With the old fashioned film you are probably referring to, projectors are designed to flash each image twice or even three times, meaning that a 24fps movie has a flicker rate of 48 or 72 Hz[0].
The takeaway is that the perceptual frame rate for brightness is higher than the perceptual frame rate for motion.
The takeaway is that the perceptual frame rate for brightness is higher than the perceptual frame rate for motion.
[0] http://en.wikipedia.org/wiki/Movie_projector#Shutter