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Cell Structure: Question 1

Syllabus 1.1

Multiple choice AS 1 mark

A biology class compares a light microscope and an electron microscope, both used to study the same prepared slide of onion epidermis cells. The teacher increases the magnification on the light microscope to its maximum setting, but no extra internal detail becomes visible inside the cells. The image simply becomes larger and more blurred. Later, a similar numerical magnification is reached on an electron microscope, and fine internal detail, such as the double membrane surrounding a mitochondrion, becomes clearly visible.

Which property of the electron microscope explains why it reveals this extra detail, even at a magnification the light microscope could also reach?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Separate magnification from resolution

Magnification is how many times larger an image is than the real, actual object. Resolution is the smallest distance between two points that can still be seen as two separate points, rather than blurring into one. These are two different properties of a microscope, and a high magnification is useless if the resolution is too poor to show separate detail at that size.

Step 2: Why wavelength matters

A light microscope forms an image using light waves. The wavelength of light limits the resolution of a light microscope to around 200 nanometres (0.2 micrometres). Two points closer together than this cannot be distinguished as separate, no matter how much the image is enlarged.

An electron microscope forms an image using beams of electrons, which have a far shorter wavelength than light. This gives the electron microscope a much better resolution, able to distinguish points only a few nanometres apart. Structures such as the two membranes of a mitochondrion’s envelope, which are only tens of nanometres apart, can therefore be seen as two separate lines with an electron microscope but never with a light microscope.

Step 3: Apply this to the scenario

At the same numerical magnification, the electron microscope image stays sharp and detailed because its underlying resolution is much finer. The light microscope image, once magnified past its resolution limit, just becomes a bigger blur, the extra magnification adds no genuine new detail (“empty magnification”). This is exactly why enlarging the light microscope image further did not reveal the mitochondrial membranes, while the electron microscope image did.

Why the other options are wrong

  • B: staining brightness affects contrast, not the fundamental resolving power of the microscope, so it cannot explain why the light microscope’s image stayed blurred at high magnification.
  • C: an electron microscope does not use a light source at all, it uses electron beams, and it is resolution, not “a stronger source”, that allows genuinely useful magnification.
  • D: standard electron micrographs are not moving, three-dimensional images; they are still, two-dimensional images, just formed with much finer resolution.

Final answer

  • The electron microscope reveals more detail because electrons have a much shorter wavelength than light, giving a much better resolution, option A.