Biology 9700/11 — October/November 2023

Background Concept

Resolution (or resolving power) is the minimum distance between two points at which they can still be seen as separate objects. It is not the same as magnification: a microscope can magnify enormously, but if its resolution is poor, the enlarged image will simply be a blurred blob.

• A light microscope is limited by the wavelength of visible light, giving a maximum resolution of about $0.2\text{–}0.25\ \mu\text{m}$ ($200\text{–}250\ \text{nm}$). Anything smaller than this appears as a single point and cannot be made out, no matter how high the magnification.
• An electron microscope uses a beam of electrons, which has a much shorter effective wavelength. Its resolution is around $0.1\text{–}0.5\ \text{nm}$, so it can resolve structures thousands of times smaller than a light microscope can.

Rule of thumb: to be visible, an object must be at least as large as the resolution of the microscope being used.

Understanding the Question

The question gives the sizes of three categories of virus and asks which of them satisfy the visibility criteria of a light microscope (resolution $0.25\ \mu\text{m}$) and an electron microscope. The answer table uses a single ✓/✗ per virus, so for each virus we must decide whether it is visible using both microscopes together (i.e. visible on the light microscope, since all viruses are visible on an electron microscope).

Approach

1. Convert the light microscope's resolution into the same units as the virus sizes (nanometres).
2. Compare each virus size to that resolution.
3. Decide for each virus whether it satisfies the joint visibility condition.

Step-by-Step Reasoning

Step 1 — Unit conversion.

$$0.25\ \mu\text{m} = 0.25 \times 1000\ \text{nm} = 250\ \text{nm}$$

So the light microscope can only resolve objects of about $250\ \text{nm}$ or larger.

Step 2 — Apply the rule to each virus.

Step 3 — Match to an option.
The pattern (✗, ✓, ✓) corresponds to row B.

Key Takeaways

• Resolution, not magnification, sets the limit of what a microscope can show.
• $0.25\ \mu\text{m} = 250\ \text{nm}$ is the threshold for the light microscope used in CIE practical questions.
• Anything smaller than the resolution of the light microscope (e.g. typical viruses, ribosomes, membrane details) needs an electron microscope.
• The "giant" viruses Mimivirus and Pandoravirus were remarkable partly because they crossed this size threshold and can be seen with a light microscope — most viruses cannot.

Common Mistakes

• Confusing magnification with resolution — assuming that simply increasing magnification would make a typical virus visible. It would not; the image would just be a bigger blur.
• Forgetting to convert units: $0.25\ \mu\text{m}$ must be changed to $250\ \text{nm}$ before being compared with virus sizes given in nanometres.
• Assuming the question is asking about the electron microscope only — in which case every virus would be visible and the answer would be A. The mark scheme shows the question requires visibility on both microscopes, so the light microscope is the discriminating factor.

Things to Be Careful About

• Note the wording of the question: "can be seen using a light microscope … and an electron microscope". Treat this as a combined criterion — a virus is only "can be seen" if both instruments can resolve it.
• Even though Mimivirus and Pandoravirus are much larger than the $250\ \text{nm}$ threshold, they are still viruses and would also be visible on an electron microscope; the ✓ captures both.
• Don't be distracted by the unusual fact that "Pandoravirus is bigger than the light microscope's resolution" — this is precisely the point the question is testing.