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When the voltage applied to a Crookes tube is high enough, around 5,000 volts or greater, it can accelerate the electrons to a high enough velocity to create X-rays when they hit the anode or the glass wall of the tube. The fast electrons emit X-rays when their path is bent sharply as they pass near the high electric charge of an atom's nucleus, a process called bremsstrahlung, or they knock an atom's inner electrons into a higher energy level, and these in turn emit X-rays as they return to their former energy level, a process called X-ray fluorescence. Many early Crookes tubes undoubtedly generated X-rays, because early researchers such as Ivan Pulyui had noticed that they could make foggy marks on nearby unexposed photographic plates. On November 8, 1895, Wilhelm Röntgen was operating a Crookes tube covered with black cardboard when he noticed that a nearby fluorescent screen glowed faintly. He realized that some unknown invisible rays from the tube were able to pass through the cardboard and make the screen fluoresce. He found that they could pass through books and papers on his desk. Röntgen began to investigate the rays full-time, and on December 28, 1895, published the first scientific research paper on X-rays. Röntgen