AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The principles and limitations of optical microscopes, transmission…
Full specification wording
Methods of studying cells: The principles and limitations of optical microscopes, transmission electron microscopes and scanning electron microscopes.
An optical microscope passes visible light through a specimen using glass lenses.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 1 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An optical microscope passes visible light through a specimen using glass lenses. It is limited by the relatively long wavelength of light, giving a maximum resolution of about 0.2 micrometres. It cannot resolve small organelles but can view living, coloured specimens. Electron microscopes use electron beams with much shorter wavelengths, providing significantly higher resolution. A transmission electron microscope (TEM) transmits electrons through an ultra-thin section to reveal 2D internal ultrastructure. A scanning electron microscope (SEM) sweeps a beam across a surface to produce a 3D image. Electron microscopy requires a vacuum, meaning specimens must be dead, and involves complex preparation that can create artefacts.
What earns the marks
- Optical microscopes are limited by the relatively long wavelength of light, restricting resolution to approximately 0.2 micrometres.
- Electron microscopes achieve much higher resolution because electrons have a significantly shorter wavelength than visible light.
- TEM produces a high-resolution two-dimensional image of internal ultrastructure from ultra-thin sections.
- SEM produces a three-dimensional image of the specimen's surface by detecting scattered electrons.
- Limitations of electron microscopy include the requirement for a vacuum (specimens must be dead), complex staining, and the risk of artefacts.
Common mistakes and how to put them right
Using magnification and resolution interchangeably.
Put it right: Magnification is how much larger the image is, while resolution is the ability to distinguish two close points; higher magnification does not improve resolution.
Claiming a TEM gives a 3D surface image or an SEM shows internal structure.
Put it right: TEM shows 2D internal ultrastructure, whereas SEM shows 3D surface topography.
Stating living processes can be observed using an electron microscope.
Put it right: Electron microscopes require a vacuum, so all specimens must be dead.
Explaining better resolution by stating electrons are smaller.
Put it right: Higher resolution is due to electrons having a much shorter wavelength than light, not their physical size.
Exam tips
- Clearly distinguish between instrument limitations (e.g., wavelength affecting resolution) and specimen preparation limitations (e.g., vacuum requirement, artefacts).
- Always link the shorter wavelength of electrons directly to the higher resolution achieved by electron microscopes.
Try the lesson
The start of the Methods of studying cells: The principles and limitations of optical microscopes, transmission electron microscopes and scanning electron microscopes. lesson, made from checked teaching notes for this specification point.
Have a go first
Which type of microscope uses a beam of electrons passed through a thin specimen to produce an image?
Listen
An optical microscope passes visible light through a specimen using glass lenses. It is limited by the relatively long wavelength of light, giving a maximum resolution of about 0.2 micrometres. It cannot resolve small organelles but can view living, coloured specimens. Electron microscopes use electron beams with much shorter wavelengths, providing significantly higher resolution. A transmission electron microscope (TEM) transmits electrons through an ultra-thin section to reveal 2D internal ultrastructure. A scanning electron microscope (SEM) sweeps a beam across a surface to produce a 3D image. Electron microscopy requires a vacuum, meaning specimens must be dead, and involves complex preparation that can create artefacts. A strong answer earns marks for these points. Optical microscopes are limited by the relatively long wavelength of light, restricting resolution to approximately 0.2 micrometres. Electron microscopes achieve much higher resolution because electrons have a significantly shorter wavelength than visible light. TEM produces a high-resolution two-dimensional image of internal ultrastructure from ultra-thin sections. SEM produces a three-dimensional image of the specimen's surface by detecting scattered electrons. A common slip: Using magnification and resolution interchangeably. Instead, Magnification is how much larger the image is, while resolution is the ability to distinguish two close points; higher magnification does not improve resolution. Exam tip: Clearly distinguish between instrument limitations (e.g., wavelength affecting resolution) and specimen preparation limitations (e.g., vacuum requirement, artefacts).
Key words
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wavelength and resolution
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optical vs electron microscopes
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TEM and SEM principles
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microscopy limitations
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See every definition
- wavelength and resolution
- The relatively long wavelength of light limits an optical microscope to a maximum resolution of about 0.2 micrometres, while electrons have a much shorter wavelength and so provide significantly higher resolution.
- optical vs electron microscopes
- An optical microscope passes visible light through a specimen using glass lenses and cannot resolve small organelles, whereas electron microscopes use electron beams with much shorter wavelengths for significantly higher resolution.
- TEM and SEM principles
- A transmission electron microscope transmits electrons through an ultra-thin section to reveal 2D internal ultrastructure, while a scanning electron microscope sweeps a beam across a surface to produce a 3D image.
- microscopy limitations
- Electron microscopy requires a vacuum, meaning specimens must be dead, and involves complex preparation that can create artefacts.
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