Video: Doppler's Effect Video Lecture | Physics Class 11 - NEET

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.