Name: 多普勒效應：運動對波有什麼影響？ (The Doppler Effect: what does motion do to waves?)
Uploaded: 2021-01-14T06:23:02.000Z
Duration: 3 min 3 s
Description: 【看影片學英語】數萬部 YouTube 影片，搭配英漢字典即點即查，輕鬆掌握單字發音與用法，長久累積看電影不必再看字幕。

"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

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-

at a lower frequency. So that is the Doppler effect.

The apparent change in the frequency of a wave caused by relative motion between

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

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

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

So that's how the Doppler effect works with sound. It also affects another kind of

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

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

Higher-frequency light waves means bluer light, and lower-frequency light waves

This is called redshift, and it may possibly be among the weirdest and coolest things

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

or towards, Earth. As it turns out, almost everything we can see in the universe is

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.