The Doppler effect. In Sheldon's words, "it's the apparent change in the frequency of a wave caused by relative motion between the source of the wave and the observer". The Doppler effect is perhaps best explained visually. So here's a thing that is emitting waves. It could be a fire truck emitting sound, it could be a star emitting light, it could be a duck creating ripples on a pond. Those are all waves, and they all look something like this. We see the Doppler effect happening when the thing that is emitting waves moves. In the direction it's moving, the wavefronts bunch up, and behind it, they spread out. If our object is moving towards a stationary observer, these bunched up waves are observed at a high frequency, and if the object is moving away from a stationary observer, the waves are ob- served at a lower frequency. So that is the Doppler effect. The apparent change in the frequency of a wave caused by relative motion between the source of the wave and the observer. It makes sense. But it gets interesting when you consider some its applications. So let's say you are standing in the middle of the road – that's – that's you – and a car drives past you very fast. As it does so, it honks its horn, because you're standing in the middle of the road. The horn to you might sound something like this. So it starts at a high pitch and moves to a lower pitch, even though from the driver's perspective, the horn is playing the same pitch the entire time. So what's going on? As the vehicle's coming towards you, the sound waves that it's emitting bunch up, and so are delivered to you at a higher frequency, which you interpret as a higher pitch, because the frequency of sound waves is pitch, and then when the vehicle passes you and is moving away from you, the sound waves spread out, and so you hear them at a lower frequency – a lower pitch. So that's how the Doppler effect works with sound. It also affects another kind of wave – light. So let's say you look out your observatory and you see a star. Just like the car's sound waves, if the star is moving towards you, even just a little bit, the light waves that it emits will be bunched up, meaning that you see the light at a higher frequency than it actually is. Frequency in sound is pitch, so what does it mean for light? Well, if we look at our Handy Pocket Electromagnetic Spectrum Chart, we'll see that a small change in frequency for visible light will change its colour. Higher-frequency light waves means bluer light, and lower-frequency light waves means redder light. This is called redshift, and it may possibly be among the weirdest and coolest things of all time. Stars, or anything that you can see, change colour depending on their relative motion to you. Of course, you can't see this minute difference with your eyes, but astronomers, with the right equipment, can use this effect to tell whether stars are moving away from, or towards, Earth. As it turns out, almost everything we can see in the universe is moving away from us very quickly, which is both an important piece of evidence for the Big Bang and an indication that the Earth might get very lonely in the distant, distant, future. Thanks for watching this video. Let me know if you got something out of it. Bye.