This document discusses calculating magnification of drawings and images. It provides examples of calculating magnification by measuring the drawn/image size and actual size. Magnification is the ratio of drawn/image size to actual size. A scale bar can also be used to determine actual size and magnification, by measuring the scale bar in the image and calculating its magnification based on its known actual size.

1. magnification
Core and extended

2. Suppose you were
to draw this ant
It’s a huge ant but we are going
to say it is 10mm long

3. The drawing looks
about three times the
size of the ant

4. Let us check that
more scientifically
We need to measure both
the drawing and the ant
Using a ruler measure in
millimetres

5. Measure the longest part
but ignore appendages
Length of ant = 10mm

6. Measure the longest part
but ignore appendages
Length of ant = 10mm
Length of drawing = 25mm

7. Magnification is calculated by:
Magnification
Drawn size
Actual size
= 25mm
10mm
Units cancel
Mag X2.5

8. Length of ant = 10mm
Length of drawing = 25mm
Mag = 25mm
10mm
Mag X2.5

9. Mag X2.5
Ask yourself “does this sound about right?”
In this example we are saying the drawing is magnified X2.5
Or it is two and a half times bigger than the actual ant
And that seems about right!

10. If we draw something
smaller than it actually is
the magnification should
be a fraction

12. Measure the length but
not the shaft
Actual length = 80mm

13. Measure the length but
not the shaft
Drawn length = 40mm

14. Mag = drawn size
actual size
= 40mm
80mm
Mag X1/2
In other words the drawing was half
the size of the actual feather
It was magnified x1/2

15. Mag X1/2
That sounds
about right

16. We can use a scale bar to find the actual size of an
image

17. First we measure the scale bar
100µm = 20mm
100µm = 20 000µm

18. Next we calculate how much the image has been
magnified by seeing how much the scale bar is
magnified
100µm = 20mm
100µm = 20 000µm
The scale bar is magnified
20 000/100
=X200

19. What this means is the picture has been magnified
200 times
100µm = 20mm
100µm = 20 000µm
The scale bar is magnified
20 000/100
=X2000

20. Next we measure the cell we are interested in
100µm = 20mm
100µm = 20 000µm
The scale bar is magnified
20 000/100
=200times
The cell is 17mm
Or 17 000µm

21. Now we can find the actual length of the cell
100µm = 20mm
100µm = 20 000µm
The scale bar is magnified
20 000/100
=200times
Mag = image size/ actual size
Actual size = image size/mag
= 17000µm/200
= 85µm

22. This cell has a diameter of 85µm

23. Can you find the magnification of this cell using the scale
bar provided?
Remember to convert mm to µm