Microscopy: An electron microscope has much higher magnification and resolving power than a light microscope. This means that it can be used to study cells in much finer detail. This has enabled biologists to see and understand many more sub-cellular structures.

The idea

Magnification tells you how many times larger an image is than the object, while resolving power is the smallest distance at which two points can still be seen as separate. A light microscope uses visible light with a relatively long wavelength, so its resolving power is limited and useful magnification is modest. An electron microscope uses a beam of electrons with a much shorter wavelength, giving far greater resolving power and therefore much higher useful magnification. For example, a light microscope can show a nucleus and cytoplasm, but an electron microscope reveals mitochondria, ribosomes and the internal membranes of chloroplasts. This finer detail has allowed biologists to identify and understand many more sub-cellular structures and how they function.

What earns the marks

• State that magnification is how many times larger the image is than the real object, whereas resolving power is the ability to distinguish two separate points as distinct.
• Explain that electrons have a much shorter wavelength than visible light, so an electron microscope has a much smaller limit of resolution.
• Link greater resolving power to the ability to see finer detail, not simply to a larger image.
• Give a named sub-cellular structure visible only with an electron microscope, such as a ribosome or the internal membranes of a mitochondrion.
• Recognise that increased detail has allowed biologists to identify and understand more sub-cellular structures and their functions.

Common mistakes and how to put them right

• Confusing magnification with resolving power.

  Put it right: Magnification enlarges an image, but resolving power determines whether fine detail can be distinguished; a blurred image enlarged further does not reveal more detail.

• Claiming that an electron microscope simply magnifies more because it is more powerful, without referring to the shorter wavelength of electrons.

  Put it right: Link resolving power to wavelength.

• Stating that light microscopes cannot show any sub-cellular structures.

  Put it right: They show larger structures such as the nucleus, but not the smallest ones such as ribosomes.

Exam tips

• Define both magnification and resolving power before comparing the two microscopes, so each term is used precisely.
• When asked why electron microscopes reveal more detail, refer explicitly to the shorter wavelength of electrons and the smaller limit of resolution.
• Support your answer with a named example of a sub-cellular structure that only an electron microscope can resolve.