Final answer:
X-rays are usually used to study black hole jets as they have shorter wavelengths and higher energy than visible light.
Step-by-step explanation:
The part of the spectrum that is usually used to study black hole jets is X-rays. X-rays have shorter wavelengths and higher energy than visible light, which make them suitable for studying high-energy phenomena such as black hole jets. By analyzing X-ray emission from black hole jets, scientists can gain insights into the physical processes happening near the event horizon of black holes.