what happens to an x-ray spectrum when the tube voltage is changed?
An X-ray tube is a vacuum tubethat produces X-rays. It consists of a cathode, which emits electrons into the vacuum and an anode to collect the electrons, thus establishing a flow of electrical current through the tube. A high voltage power source, for example 30 to 150 kilovolts (kV), is connected across cathode and anode to accelerate the electrons. The X-ray spectrum depends on the anode material and the accelerating voltage. The x-ray emission is a result of "braking radiation" ("Bremsstrahlung") on the catching plate of the anode. Generally speaking, the higher the energy of the particle when it meets the anode, the higher the energy of an x-ray photon emitted, i.e. the wavelength is shorter. The spectrum in this case is continuous and it's low wavelength cut-off (shortest wavelength in the spectrum) is inversely proportional to the voltage.