How big is that cell really

Science · Organisms & Life Processes · ages 14-15

Name ______________________ Date ____________

If you magnify a photo of a cell more, the real size of the cell also gets bigger.
Circle one: True False
A paramecium is really 0.2 mm long. In a micrograph it looks 40 mm long. What is the magnification of the image?
- ×20
- ×200
- ×2000
- ×8
An image is 40 mm across and the real cell is 0.2 mm. What is the magnification?
- 200 times
- 8 times
- 40.2 times
You want to watch a living bacterium move. Which microscope do you pick?
- A hand lens
- An electron microscope
- A light microscope
An image is 60 mm across at 300 times. What is the real size?
- 0.5 mm
- 0.2 mm
- 18000 mm
A micrograph has a scale bar labelled 10 µm. On the printed image, that scale bar is 20 mm long. A bacterium in the same image measures 6 mm across on the page. How wide is the real bacterium?
- 1 µm
- 3 µm
- 6 µm
- 12 µm
A micrograph of a mitochondrion is magnified ×8000. The image measures 24 mm across. What is the real width of the mitochondrion in micrometres?
Answer: ______________
A mitochondrion photo is 24 mm across at 8000 times. What is the real width in micrometres?
Answer: ______________
Tom converts 0.003 mm to micrometres and writes 0.000003. What did he do wrong?
- He used the wrong magnification
- He read the scale bar backwards
- He divided by 1000 instead of multiplying
A cell photo is 50 mm across at 250 times. Give the real width in micrometres.
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

False · Zooming in only changes how big the image looks in the formula, not the object itself.
×200 · Dividing the image length by the real length gives the magnification.
200 times · Divide image size by actual size: 40 divided by 0.2 is 200.
A light microscope · A living sample rules out electron microscopes, and bacteria show well near 400 times, which a light microscope handles.
0.2 mm · Actual size equals image divided by magnification: 60 divided by 300 is 0.2 mm.
3 µm · The scale bar tells you what one printed millimetre stands for in real life, so you can scale the bacterium's measurement the same way.
3 · Undoing the magnification gives the real size, and converting to micrometres keeps the units sensible for something this small.
3 · 24 divided by 8000 is 0.003 mm, and times 1000 gives 3 micrometres.
He divided by 1000 instead of multiplying · Going from milli to micro steps down to a smaller unit, so you multiply by 1000 to get 3.
200 · 50 divided by 250 is 0.2 mm, and times 1000 gives 200 micrometres.