Just out of curiosity..

When a table fan is placed in front of a TV that is swtiched on, what makes us see the fan moving slowly,but when there is no light behind the fan we cannot see the fan's motion.

What exactly makes the fan motion seem to slow down when there is light behind it? Is it that the light is actually coming through even though the fan is in motion?

If anyone can explain that would be wonderful:)

Thanks in advance.