when a helicopter’s blades are spinning incredibly fast, why do they sometimes look distinguished as if they’re actually barely moving at all?

Question

when a helicopter’s blades are spinning incredibly fast, why do they sometimes look distinguished as if they’re actually barely moving at all?

In: Physics

Answer 1

This is an effect called “Aliasing”.
You are looking at a continuous motion, but you only get to see individual frames of it: Videos are only taken at 24/60hz, most lighting is around the 50/100hz area.

Imagine you’re taking a video of the helicopter, but it spins at 24hz as well. That means that every frame you capture, the blades would be in the exact same position. When you would watch the video, it would seem like they are completely still.