged photograph show?
- Aone line, because enlarging the image does not improve the resolution
- Btwo lines 1.5 mm apart, because the total magnification is ×30 000
- Ctwo lines, because ×1500 is enough to resolve objects 50 nm apart
- Dtwo lines, but only if the photograph is enlarged a further 10 times
In an electron micrograph a bacterium that is 2.0 μm long appears 50 mm long. A ribosome in the same micrograph has a diameter of 25 nm. What is the diameter of the ribosome in the image?
- A0.63 mm
- B6.3 mm
- C0.063 mm
- D63 mm
Quiz key
Magnification is how many times larger the image is than the object; resolution is the smallest distance between two points that can still be seen as separate. Once the resolution limit is reached, further magnification only enlarges the blur (empty magnification), so no new detail appears. Magnification and resolution are independent: a light microscope cannot resolve below about 200 nm however much it magnifies. The definition given in the third option is the definition of magnification, not resolution. The electron microscope has both a higher magnification and a much better resolution (about 0.5 nm for a TEM), because electrons have a far shorter wavelength than light.
Convert the image size to μm: 54 mm = 54 000 μm. Magnification = 54 000 ÷ 60 = ×900. ×0.9 comes from dividing 54 by 60 without converting mm to μm. ×90 comes from converting mm to μm with a factor of 100 instead of 1000. ×1.1 inverts the equation (actual ÷ image, 60 ÷ 54); a drawing larger than the cell must have a magnification greater than 1.
The 20 mm (20 000 μm) bar represents 2 μm, so the magnification is 20 000 ÷ 2 = ×10 000. Actual length = 64 000 μm ÷ 10 000 = 6.4 μm (equivalently, 64 mm is 3.2 bar lengths, and 3.2 × 2 μm = 6.4 μm). 3.2 μm counts the bar lengths but forgets that each bar represents 2 μm, not 1 μm. 0.63 μm inverts the ratio (20 ÷ 64 × 2). 128 μm multiplies the measured 64 mm by the bar label of 2, mixing up image and actual sizes. A chloroplast of a few μm is the expected size.
1 μm = 1000 nm, which is well above the 200 nm limit, so a mitochondrion can be seen (though its internal cristae cannot). A ribosome (25 nm) and a microtubule (25 nm wide) are about eight times smaller than the limit, so they can only be resolved with an electron microscope. The bilayer of a membrane is only about 7 nm thick; in a light microscope the cell surface membrane shows only as the boundary of the cell, never as a resolved structure.
Lowering the coverslip from one edge pushes the liquid and air out ahead of it, so no air bubbles are trapped; bubbles look like dark-rimmed circles and hide the cells. The stain reaches the cells by diffusion, not by the way the coverslip is lowered. The epidermis is already a single layer of cells because it is peeled off as a thin strip, not because the coverslip squashes it. Water loss by osmosis depends on the solution the tissue is mounted in, not on how the coverslip is placed.
At ×10: 0.36 mm = 360 μm, so 1 division = 360 ÷ 40 = 9 μm. The ×40 objective magnifies four times more, so each division now covers 9 ÷ 4 = 2.25 μm of the specimen. Cell length = 12 × 2.25 = 27 μm. 108 μm uses the ×10 calibration without recalibrating. 432 μm multiplies the division size by 4 instead of dividing (a higher objective makes each division represent a smaller length). 2.7 μm misreads 0.36 mm as 36 μm, a factor of 10 error in the unit conversion.
The light microscope cannot resolve points closer than about 200 nm, so at the moment the photograph was taken the two membranes were already merged into one image. Enlarging the photograph only enlarges that single blurred line. 50 nm × 30 000 = 1.5 mm is correct arithmetic, but it assumes the membranes were resolved, which they were not. ×1500 is a magnification, and magnification does not set the resolution. Further enlargement is still empty magnification; only a shorter wavelength (an electron microscope) can separate them.
Magnification = 50 000 μm ÷ 2.0 μm = ×25 000. 25 nm = 0.025 μm, so the image diameter = 0.025 × 25 000 = 625 μm = 0.63 mm. 6.3 mm treats 25 nm as 0.25 μm (dividing by 100 instead of 1000). 0.063 mm treats 25 nm as 0.0025 μm. 63 mm treats 25 nm as 2.5 μm, which would make a ribosome larger than the whole bacterium.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.