Which microscope is focused by varying the strengths of magnetic fields?

• A. Light microscope
• B. Electron microscope
• C. Scanning electron microscope
• D. Phase-contrast microscope

Answer: (B)

Focusing a beam of charged particles requires magnetic fields. An electron microscope uses electromagnetic lenses—coils that generate magnetic fields—to bend and converge the electron beam. By changing the current in these lenses, the focal length changes, allowing you to bring the image into focus. Light microscopes don’t use magnetic focusing; they rely on glass optical lenses to bend light and adjust focus by moving lenses or changing spacing. Phase-contrast microscopy also works with light, exploiting phase shifts rather than magnetic fields. While a scanning electron microscope is indeed an electron-based instrument and uses magnetic lenses to focus the beam, the question points to the broader category that relies on magnetic focusing: the electron microscope.