Microscopy and Magnification

This lesson covers microscopy, magnification and resolution as required by the AQA GCSE Combined Science Trilogy specification (8464). You need to understand how light and electron microscopes work, carry out magnification calculations and express answers in standard form.

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Why Do We Need Microscopes?

Most cells are too small to be seen with the naked eye. The invention of microscopes has allowed scientists to observe cells and subcellular structures in detail. The development of microscopy has directly led to our understanding of cell biology.

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Light Microscopes

Light microscopes use visible light and glass lenses to magnify specimens.

Key Features

Property	Detail
Maximum magnification	Approximately ×1500
Maximum resolution	Approximately 200 nm (0.2 μm)
Type of specimen	Living or dead; can be stained
Size	Relatively small and portable
Cost	Relatively inexpensive

How a Light Microscope Works

1. Light passes through the specimen on a glass slide.
2. The objective lens magnifies the image (typically ×4, ×10, or ×40).
3. The eyepiece lens (ocular lens) magnifies the image further (typically ×10).
4. The total magnification is calculated by multiplying the two lens magnifications.

$\text{Total magnification} = \text{Eyepiece lens magnification} \times \text{Objective lens magnification}$Total magnification=Eyepiece lens magnification×Objective lens magnification

Example: If the eyepiece lens is ×10 and the objective lens is ×40:

$\text{Total magnification} = 10 \times 40 = \times 400$Total magnification=10×40=×400

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Electron Microscopes

Electron microscopes use a beam of electrons instead of light to produce an image. Because electrons have a much shorter wavelength than visible light, electron microscopes have a much higher resolution.

Key Features

Property	Detail
Maximum magnification	Over ×1,000,000
Maximum resolution	Approximately 0.2 nm
Type of specimen	Dead only (specimens must be placed in a vacuum)
Size	Very large, fills a room
Cost	Very expensive

Types of Electron Microscope

Type	What It Shows
Transmission Electron Microscope (TEM)	Produces a 2D image of a thin slice through the specimen. Shows internal structures in great detail.
Scanning Electron Microscope (SEM)	Produces a 3D image of the surface of the specimen. Gives a lower resolution than TEM but shows surface features.

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Comparing Light and Electron Microscopes

Feature	Light Microscope	Electron Microscope
Radiation used	Visible light	Beam of electrons
Magnification	Up to ×1500	Up to ×1,000,000+
Resolution	~200 nm	~0.2 nm
Specimen	Living or dead	Dead only (vacuum required)
Image	Colour image	Black and white (can be false-coloured)
Cost	Low	Very high
Size	Portable	Very large

Exam Tip: Resolution is the ability to distinguish between two points that are very close together. A higher resolution means you can see finer detail. Do not confuse resolution with magnification — you can magnify an image as much as you like, but without sufficient resolution the image will just look blurry.

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Magnification, Image Size and Actual Size

The relationship between magnification, image size and actual size is given by the formula:

$\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}}$Magnification=Actual sizeImage size​

This can be rearranged to find any of the three values:

$\text{Image size} = \text{Magnification} \times \text{Actual size}$Image size=Magnification×Actual size

$\text{Actual size} = \frac{\text{Image size}}{\text{Magnification}}$Actual size=MagnificationImage size​

Exam Tip: A useful way to remember this is the triangle method. Place I (Image size) at the top, and M (Magnification) and A (Actual size) at the bottom. Cover the one you want to find: I = M × A, M = I ÷ A, A = I ÷ M.

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Worked Example 1 — Calculating Magnification

Question: A cell has an actual length of 50 μm. In a microscope image, the cell appears to be 25 mm long. Calculate the magnification.

Solution:

Step 1: Convert both measurements to the same unit.

$25 \text{ mm} = 25 \times 1000 = 25{,}000 \text{ μm}$25 mm=25×1000=25,000 μm

Step 2: Apply the formula.

$\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}} = \frac{25{,}000}{50} = \times 500$Magnification=Actual sizeImage size​=5025,000​=×500

The magnification is ×500.

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Worked Example 2 — Calculating Actual Size

Question: A microscope image of a mitochondrion is 30 mm long. The magnification used was ×15,000. Calculate the actual length of the mitochondrion in μm.

Solution:

Step 1: Convert the image size to μm.

$30 \text{ mm} = 30 \times 1000 = 30{,}000 \text{ μm}$30 mm=30×1000=30,000 μm

Step 2: Apply the rearranged formula.

$\text{Actual size} = \frac{\text{Image size}}{\text{Magnification}} = \frac{30{,}000}{15{,}000} = 2 \text{ μm}$Actual size=MagnificationImage size​=15,00030,000​=2 μm

The actual length of the mitochondrion is 2 μm.

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Worked Example 3 — Calculating Image Size

Question: A bacterium has an actual length of 3 μm. It is viewed under a microscope at ×5000 magnification. Calculate the image size in mm.

Solution:

Step 1: Calculate the image size in μm.

$\text{Image size} = \text{Magnification} \times \text{Actual size} = 5000 \times 3 = 15{,}000 \text{ μm}$Image size=Magnification×Actual size=5000×3=15,000 μm

Step 2: Convert to mm.

$15{,}000 \text{ μm} = \frac{15{,}000}{1000} = 15 \text{ mm}$15,000 μm=100015,000​=15 mm

The image size is 15 mm.

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Standard Form

Very large and very small numbers are often expressed in standard form (scientific notation):

$a \times 10^n$a×10n

where a is a number between 1 and 10, and n is a whole number (positive or negative).

Number	Standard Form
0.000025 m	$2.5 \times 10^{-5}$2.5×10−5 m
1500	$1.5 \times 10^{3}$1.5×103
0.002 mm	$2 \times 10^{-3}$2×10−3 mm
50,000 μm	$5 \times 10^{4}$5×104 μm

Exam Tip: When converting between units, always show your working clearly. Examiners award marks for method even if your final answer is wrong. Always check that your answer is sensible — if you calculate an actual cell size of 5 m, something has gone wrong!

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Unit Conversions

Conversion	Method
mm → μm	Multiply by 1000
μm → mm	Divide by 1000
μm → nm	Multiply by 1000
nm → μm	Divide by 1000
mm → nm	Multiply by 1,000,000

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Using a Light Microscope — Method

The AQA specification expects you to know how to use a light microscope to observe cells: