International A Level Biology / Cell structure
Light and electron microscopes: magnification and resolution revision notes
Edexcel International A Level Biology (YBI11), chapter 7: Cell structure, specification point 3.7. Free lesson.
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Magnification tells you how much bigger an image is, while resolution tells you how much detail you can see. Electron microscopes have much higher resolution than light microscopes because electrons have a shorter wavelength.
Magnification and resolution are different
Magnification describes image size. Resolution describes whether two close points can still be seen separately. This distinction matters because making an image bigger does not reveal detail that the microscope cannot resolve. Magnification is how many times bigger the image is than the real object. It has no unit and is written with a × sign, such as ×400. Resolution is the smallest distance between two points that can still be seen as separate. It is a length, measured in units such as nm or µm. A higher magnification means a bigger image. A better resolution means a smaller…
How resolution separates two points
A microscope can show two points separately only when they are far enough apart. If they are closer than its resolution limit, their images overlap and look like one blur. Points further apart than the resolution limit appear as two separate points. Points closer than the limit merge into one blur, so you cannot tell that there are two.
Why electrons resolve more detail
Resolution is limited by the wavelength of the beam. Points closer than about half the wavelength merge. Visible light has a much longer wavelength than electrons, so electron microscopes can distinguish much closer points. Visible light has wavelengths of about 400 to 700 nm. Half of 400 nm is 200 nm, so a light microscope resolves down to about 200 nm. Electrons have a wavelength well under 1 nm. A TEM can have a resolution down to about 0.1 nm. Ribosomes are smaller than about 200 nm, so a light microscope cannot resolve them. A TEM can, because its resolution is much higher.…
How light microscopes, TEMs and SEMs compare
The three microscopes differ in how they focus the beam and form an image. These differences affect what the image shows and whether the specimen can be alive. A light microscope uses glass lenses. Its magnification is eyepiece magnification × objective magnification. Light passes through the specimen, giving a coloured, flat image. It can show living cells. A transmission electron microscope (TEM) uses electromagnets to focus electrons passing through a very thin section. It gives a flat, black-and-white view of internal detail, with resolution about 0.1 nm. A **scanning electron…
How stains make cell structures visible
Most cell structures are nearly transparent, so there is little contrast between them without a stain. A stain is a dye that binds more strongly to some structures than others, making structures within the specimen easier to distinguish. Methylene blue stains the nucleus dark blue and the cytoplasm pale blue. It can be used on animal cells, such as cheek cells in core practical 5. Iodine solution turns starch grains blue-black. It is used on plant tissue, such as potato cells. Acetic orcein stains chromosomes dark red. It is used for a root tip squash in core practical 6. **Heavy…