Calculations of magnification, real size and image size
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of using equations in science and understanding of cell scale to solve size-related problems.
What is magnification?
How many times larger the image is than the actual size.
The total magnification of an image is x500. The eye piece lens is x10. What is the objective lens?
Total magnification = eyepiece lens x objective lens
500 = 10 x objective lens
Objective lens = x50
Why would an electron microscope be better for viewing ribosomes?
It has higher magnification and resolution (and ribosomes are very small sub-cellular structures).
Topic Explainer Video
Check out this @JoeDoesBiology video that explains calculations of magnification then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Calculating Magnification
Formula:
Magnification = Image size ÷ Actual size
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All units must be the same before calculating.
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You may need to convert units or use standard form.
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You may need to rearrange the equation. Use the formula triangle to help.
Common Unit Conversions
Questions frequently require you to convert between units:
Make sure to check out the study note ‘Unit conversions and orders of magnitude’ in the exam skills section for more help with this.
Example 1: Simple Calculation
A red blood cell appears 12 mm in an image. Its actual size is 8 µm. What is the magnification?
Image = 12 mm, Actual = 8μm
Convert mm → µm:
12 mm = 12,000 µm
Magnification = Image size ÷ Actual size
Magnification = 12,000 ÷ 8 = 1500
The image size is therefore 1500x larger than the actual size.
No answer provided.
Example 2: Challenging Calculation
The image of the embryo below was produced using a magnification of x400. Calculate the actual size of the embryo.
Give your answer in micrometers.
First measure the real size of the image. e.g 23 mm
Image = 23 mm, Magnification = 400
Convert mm → µm:
23 mm = 23,000 µm
Magnification = Image size ÷ Actual size
400 = 23,000 ÷ A
A = 23,000 ÷ 400
Actual size = 57.5 µm
No answer provided.
Using Standard Form
Standard form is a way of writing very large or small numbers using powers of ten:
Standard form:
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a is a number between 1 and 10 (including decimals)
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n is an integer (positive or negative)
Step-by-Step Guide
Converting a Large Number to Standard Form
Example: 1250000
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Place the decimal after the first non-zero digit:
→ 1.25 -
Count how many places the decimal has moved:
→ 6 places -
Write it as:
Converting a Small Number to Standard Form
Example: 0.00047
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Place the decimal after the first non-zero digit:
→ 4.7 -
Count how many places the decimal has moved:
→ 4 places -
Write it as:
Key Term
- Magnification – how much larger the image is than the actual object.
No answer provided.
Exam Tips
- Watch your units! You must convert units to match before using the formula. Write your answers with correct units and in standard form when required.
- Watch out for questions that require you to give your answer in different units or in standard form. You will lose marks if you don't do this.
No answer provided.
Practice Question
An image of a bacterium measures 2.5 cm, when viewed at a magnification of x5000. Calculate the real size of the bacterium.
Give your answer in mm
Give your answer in standard form (3 marks)
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Convert cm to mm: 25 cm = 25 mm
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Magnification = Image size ÷ Actual size
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5000 = 25 ÷ Actual
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Actual = 25 ÷ 5000 = 0.005 mm
- Answer: 5 x10-3 mm
More Practice
Try to answer these practice questions from the TikTok videos on your own, then watch the videos to see how well you did!