Hertz and framerate

[sunnyl]

Hello, after doing some reading i cant seem to understand how hertz and framerates of videos/movies work....

For example:

PAL 25p movie, at what Hertz should it play? i keep reading that should be multiple, so 50hz, but why multiple and not 25hz? if it is progressive, should not be 25hz?

 

[iceblitzed]

a 25p movie is 25 frames per second so in other words 25hz

 

[sunnyl]

But If it is 25 frames per second, then why it plays at 50hz on both my projector and TV????

 

[iceblitzed]

For example, most movie projectors advance from one frame to the next one 25 times each second. But each frame is illuminated two or three times before the next frame is projected using a shutter in front of its lamp. As a result, the movie projector runs at 25 frames per second, but has a 50 or 75 Hz refresh rate.