Biology 9700/11 — May/June 2014

Background Concept

Microscopes form images by detecting something that has interacted with a specimen. A light microscope uses a beam of visible light; a transmission electron microscope (TEM) or scanning electron microscope (SEM) uses a beam of electrons. Two properties of any microscope matter for what we can see:

• Magnification — how much larger the image is than the object. A light microscope can typically reach about $\times 400$ for a school/college instrument and up to roughly $\times 1500$ for a high-quality research light microscope. Electron microscopes can reach $\times 500{,}000$ or more.
• Resolution (resolving power) — the smallest distance apart two points can be and still be seen as two separate points. This is limited by the wavelength of whatever is illuminating the specimen: shorter wavelength = better resolution. The wavelength of visible light is about $400$–$700\ \text{nm}$, so a light microscope can resolve down to around $200\ \text{nm}$. Electrons accelerated through a high voltage have a wavelength of about $0.005\ \text{nm}$ (or less), so the electron microscope can resolve down to about $0.2\ \text{nm}$ — a thousand times better than a light microscope.

Electron microscopes therefore beat light microscopes on both magnification and resolution.

Understanding the Question

The question gives a small table of four options, each combining a magnification judgement (higher/lower) and a resolution judgement (higher/lower) for an electron microscope compared with a light microscope. You must pick the row that correctly describes the electron microscope on both counts.

Approach

Recall the two key differences between electron and light microscopes and match them to the right option:

• Electron microscopes have higher magnification (so eliminate C and D).
• Electron microscopes have higher resolution (so eliminate B).

Only option A is consistent with both.

Step-by-Step Reasoning

1. Magnification comparison: Light microscopes are limited to about $\times 1500$ at best; electron microscopes can magnify hundreds of thousands of times. So the electron microscope has the higher magnification.
2. Resolution comparison: Resolution is limited by wavelength. Electrons have a much shorter wavelength than visible light, so the electron microscope can distinguish much smaller structures — it has the higher resolution.
3. Both comparisons point to option A (higher magnification, higher resolution).

Key Takeaways

• Electron microscopes surpass light microscopes in both magnification and resolution.
• Resolution is the more important of the two: increasing magnification without improving resolution just gives a bigger blurry image (the "empty magnification" concept).
• The reason resolution improves is the much shorter wavelength of the electron beam compared with visible light.

Common Mistakes

• Confusing magnification with resolution. A higher magnification does not automatically mean a higher resolution — but for the electron vs light comparison it just happens that the electron microscope wins on both.
• Thinking the electron microscope is only better at magnification and not resolution (or vice versa).

Things to Be Careful About

• "Higher resolution" means resolving power is better, i.e. smaller structures can be distinguished — not that the resolution number is larger.
• The comparison here is electron microscope vs light microscope; for TEM vs SEM the comparison is different (SEM has lower resolution than TEM but gives 3D surface images).