Science Puzzle
Atmospheric X-Ray
Astronomers want to observe X-rays coming from a black hole. They build a sensitive X-ray detector and point it at the sky.
If the detector is on the ground, will it detect anything? Why or why not?
The Answer
It will detect nothing from space. The Earth's atmosphere is almost completely opaque to X-rays. The oxygen, nitrogen and other molecules in the upper atmosphere absorb X-ray photons before they reach the ground.
This is why all X-ray astronomy is done from space. Telescopes like the Chandra X-ray Observatory and XMM-Newton orbit above the atmosphere where they can detect the X-rays that flow freely through the vacuum of space.
The atmosphere blocks different parts of the electromagnetic spectrum for different reasons. Ozone blocks most ultraviolet. Water vapour and carbon dioxide absorb infrared. X-rays and gamma rays are stopped by atoms in the upper atmosphere through the photoelectric effect and Compton scattering.
Only two "windows" are reliably transparent: the visible light window (roughly 380 to 700 nm) and the radio window (a wide band from about 1 cm to 10 m wavelength). Everything else requires space-based observation.
The principle: Atmospheric opacity. The atmosphere blocks most of the electromagnetic spectrum, including X-rays and gamma rays. Only visible light and radio waves have reliable ground-level windows. X-ray astronomy requires space-based telescopes.