1.1.21 · Cell Structure
Magnification Calculations — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
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Lesson summary
1.1.2 Magnification Calculations — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Use the magnification equation to calculate magnification, image size or actual size, converting units correctly.Definition — magnification is the number of times a specimen has been en…
- Equation triangle
- Same units
- Ruler readings
- Two lens types
- Field of view
- Micrometre (µm)
Section 1 of 8What magnification means
Definition
- Definition magnification is the number of times a specimen has been enlarged to give the image.
- A magnification of x100 means the image is 100 times larger than the specimen.
Equation
- Equation magnification (M) = image size (I) ÷ actual size (A).
- RearrangedA = I ÷ M and I = A × M.
Equation triangle
- Equation triangle put I at the top and A and M below; cover the quantity to find.
No units
- No units magnification is a ratio, so it is written as x100.
Section 2 of 8Units and conversions
| Unit | Symbol | Relationship |
|---|---|---|
| Millimetre | mm | 1 mm = 1000 µm |
| Micrometre | µm | 1 µm = 1000 nm |
| Nanometre | nm | 1 nm = 0.001 µm |
Same units
- Same units image size and actual size must be in the same unit before dividing.
- It is best to use the smallest unit shown in the question.
Converting
- Converting multiply when going from a larger unit to a smaller unit and divide for the reverse.
Ruler readings
- Ruler readings measurements in cm must be converted to mm before multiplying by 1000 to give µm.
Section 3 of 8Steps for a magnification calculation
1
Rearrange the equation for the quantity needed.
2
Read and measure the values from the question and image.
3
Convert units so they match.
4
Substitute the numbers and calculate.
5
Check that the answer is sensible for the structure.
Section 4 of 8Using a scale bar
Scale bar
- Scale bar a line labelled with the real length it represents.
Image size
- Image size the measured length of the bar on the page.
Actual size
- Actual size the length written on the bar.
Magnification
- Magnification measured bar length ÷ labelled length.
- (converted to the same unit)
Section 5 of 8Total magnification of a light microscope
Total magnification of a light microscope
Two lens types
- Two lens types an eyepiece lens (often x10) and objective lenses.
- (e.g. x4, x10, x40, x100)
Calculation
- Calculation total magnification = eyepiece magnification × objective magnification.
Field of view
- Field of view a higher power objective narrows the field of view, so find the specimen on low power first.
Section 6 of 8More from the specification
1
Magnification ratio of observed sample
- Magnification ratio of observed sample dividing measured image size by actual specimen dimensions determines total linear magnification for any microscope sample.
2
Microscopic magnification formula
- Linear magnification of a specimen is calculated using the relationship Magnification = Image size / Actual size (M = I / A).
3
Stage micrometer calibration of graticules
- An eyepiece graticule scale is calibrated against a stage micrometer of known division lengths to determine the true micrometre value per graticule unit.
4
Resolution limits of light vs electron microscopy
- Light microscopy is limited by the wavelength of visible light (~200 nm), whereas electron beams achieve sub-nanometre resolution.
5
Eyepiece graticule calibration
- Eyepiece graticule calibration calibrating graticule scale divisions against a stage micrometer allows true actual sizes of viewed cells to be measured.
6
Light microscope resolution
- Light microscope resolution resolution limit is determined by illumination wavelength, distinguishing individual specimen structures under high magnification.
7
Slide specimen preparation
- Slide specimen preparation placing thin tissue layers onto a glass slide under a coverslip prevents lens contamination and optical distortion.
8
Light microscope
- Microscope using light, magnifying up to about x1500.
9
Magnification factor
- The µm per graticule division at a given objective lens.
10
Starch grain
- Store of starch in the stroma.
11
Graticule
- An eyepiece scale calibrated against a stage micrometer to measure specimens.
12
Resolution
- The ability to distinguish between two objects that are close together.
13
Coverslip
- A thin glass or plastic square placed over the specimen to flatten it and protect the lens.
14
Starch
- An insoluble polysaccharide of glucose used by plants for energy storage.
15
Eyepiece graticule
- A scale in the eyepiece that is calibrated with a stage micrometer to measure specimens.
16
Graticule division
- One division of the eyepiece graticule scale.
17
Biological drawing
- A labelled, clear line drawing recording what is observed.
18
Stage micrometer
- A slide with an accurately known scale used to calibrate the graticule.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Magnification | How many times larger an image is than the actual object; I ÷ A. |
| Actual size | The real dimension of the specimen. |
| Image size | The measured dimension of the drawing or micrograph. |
| Scale bar | A line on a micrograph labelled with the real length it represents. |
| Micrometre (µm) | One thousandth of a millimetre. |
| Nanometre (nm) | One thousandth of a micrometre. |
| Objective lens | The lens nearest the specimen on a light microscope. |
Section 8 of 8Exam technique
1
Dividing without converting units first — image and actual size must be in the same unit.
2
Giving units for magnification — it is a ratio, so write x1200.
3
Rearranging the equation wrongly — actual size = image size ÷ magnification.
4
Measuring in cm and multiplying by 1000 to get µm — convert cm to mm first.
5
Adding the eyepiece and objective magnifications — total magnification is their product.
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