Which scientific instrument is specifically designed for photographing the Sun in selected wavelengths?

Introduction / Context:
Astronomy uses specialised instruments to observe different celestial bodies. The Sun, being our nearest star, is studied in great detail using tools that can isolate specific wavelengths of light to reveal features such as sunspots, prominences, and chromospheric structures. This question asks which instrument is designed to photograph the Sun, especially at selected wavelengths, linking general science to basic astronomical instrumentation.

Given Data / Assumptions:

• The options are galvanometer, potentiometer, spectrophotometer, and spectroheliograph.
• The question is about photographing the Sun, not about measuring electric current or voltage.
• We assume standard definitions of these instruments from physics and astronomy.

Concept / Approach:
A spectroheliograph is an instrument that produces images of the Sun at a single wavelength of light, usually in a spectral line such as hydrogen alpha. It uses a spectroscope and a moving slit or scanning method to build up a picture of the solar disk at that wavelength. This allows astronomers to study specific layers or phenomena in the solar atmosphere. A galvanometer measures small electric currents, a potentiometer measures voltage or potential difference, and a spectrophotometer measures intensity of light at different wavelengths but is not specifically a solar imaging instrument.

Step-by-Step Solution:
1. Focus on the key requirement: an instrument used for photographing the Sun, usually with spectral selection.2. Recognise that the term spectroheliograph combines spectrum, Helios (Greek for Sun), and graph (writing or recording), indicating a device for recording the Sun's spectrum.3. A spectroheliograph isolates a narrow band of wavelengths and scans the solar disk to produce an image in that line, revealing features in the solar atmosphere.4. A galvanometer is purely an electrical instrument that detects and measures small currents, unrelated to solar photography.5. A potentiometer measures voltage or potential difference and is used in electrical circuits, not in direct solar imaging.6. A spectrophotometer measures the intensity of light at different wavelengths for samples in laboratories but does not typically form images of the Sun.7. Therefore, the correct instrument for photographing the Sun is the spectroheliograph.

Verification / Alternative check:
Astronomy references describe the spectroheliograph as an invention used to create monochromatic images of the Sun in specific spectral lines, allowing study of active regions and the chromosphere. Historical solar observatories often had dedicated spectroheliographs. By contrast, galvanometers and potentiometers appear in chapters on electricity, and spectrophotometers appear in chemistry or physics labs for analysing light passing through samples. These contexts confirm that spectroheliograph is the unique solar imaging instrument among the choices.

Why Other Options Are Wrong:
Option A, galvanometer, is incorrect because it measures electrical current and has nothing to do with photographing the Sun. Option B, potentiometer, measures potential difference and is again unrelated to solar imaging. Option C, spectrophotometer, does involve light and spectra but is used to measure light intensity, not to form direct images of the Sun.

Common Pitfalls:
Students may be tempted by spectrophotometer because it contains the word spectro, suggesting a connection to light. However, the inclusion of Helio in spectroheliograph is the clue that it is Sun specific. Another pitfall is to ignore the context of astronomy and think only about general laboratory instruments. Remembering that spectroheliograph literally means instrument to record the Sun in spectral lines will help you select the correct answer.

Final Answer:
The instrument used for photographing the Sun in selected wavelengths is the spectroheliograph.