3.1.7Unit 3: Practical Skills in Biology I
Microscopy – Magnification and Resolution — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 9 of 27 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Magnification
Magnification tells you how much bigger an image is, but not how clear it is.
Definition
- Definition Magnification is how many times larger an image is than the actual size of the specimen.
Formula
- Formula Magnification = size of image ÷ size of real object, which can be rearranged to find any one of the three quantities.
Total magnification
- Total magnification In a light microscope this is the magnification of the eyepiece lens multiplied by that of the objective lens.
Unit conversion
- Unit conversion Convert all lengths to the same unit before calculating, using 1 mm = 1000 μm and 1 μm = 1000 nm.
Scale bars
- Scale bars A scale bar on an image shows the real length it represents, so magnification can be found by dividing its measured length by that length.
Standard form
- Standard form Very small sizes are best written in standard form, such as 2.5 × 10⁻⁶ m.
Part 2 of 5Resolution
Resolution
Definition
- Definition The ability to distinguish between two separate points that are close together.
Limit
- Limit Light microscopes cannot resolve points closer than about 200 nm because of the wavelength of light.
Electron microscopes
- Electron microscopes The wavelength of electrons is much shorter, so they can resolve structures far smaller than a light microscope can.
Empty magnification
- Empty magnification Increasing magnification beyond the resolution just makes a bigger blurred image with no extra detail.
Part 3 of 5Comparing Microscopes
1
A light microscope shines light through a specimen and can be used for living or dead specimens larger than about 200 nm.
2
A light microscope is used to study whole cells, tissues and small organisms, and is cheap, portable and easy to use.
3
Electron microscopes can resolve very small structures, such as organelles and the internal structure of mitochondria.
4
Electron microscopes are expensive, need trained operators and require specimens to be dead and in a vacuum.
5
Specimen preparation for either type can create artefacts, which are features that are not present in the living cell.
Part 4 of 5Worked Example
Question
- A light microscope has a ×10 eyepiece and a ×40 objective.
- A cell is 8 mm long in the image.
- Calculate the total magnification and the real length of the cell in micrometres.
- Total magnification = 10 × 40 = 400.
- Real size = image size ÷ magnification = 8 mm ÷ 400 = 0.02 mm.
- Convert to micrometres0.02 mm × 1000 = 20 μm.
Answer
- Magnification ×400;
- the cell is 20 μm long.
Part 5 of 5Exam Checklist & Tips
3.1.7 – Key Points for the Exam
- Magnification The number of times larger the image is than the specimen.
- Resolution The minimum distance at which two objects can be seen as separate.
- Scale bar A line on an image that shows a known real length.
- Artefact A feature seen in an image that was caused by preparation and is not part of the living specimen.
Common Mistakes
- Treating magnification and resolution as the same thing.
- Failing to convert units, such as mixing millimetres and micrometres in one calculation.
- Writing that a light microscope can resolve the phospholipid bilayer, which is around 10 nm thick.
- Multiplying instead of dividing when asked for actual size.
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Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.