Biology 9700/11 — May/June 2018

Background Concept

Two of the most important properties of any microscope are magnification and resolution, and they are often confused.

• Magnification is simply how much bigger the image is than the real specimen. It is a ratio (e.g. $\times 400$ means the image is 400 times the actual size of the object) and is calculated by multiplying the magnification of the eyepiece lens by the magnification of the objective lens. Magnification tells you how big the image is, but says nothing about how clear or detailed it is.

• Resolution (also called resolving power) is the ability of the microscope to distinguish two points that are very close together as separate points. If a microscope cannot resolve two objects, they blur into one, regardless of how much you magnify them. Resolution is limited mainly by the wavelength of the light (or electrons) used to form the image — shorter wavelengths give better resolution. This is why electron microscopes, which use a beam of electrons with a much shorter effective wavelength than visible light, can resolve structures down to about $0.2\ \text{nm}$, while light microscopes are limited to about $200\ \text{nm}$.

Understanding the Question

This is a multiple-choice question with a table of four options. Each option gives a definition for resolution and a definition for magnification. The candidate must pick the row in which both definitions are correct. The mark scheme confirms the answer is A.

Approach

Recall the precise definitions of the two terms and check each option against them. The correct definitions are:

• Resolution: the ability to distinguish between two separate objects that are very close together.
• Magnification: the number of times larger an image is compared with the real size of the object.

Only option A matches both definitions correctly.

Step-by-Step Reasoning

• Option A — resolution: "the ability to distinguish between two separate objects that are very close together" ✓ (correct). Magnification: "the number of times larger an image is compared with the real size of the object" ✓ (correct). Both definitions are spot on, so A is correct.
• Option B — resolution: "the clarity of the image formed by the microscope" ✗ (this is a vague, non-technical rewording and not the accepted definition). Magnification: "the power of the microscope to focus on very small objects" ✗ (focusing is unrelated to magnification). Both wrong.
• Option C — the two definitions are swapped. Resolution is given the magnification definition, and magnification is given the resolution definition. Wrong.
• Option D — resolution: "the power of the microscope to focus on very small objects" ✗. Magnification: "the clarity of the image formed by the microscope" ✗ (this actually sounds more like resolution). Both wrong.

Key Takeaways

• Resolution and magnification are independent properties of a microscope. You can have very high magnification with poor resolution (you just see a bigger blurry blob).
• A microscope is useful only when both magnification and resolution are sufficient to make a structure visible and distinct.
• For light microscopy, the practical resolution limit is about half the wavelength of light used (~200 nm). Electron microscopes push this down to about 0.2 nm.

Common Mistakes

• Confusing magnification with resolution: many students treat "more detail" and "bigger image" as the same thing. They are not — you can magnify a feature without resolving it.
• Equating resolution with "clarity". Clarity is a vague, everyday term; the technical definition specifies the ability to distinguish two separate points.
• Equating magnification with "power to focus". Focusing is part of producing a sharp image at a given plane and is unrelated to how much the image is enlarged.

Things to Be Careful About

• On multiple-choice questions of this kind, always check both parts of the option, not just one. Options B and D each have one definition that sounds like the right idea, which can trap a candidate who reads too quickly.
• The order of the columns is the same in every option, so swapping the two definitions (as in option C) is a common distractor — read each column carefully against the correct term.