The timeframes of the camera are actually visible. If the back of a single lens reflex camera is opened, when there is no film inside, the shutter can be activated to watch the timeframe of the camera. At 1/250 of a second a flicker is discernible as the curtains of the shutter open and close again more quickly than the human eye can clearly see.
At slower speeds such as 1/60 of a second or 1/30 of a second the sense of the curtains moving is much easier to discern.
If the camera’s shutter activation is set to a much lower speed such as 2 seconds then the shutter curtains can clearly be seen to open, to stay open allowing sight towards the back of the lens, and then to close again.
Looking at the mechanism of the camera can help us to understand some of the technical aspect of what is happening within the camera after the light is resolved by the lens, subsequently is allowed to act for a period of time on either the film or digital sensor which then allows for a negative or digital raw file to be recorded.