For about 400 years, nearly all observations of the universe were in what form?

• A. Gravitational waves
• B. Visible light
• C. Radio waves
• D. X-rays

Answer: (A)

Observations were made using visible light for about 400 years because early astronomy relied on what human eyes can see and on instruments designed to work with those wavelengths. The first telescopes collected and focused visible photons, and ground-based astronomy could study positions, brightness, and spectra using photographic plates and later detectors that were sensitive to visible light. The atmosphere is relatively transparent to many visible wavelengths, making it practical to observe from Earth with relatively simple equipment. It wasn’t until the 20th century that people developed reliable instruments for other parts of the electromagnetic spectrum (radio, infrared, ultraviolet, X-ray, gamma-ray) and, for gravitational waves, incredibly precise interferometers. So, for most of its history, the universe was studied through visible light.