The shape of the lesson
By the end of the lesson, learners can
- Measure, then divide: Choose find magnification.
- Working back to the actual size: Switch the simulator to find actual size and find image size.
- Millimetres and micrometres: Extended content.
- M = I ÷ A; rearranged, A = I ÷ M and I = M × A.
- Measure the image in mm: 1 cm = 10 mm.
- Extended: 1 mm = 1000 μm; convert both sizes to the same unit before dividing.
- Magnification has no unit: write ×600, and check the actual size is smaller than the image.
One picture every idea returns to
magnification = image size ÷ actual size. Rearranged, actual size = image size ÷ magnification, and image size = magnification × actual size. Measure the image with a ruler in millimetres and put both sizes in the same unit before dividing. Magnification has no unit; it is written as ×600, for example. Extended: 1 mm = 1000 μm, so multiply by 1000 to go from mm to μm and divide by 1000 to go from μm to mm.
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, Image size over actual size.. A drawing or photograph of a cell is far bigger than the cell itself. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Measure, then divide | Choose find magnification. Project tab 6 of the lab (Cells lab, magnification) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Working back to the actual size | Switch the simulator to find actual size and find image size. | Pairs |
| 21 to 30 min | Teach and check: Millimetres and micrometres | Extended content. Project tab 7 of the lab (Cells lab, mm and micrometres) and let a student drive it while the class predicts. | Rally Coach |
| 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, 14 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 Millimetres and micrometres, 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.
Rally Coach
Pairs share one worksheet. One partner solves the first question aloud while the other coaches, checking each line and offering a prompt rather than the answer; they swap roles for the next question.
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 |
|---|---|
| Measure the image in millimetres, not centimetres. | A drawing 3.6 cm long is 36 mm. Using 3.6 with an actual size in mm gives a magnification ten times too small. |
| Never divide a size in mm by a size in μm. | Dividing 40 mm by 8 μm as if they were the same unit gives ×5, a thousand times too small. Convert both sizes to the same unit first. |
Magnification and size of specimens: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
State the formula that links magnification, image size and actual size.
A drawing of a cheek cell is 45 mm wide. The actual width of the cell is 0.06 mm. Calculate the magnification of the drawing.
A photograph of an ant is magnified ×12. The image of the ant is 48 mm long. Calculate the actual length of the ant in mm.
A student draws a leaf cell. She measures the drawing between two marked points as 7.2 cm. The magnification of the drawing is ×1200. Calculate the actual length of the cell in mm.
Extended. A bacterium is 2 μm long. An image of it in a textbook is 30 mm long. Calculate the magnification of the image.
Extended. A photomicrograph of an Amoeba is printed at ×150. The image is 67.5 mm long. Calculate the actual length of the Amoeba, giving your answer in μm.
Answer key and marking notes
magnification = image size ÷ actual size.
- magnification = image size ÷ actual size = 45 ÷ 0.06.
- = ×750.
- actual size = image size ÷ magnification = 48 ÷ 12.
- = 4 mm.
- Convert the image to mm: 7.2 cm = 72 mm.
- actual size = 72 ÷ 1200.
- = 0.06 mm.
- Convert to the same unit: 2 μm = 2 ÷ 1000 = 0.002 mm.
- magnification = 30 ÷ 0.002.
- = ×15 000.
- actual size = 67.5 ÷ 150 = 0.45 mm.
- Convert to μm: 0.45 × 1000.
- = 450 μm.
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.
A cheek cell is 0.15 mm wide. In a drawing it is 60 mm wide. What is the magnification of the drawing?
- A×400
- B×9
- C×40
- D×4000
A photograph of a ladybird is magnified ×8. The image of the ladybird is 24 mm long. What is the actual length of the ladybird?
- A3 mm
- B192 mm
- C0.33 mm
- D16 mm
A student measures her drawing of a leaf cell with a ruler and finds it is 4.5 cm long. The drawing is magnified ×300. What is the actual length of the cell?
- A0.15 mm
- B0.015 mm
- C1350 mm
- D6.7 mm
A human hair is 0.08 mm wide. It is drawn at a magnification of ×250. How wide is the drawing of the hair?
- A20 mm
- B3125 mm
- C0.00032 mm
- D2 mm
Drawing P of a cell is 40 mm long and is magnified ×400. Drawing Q of a different cell is 30 mm long and is magnified ×200. Which statement is correct?
- Athe cell in Q is longer than the cell in P
- Bthe cell in P is longer, because its drawing is longer
- Cthe two cells are exactly the same length
- Dthe cell in P is longer, because it is magnified more
A bacterium is 2.5 μm long. What is this length in millimetres?
- A0.0025 mm
- B2500 mm
- C0.025 mm
- D0.25 mm
A photomicrograph of a cheek cell is printed at a magnification of ×800. The image of the cell is 48 mm wide. What is the actual width of the cell in μm?
- A60 μm
- B0.06 μm
- C38 400 μm
- D600 μm
The drawing shows a cell with a scale bar. The scale bar is 15 mm long on the drawing and represents 10 μm. The image of the cell is 60 mm long. What is the actual length of the cell?
- A40 μm
- B90 μm
- C6 μm
- D400 μm
Quiz key
magnification = image size ÷ actual size = 60 ÷ 0.15 = ×400. ×9 comes from multiplying 60 by 0.15 instead of dividing. ×40 and ×4000 are slips with the decimal point; check by working back: 0.15 × 400 = 60 mm.
actual size = image size ÷ magnification = 24 ÷ 8 = 3 mm. 192 mm multiplies instead of dividing, which would make the real ladybird bigger than its photograph. 0.33 mm divides the wrong way round (8 ÷ 24). 16 mm subtracts the magnification from the image size.
4.5 cm = 45 mm. actual size = 45 ÷ 300 = 0.15 mm. 0.015 mm uses 4.5 as if it were already in mm, giving an answer ten times too small. 1350 mm multiplies instead of dividing. 6.7 mm divides 300 by 45, the wrong way round.
image size = magnification × actual size = 250 × 0.08 = 20 mm. 3125 mm divides 250 by 0.08. 0.00032 mm divides 0.08 by 250, which would make the drawing smaller than the hair. 2 mm is a slip with the decimal point.
Cell P: 40 ÷ 400 = 0.1 mm. Cell Q: 30 ÷ 200 = 0.15 mm, so the cell in Q is longer. A longer drawing does not mean a longer cell if the magnifications differ. The two lengths are different, not equal. A higher magnification makes the real cell smaller for the same image size, not larger.
To change μm into mm, divide by 1000: 2.5 ÷ 1000 = 0.0025 mm. 2500 mm multiplies by 1000, the wrong direction. 0.025 mm divides by 100 and 0.25 mm divides by 10, as if there were 100 or 10 μm in a millimetre.
actual size = 48 ÷ 800 = 0.06 mm, and 0.06 × 1000 = 60 μm. 0.06 μm forgets to convert from mm. 38 400 multiplies 48 by 800 instead of dividing. 600 μm is a slip with the decimal point when converting.
10 μm = 0.01 mm, so the magnification is 15 ÷ 0.01 = ×1500. Actual length = 60 ÷ 1500 = 0.04 mm = 40 μm. (Quicker: the cell is 60 ÷ 15 = 4 scale bars long, and 4 × 10 = 40 μm.) 90 μm multiplies 60 by 15 and divides by 10. 6 μm divides 60 by 10, ignoring the length of the bar. 400 μm is a slip of ten when converting mm to μm.
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.