Light vs electron microscopes, magnification calculations, preparing slides, scale and units. Paper 1.
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CognitoMicroscopy — light vs electron, magnification
Key facts & methods
Types of microscope
Light microscope: uses visible light. Max magnification ×2,000. Max resolution ~200 nm. Can view living cells. Cheap and portable.
Electron microscope: uses beam of electrons. Max magnification ×2,000,000. Max resolution ~0.2 nm. Cannot view living cells (sample must be dead and in vacuum). Reveals subcellular structures (ribosomes, membranes).
Resolution: the ability to distinguish two points as separate. Higher resolution = more detail.
Electron microscopes revealed structures too small for light microscopes: ribosomes, mitochondrial cristae, endoplasmic reticulum.
Magnification and calculations
Magnification = image size ÷ actual size.
Rearranged: actual size = image size ÷ magnification. Image size = actual size × magnification.
Units: μm (micrometres, 10⁻⁶ m), nm (nanometres, 10⁻⁹ m). 1 mm = 1000 μm. 1 μm = 1000 nm.
Scale bars: use the scale bar to calculate actual size. Measure the bar on the image, compare to its stated real length.
Example: image of cell = 30 mm wide. Actual width = 0.03 mm. Magnification = 30 ÷ 0.03 = ×1000.
Preparing slides and staining
To view cells under light microscope: place sample on glass slide, add a cover slip.
Staining: stains colour different structures. Iodine stains starch blue-black. Methylene blue stains nucleus blue.
Staining makes transparent structures visible.
Onion skin / cheek cell practical: stain with iodine (plant) or methylene blue (animal).
AQA exam tip: know how to calculate magnification and actual size. Convert units carefully.
⚡ Show all reasoning — the AI marks against the AQA mark scheme. Partial credit for correct biology even if wording differs.
1 markLight vs electronAQA 8461 style
Which of the following is an advantage of electron microscopes over light microscopes?
A They can be used to view living cells
B They are cheaper and more portable
C They have much greater resolution and magnification, revealing subcellular structures
D They produce colour images automatically
3 marksMagnification calculationAQA 8461 P1 style
A student views a cell under a microscope. The image of the cell is 45 mm wide. The actual width of the cell is 0.015 mm. (a) Calculate the magnification. (b) Give the actual width in micrometres (μm). (3 marks)
Hint: (a) M = image ÷ actual. (b) To convert mm to μm: multiply by 1000.
+30 XP
3 marksScale bar calculationAQA 8461 style
An electron micrograph shows a cell. The scale bar represents 2 μm and measures 20 mm on the image. The nucleus in the image measures 14 mm across. (a) Calculate the actual width of the nucleus. (b) State this in nm. (3 marks)
Hint: First find the scale (how many μm per mm of image). Multiply nucleus image size by the scale. Convert to nm by ×1000.
+30 XP
1 markResolution definitionAQA 8461 style
What does "resolution" mean in the context of microscopy?
A The maximum size of specimen that can be placed under the microscope
B The ability to distinguish two points as separate (detail visible)
C The total number of times an image is enlarged
D The brightness of the image produced
1 markStaining purposeAQA 8461 style
Why are biological stains used when preparing microscope slides?
A They kill the cells so they cannot move
B They make transparent structures visible by adding colour to specific cell components
C They increase the magnification of the microscope