Microscopy and Magnification
Simple Explanation
Cells are far too small to see with the naked eye, so biologists use microscopes to magnify them. Magnification tells you how many times bigger the image is compared to the actual, real-life size of the object.
Why Do We Need It?
Understanding magnification lets you correctly interpret what you are seeing under a microscope, including how big the real structures actually are.
Formula
Magnification Formula
Magnification = Image size / Actual size
How many times larger a microscope image is compared to the real, actual size of the object.
- Image size
- β the measured size of the image under the microscope
- Actual size
- β the object's real, true size
When to use it: Whenever you need to calculate how much a microscope image has been magnified, or work backwards to find an object's real size.
Worked Example
Calculate magnification
A cell has an actual diameter of 20 micrometres (ΞΌm). Its image under the microscope measures 60 mm across. What is the magnification?
Why Does This Work?
Magnification is simply a ratio β how many times larger the image is than the real object β so as long as both measurements are converted to the same unit first, dividing image size by actual size always gives that ratio correctly.
Real-Life Example
Reading the magnification on a microscope
A school microscope objective lens is labelled Γ40, combined with a Γ10 eyepiece.
Multiplying the two (40 Γ 10 = Γ400 total magnification) tells you how many times bigger the image you see is compared to the real specimen β using this same idea.
Practice
An image of a cell measures 5 mm across. The actual cell is 0.05 mm across. What is the magnification? (Give just the number.)
EasyA cell's actual size is 25 micrometres. Under a microscope its image measures 2.5 mm. What is the magnification? (Convert units first; give just the number.)
MediumCommon mistake
Forgetting to convert both measurements to the same unit before dividing β mixing millimetres and micrometres gives a magnification that is wrong by a factor of 1000.
Quick Review
- Magnification = Image size / Actual size
- Always convert both sizes to the same unit first.
- Magnification tells you how many times bigger the image is than the real object.