Final answer:
The envelope of an x-ray tube is typically made from glass or metal and is designed to maintain a vacuum to facilitate efficient electron travel and x-ray production when high filament voltage is applied.
Step-by-step explanation:
The envelope in an x-ray tube is commonly made from glass or metal to maintain a vacuum environment. The vacuum allows electrons to travel from the filament to the anode without interacting with air molecules, which could diminish the efficiency and quality of the x-rays produced. When a high filament voltage is applied, electrons are emitted from a hot filament and are accelerated towards the anode, often made of tungsten due to its high melting point. The high-melting-point material is essential because when the energetic electrons strike the tungsten anode, a significant amount of heat is generated alongside the x-rays. X-rays are produced as a result of the electrons interacting with the material and causing inner-shell vacancies, which are then filled, releasing x-ray photons with energies characteristic to the anode material.