Vsauce is one of my favorite YouTube channels. Their quirky, engaging videos make understanding some tricky scientific ideas straightforward and fun. If you've ever wondered about rolling shutter or love learning about all the funky properties of light (which is the main ingredient of photography, after all), this is the video for you.
Rolling shutter is a phenomenon caused by the fact that most cameras do not read all the data from the sensor at the same time (a camera that does do that has a "global shutter"). Instead, they read the data in lines that advance across the frame (think of an inkjet printer). When an object in the frame moves relatively quickly compared to how fast the readout proceeds across the frame, it appears in multiple places during the same exposure and thus, the rolling shutter effect kicks in. Airplane propellers are one especially notorious offender:
The rolling shutter effect is a relatively simple phenomenon to explain; on the other hand, nothing about light is simple to explain, which is what makes it one of the coolest things in the universe. One peculiar property is that as fast as light is, it still only moves at a finite speed. This has all sorts of consequences, from the rolling shutter effect on the scale of galaxies to the fact that everything our eyes or cameras see is actually something that happened in the past. Check out the video for a more in-depth explanation of these fascinating properties.