Question

In an X-ray tube, electrons accelerated through a very high potential difference strike a metal target. If the potential difference is increased, the speed of the emitted X-rays

• A. increases
• B. decreases
• C. increases or decreases depending on the target material
• D. is always equal to $3\times {10}^{8}m{s}^{-1}$

Answer 1

The correct option is D

is always equal to $3\times {10}^{8}m{s}^{-1}$

X- rays, being electromagnetic waves, always travel with the speed of light, i.e., $3\times {10}^{8}m{s}^{-1}$ in vacuum. It is only the energy of the incident electrons (and, therefore the energy of the emitted X-rays) that depends on the accelerating potential difference. The speed of X-rays has nothing to do with this potential difference, it is a characteristic only of the medium of propagation. Hence, the correct choice is (d).