Why do those moving black bars appear when trying to take a photo of an older tv or computer screen? And not on modern ones?
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Older cathode ray tube (CRT) TVs drew each image row by row. The phosphor coating briefly lit up very brightly as it was drawn but then quickly faded. If you could take a very short exposure photograph then only a narrow horizontal bar would be bright and the rest close to black. With the longer exposures common with indoor photography, you can see more of the picture but typically a black bar will remain. Modern LCD and OLED TVs can display a complete image all at once so problems of this kind are less likely.
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