Why does pitch change when you speed up a video?

Ministry Of Sound

Question: 

Why does pitch change when you speed up a video?  

Alex, Fulham 

Answer:

Name: Professor Martin, Manor Park

Qualification: Professor of Robotics at the University of Middlesex. First degree was in Physics.

Answer: Sound is a wave form which means it has peaks and troughs. If you speed up the tape you receive more peaks in your ear per second than you would otherwise and that makes the pitch go higher. Similarly if you slow down a tape you get fewer peaks and troughs arriving at your ear per second, slowing down the pitch.

It also works with light. This is how we know that the universe is expanding because all the stars are shifting in colour a little bit to red so we know the stars are moving away from us.  By the amount of shift in red we can measure how fast the universe is expanding, that is called the ‘Doppler shift.’

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