Microscopy: resolution, magnification and graticules
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 1 worked example, 5 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Seeing the invisible
- Magnification and resolution
- Light and electron microscopes
- Units
Read the transcript of the teaching part
Last time
Before we start: last time, in The secretory pathway and viruses, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Seeing the invisible
A bacterium is a few thousandths of a millimetre long. You cannot see it with your eyes, so we use microscopes. But making an image bigger is not enough. It also has to be sharp enough to show detail. Those two ideas, magnification and resolution, are the whole of this lesson.
Magnification and resolution
Magnification is how many times bigger the image is than the real object. Resolution is the ability to see two close points as separate. It is the smallest distance between two points that can be seen apart. You can magnify without limit, but if the resolution is poor, the image just becomes a bigger blur.
Light and electron microscopes
A light microscope uses light, with a long wavelength. It resolves down to about 0.2 micrometres, or 200 nanometres. An electron microscope uses a beam of electrons, with a much shorter wavelength. It resolves down to about 0.5 nanometres. That is why viruses and the inside of organelles need electron microscopes. Light cannot show them.
Units
One millimetre is one thousand micrometres. One micrometre is one thousand nanometres. To go to a smaller unit, multiply by one thousand. To go to a larger unit, divide. Always convert before you compare. Mixing units is the most common error.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Eyepiece graticule and stage micrometer · Magnification calculations · Resolution and magnification.
The whole chapter: Cell Structure, Cell Structure.
Chapter 1: Cell structure
- Microscopy: resolution, magnification and graticules 7 min
- Cell structures, plant and animal cells, and prokaryotes 5 min
- The secretory pathway and viruses 6 min
Also for A Level Biology 9700
Which topics come up most in A Level Biology 9700 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency, Paper 4 topic frequency.