You might have seen this effect before, but for those that haven't and don't know what it is, here's a quick explainer.
Sometimes, when a camera is taking footage - like the one below that's gone viral in the past day - of a helicopter in flight, it can appear as though the main rotars aren't spinning and are locked in position.
Take a look.
Looks freaky, right? No, it's not some experimental form of helicopter and no, it's not photoshopped either. There's no name, as such, for the effect but the explanation is simply down to the camera itself causing an optical illusion.
As you can see in the video, the camera has five rotors. We'll take a guess and say that a single blade is spinning at 300 rotations per minute. That means, per rotation, a blade is in a specific spot on five counts. That gives us an effective rpm of 1500. 1500rpm / 60secs = 25.
Now, cameras take 25 pictures - or frames - per second. Here, the rotor blades are shot in the exact spot every frame. Each frame then has to be shot at a fast enough shutter speed to freeze the blade for minimal motion blur.
So, put all these together and you get an optical illusion that makes it seem like the rotors are frozen in place when it's actually not because the camera is taking a picture of the rotors at the same spot in every position on every rotation.
Science. Or, y'know, just tell people it's a magic trick. Either works.