Light microscopes use VISIBLE LIGHT focused by glass lenses to produce a magnified image of a specimen.
Key facts:
• Maximum magnification: approximately ×2,000
• Maximum resolution: approximately 200 nm — cannot distinguish two points closer than this
• Can view living specimens — no special preparation needed
• Relatively cheap — used in schools and hospitals worldwide
• Images are in colour (natural or stained)
What you CAN see: cells, nucleus, cell wall, vacuole, chloroplasts, large organelles at high magnification
What you CANNOT see clearly: ribosomes (~20 nm), internal membrane detail, individual proteins
STAINING: many specimens are colourless, so dyes are used:
• Iodine solution: stains starch blue/black; stains nuclei light brown
• Methylene blue: stains cell nuclei dark blue — great for animal cells
• Toluidine blue: stains plant cell walls
Staining kills cells — so stained specimens cannot be living.
Electron Microscopes
Electron microscopes use a BEAM OF ELECTRONS instead of light.
Because electrons have a much shorter wavelength than visible light, they can produce far more detailed images.
Key facts:
• Maximum magnification: approximately ×2,000,000
• Maximum resolution: approximately 0.1 nm — 2000× better than light microscopes
• Specimens must be dead — the electron beam operates in a vacuum (no air, so no living cells)
• Very expensive — found in universities and research centres
• Images are black and white (false colour can be added digitally afterwards)
Two main types:
• Transmission electron microscope (TEM): beam passes THROUGH a very thin slice of specimen — shows internal detail of organelles
• Scanning electron microscope (SEM): beam scans the SURFACE of a specimen — produces a 3D-looking image of the external structure
Electron microscopes revealed: the detailed internal structure of mitochondria, ribosomes, the nuclear envelope, endoplasmic reticulum and many more previously unknown structures — transforming our understanding of cell biology.
Resolution vs Magnification
These two terms are often confused:
MAGNIFICATION: how many times bigger the image is compared to the actual object. A ×1000 magnification makes things appear 1000× bigger.
RESOLUTION: the ability to see fine detail — how clearly two close-together points can be distinguished as SEPARATE objects rather than blurring into one.
You can have high magnification but low resolution — like zooming in on a pixelated photo. The image is big but blurry.
Electron microscopes gave scientists BOTH very high magnification AND very high resolution — this is what made them revolutionary.
Analogy: imagine trying to read a book from across a room. You could use a magnifying glass (high magnification) but if the lens is poor quality, the words are still blurry (low resolution). A high-quality telescope gives you both magnification and resolution.
The Magnification Formula
The formula: Magnification = Image size ÷ Actual size
Rearranged:
• Actual size = Image size ÷ Magnification
• Image size = Actual size × Magnification
CRUCIAL RULE: both measurements must be in the SAME UNITS before you calculate.
If image size is in mm and actual size is in µm → convert before calculating.
Unit conversions:
• mm to µm: multiply by 1000 (1 mm = 1000 µm)
• µm to mm: divide by 1000
Tip: use the formula triangle — write M at the top, I bottom-left, A bottom-right. Cover the one you want to find.
⚠️ Common Mistake
Units must be the same before calculating. If image size is in mm and actual size is in µm, you MUST convert first or your answer will be wrong by a factor of 1000. Always write out the units in your working — this forces you to spot when they don't match.
📐 Variables
MMagnification (M) is measured in no unit (×)
IImage size (I) is measured in mm or µm (mm / µm)
AActual size (A) is measured in mm or µm (mm / µm)
📐 Key Equations
M = I ÷ A
A = I ÷ M
I = A × M
📌 Key Note
Light microscope: max ×2000, resolution 200 nm, can view living cells. Electron microscope: max ×2,000,000, resolution 0.1 nm, specimens must be dead. Resolution = sharpness. Magnification = size increase.
🎯 Matching Activity — Light Microscope or Electron Microscope?
Sort each statement to the correct type of microscope. — drag the symbols on the right to match the component names on the left.
Electron microscope
Drop here
Light microscope
Drop here
Electron microscope
Drop here
Light microscope
Drop here
Electron microscope
Drop here
Both
Drop here
Maximum magnification of approximately ×2,000
Can be used to view living cells without any special preparation
Specimens must be dead — the beam operates in a vacuum
Resolution of 0.1 nm — can distinguish structures 2000× finer than a light microscope
Used to calculate magnification using M = I ÷ A
First revealed the detailed internal structure of mitochondria and ribosomes
⚽ FIFA Worked Examples
Example 1 — Find magnification
A cell appears 54 mm wide in a diagram. Its actual width is 0.018 mm. Calculate the magnification.
F
M = Image size ÷ Actual size
I
M = 54 ÷ 0.018
F
Both values are in mm — no unit conversion needed
A
M = ×3000
Example 2 — Find actual size
A mitochondrion appears 12 mm long in an electron micrograph at ×80,000 magnification. Calculate its actual length in µm.
F
Actual size = Image size ÷ Magnification
I
A = 12 mm ÷ 80,000 = 0.00015 mm
F
Convert mm to µm: 0.00015 × 1000 = 0.15 µm
A
Actual length = 0.15 µm
Example 3 — Find image size
A bacterium has an actual diameter of 2 µm. It is drawn at ×2000 magnification. How large should the drawing be in mm?
F
Image size = Actual size × Magnification
I
I = 2 µm × 2000 = 4000 µm
F
Convert µm to mm: 4000 ÷ 1000 = 4 mm
A
Drawing should be 4 mm wide
🧪 Required Practical
🔬 RP1 — Use a light microscope to observe, draw and label plant and animal cells. Include a scale bar. Calculate the magnification of your drawing.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 5
1. What is the maximum magnification of a light microscope?
2. An image is 45 mm wide. The actual size is 0.009 mm. What is the magnification?
3. Why were ribosomes not discovered until electron microscopes became available?
4. What is the difference between magnification and resolution?
5. A cell has an actual diameter of 50 µm. At ×400 magnification, how large will the image appear in µm?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
Don't get itGetting thereNailed it!
🤖 Ask Mr Badmus AI
Stuck? Just ask! 💬
I'll use FIFA for calculations and flag Higher/Triple content clearly.