Optical, TEM and SEM Microscopes Compared
How optical, transmission electron and scanning electron microscopes work, why electron microscopes resolve more detail, and the limitations of each. Includes the 4-mark exam question, a worked model answer and the examiner trap on resolution versus magnification.
Chapters (8)
Exam question [4 marks]
Explain why an electron microscope can show more detail than an optical microscope. Compare the images from a transmission electron microscope and a scanning electron microscope. Give one limitation of electron microscopes.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Electrons have a shorter wavelength than light | "smaller wavelength", "electron beam has shorter wavelength" | "electrons are smaller"; "light is weaker" |
| 2 | So electron microscopes have a higher resolution (can distinguish points closer together) | "better resolution", "see two close points as separate" | "higher magnification" alone; "clearer" with no reference to resolution |
| 3 | TEM gives a 2D image of the inside of a thin specimen; SEM gives a 3D image of the surface (both needed) | "TEM shows internal structure / organelles", "SEM shows surface" | TEM and SEM reversed; "TEM is 3D" |
| 4 | One limitation: specimen in a vacuum so must be dead / cannot view living cells; OR preparation can create artefacts | "living specimens cannot be viewed", "artefacts may appear", "specimen must be thin (TEM)", "image is black and white" | "expensive" alone; "low magnification" |
Marks 1 and 2 are a chain, wavelength then resolution. Optical microscopes have low resolution, not low magnification. Mark 3 needs both microscopes correct. Only one limitation is credited.
Model answer (4/4)
Electrons have a shorter wavelength than light, so an electron microscope has a higher resolution and can show two points that are close together as separate. A TEM passes electrons through a thin specimen and gives a two dimensional image of the inside of the cell, whereas an SEM scans the surface and gives a three dimensional image of the surface. One limitation is that the specimen is in a vacuum, so it must be dead and living cells cannot be viewed.
Examiner trap
- Saying electron microscopes can view living specimens
- Mixing up which gives 3D surface images (SEM) and which gives internal detail (TEM)
- Saying optical microscopes have a low magnification rather than a low resolution
Key terms
- optical microscope
- transmission electron microscope
- scanning electron microscope
- resolution
- wavelength
- artefact
- 2D
- 3D
- vacuum
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:27The optical microscope
Start with the optical microscope. It uses light and glass lenses. You can use it on living specimens, it is cheap, and it shows colour. But the image is fuzzy at high power. You can only make out larger structures, like the nucleus. The reason is not magnification. It is resolution.
0:48What resolution means
Resolution is the smallest distance between two points at which you can still see them as separate points. And it depends on wavelength. The shorter the wavelength, the higher the resolution. Light has a long wavelength, so optical microscopes have a low resolution. Notice that, low resolution, not low magnification. Electrons have a much shorter wavelength, so electron microscopes have a high resolution.
That is the reason they show far more detail.
1:19TEM: internal detail
The first type is the transmission electron microscope, the TEM. A beam of electrons is fired through a very thin slice of specimen. Some parts absorb more electrons than others, so you get a dark and light image. It shows the inside of cells, the organelles in fine detail. But the image is flat, two dimensional.
1:42SEM: surface 3D images
The second type is the scanning electron microscope, the SEM. The beam sweeps across the surface of the specimen, and electrons bounce off into a detector. The specimen does not need to be thin. You get a three dimensional image of the surface.
So remember it like this. TEM, through, inside, flat. SEM, surface, three dimensional.
2:08Limits of electron microscopes
Electron microscopes have limits. Electrons are blocked by air, so the specimen sits in a vacuum. That means it must be dead. You can never view living cells. The specimen also needs complex preparation, and that can create artefacts. An artefact is something that shows up in the image but was not there in the living cell. For a long time scientists argued about whether some structures were real organelles or just artefacts.
3:09Examiner trap and recap
The trap. Never say optical microscopes have a low magnification. They have a low resolution. And never say electron microscopes can look at living specimens.
Recap. Short wavelength, high resolution. TEM is inside and flat. SEM is surface and three dimensional. Next time, we put numbers on it: magnification versus resolution, and how to calculate the size of a specimen.
Keep going
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Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-027.