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Class 11 Physics (India)
Course: Class 11 Physics (India) > Unit 19
Lesson 1: Introduction to wavesIntroduction to waves
Transverse and longitudinal waves are two types of mechanical waves, which involve the transfer of energy through a medium (e.g. water, air, a solid). Learn about transverse and longitudinal waves through the examples of a shaken rope and a sound wave. Finally learn about the difference between a single wave pulse and periodic waves. Created by Sal Khan.
Want to join the conversation?
- How does sound travel through wires?(103 votes)
- Andrew M's answer is definitely correct if you meant sound through an electrical speaker; however, if you are referring to a tin can telephone, then here is an answer. The classic kid's toy works by vibration. When you speak into one can, your voice causes the back of the can to vibrate. These vibrations transfer to the string, which functions as a conduit all the way to the other can. As long as the string is pulled tightly, when the vibrations reach the back of the other can, it resonates much like a drum, producing a sound nearly identical to the original vibration. Here's an article from Wikipedia on the toy, if you were curious: https://en.wikipedia.org/wiki/Tin_can_telephone(23 votes)
- How radio waves pass through walls, while light waves do not?(40 votes)
- But then why do gamma waves, which have a high frequency, pass through walls as well?(4 votes)
- how would you define energy?(16 votes)
- It's actually quite hard to define energy, but I would say @Ethan Dlugie gives the best definition here.(4 votes)
- this is true that light has properties of wave and matter.Then why doesn't it exerts pressure?(13 votes)
- Light can exert pressure though not much. The pressure exerted by electromagnetic radiation is called Radiation Pressure.
http://en.wikipedia.org/wiki/Radiation_pressure(21 votes)
- Could anyone give a few examples each of transverse and longitudinal waves?(3 votes)
- Transverse waves: Electromagnetic waves(light waves, Radio waves),wave in a string etc.
Longitudinal waves: compression waves in a spring, sound waves etc.(12 votes)
- because sound is made by a vibration and because atoms always vibrate, do they make even the tiniest amount of sound?(8 votes)
- yes they make the tiniest amount of sound,according to the no of vibrations,called hertz,the sound is inaudible below 20hz(3 votes)
- what kind of waves are used in our microwaves(5 votes)
- In microwaves, electromagnetic waves are used to heat up your food. The waves in your microwave are the same sort of waves that make up visible light.(5 votes)
- What happens to the wave if it is travelling through different mediums?
Example: From a medium of high density to low density(4 votes)- Great question! Unfortunately, the answer is kind of complicated. In general, if you keep EVERYTHING ELSE the same, the speed of sound in a gas (like air) is slower when the density is high, and faster when the density is low. But more commonly, what we measure about air is the temperature, and the speed of sound is faster in hot air and slower in cold air. If waves hit a boundary between two media with different speeds, the wave will be refracted, which means it will change direction. This is what causes a mirage in the desert, because the air near the ground is much hotter than the air higher up, and so the light bends (you look just below the horizon but you see the bright light from the sky coming from that direction). If the boundary between the media is sudden, like between air and water, then the refracted wave turns a sharp corner, but there will also be a partial reflection of the wave.(5 votes)
- Atwhen the "speaker" pushed air forwards, how come the molecules of air went backwards? 7:22(4 votes)
- Some molecules which are pushed forward collide with the particles which are already present in the space forward. Because of these collisions they may move backwards(most will anyway move forward , pushing other molecules forward too)(4 votes)
- because sound is made by a vibration and because atoms always vibrate, do they make even the tiniest amount of sound?(3 votes)
- Yep! Even the most tiniest amount of sound is made by the vibration of atoms, just like any other.(4 votes)
Video transcript
Let's say I've got a rope. That's my rope. And what I'm going to do is, I'm
going to take the left end of the rope, and I'm going
to jerk it up, and then back down. And we're going to talk about
what happens or what possibly gets formed. So if I take it up over here,
it's going to, obviously, take the string to the right
of it up with it. And the string is going to
look something like this. It's going to look something
like that. Now I'm going to immediately
jerk it back down. And as it passes, let's see what
the rope will look like when the left-hand point is at
its original position again. So the left-hand point-- I've
pulled it back down. But in the last time period,
this part of the rope had some type of an upward velocity. You could imagine that way. And even after that point, even
though this left-hand point starts getting pulled
down, this point right here still has some upward
momentum. So it's still going to keep
moving up, maybe at a slower pace because it's starting
to be tugged down by the rope on its left. So it's going to look
something like that. And it's going to bring the
rope to its right with it. So the rope will look
something like this. The rope might look something
like that. And then I'm going to take this
guy-- this was just an intermediate position on the
way to being pulled all the way down here. So what's the rope going
to look like now? Well. This guy, he had some momentum
that got him there. But then all of that velocity
will essentially go to zero because he's being tugged
by the rope to the left. And now, he's going to
switch directions. And he will have gotten
here, at that point. The point on the line that was
here-- on the purple period of time-- it had some
upward momentum. So it's just going to keep
going, on maybe a slower pace. It'll be there and it will bring
the rest of the rope to the right of it with it. So now my rope is going to
look something like this. And then finally, where I'm
going to jerk the rope back to its original position-- so this
left-hand point is going to be there. This guy, in the previous
time period, was moving down rapidly. So he might get there ready to
switch directions again. This guy will start
moving down. This guy, right here, he had
some upward momentum. So he's going to be up
in this position now. And he's going to be ready
to switch directions. So finally, when I've done this
whole cycle, when I've moved up, down and back there
again, my rope might look exactly like this. And I could let go
of the rope. I could just leave this little
left point right there. And this the lump is going to
propagate along the rope. Because in the next moment
of time, what's it going to look like? This guy is going to be pulled
up by this left-hand point. So he'll go back to his
resting position. This guy's being pulled down
right here by the part of the rope to the left of him, so he's
going to be pulled down. This guy's being pulled down. But this guy had some upward
momentum in the time period before, so he will
have moved up. And so, the very next time
period, my rope is going to look something like this. And this disturbance in the
rope, if I do nothing else, and if I don't lose energy to
heat and friction and all that, it'll just continue
moving down the rope. If I look at the rope at some
future period in time, maybe not that far down, the rope will
look something like this. And if I were to keep watching
it, I'll see this disturbance. I keep using the word
disturbance, because there's really no better word
to use for it. I'll see this disturbance or
perturbation, or whatever you want to call it, moving
along the rope. When we think about what a wave
is, we essentially-- I kind of jumped the gun-- I
keep calling this is a disturbance, because I didn't
want to use the word, wave. I want to say, well what
really is a wave? And a wave really is just this
disturbance that's propagating down the rope. So this is a good time to
actually define a wave. A wave. Because once I define it,
I can start calling this a wave, as opposed to
a disturbance propagating down the rope. So a wave is a disturbance
propagating through space. And you might see other
definitions of a wave. One of the most typical ones is
energy or a disturbance propagating energy
through a medium. And when they say medium,
it's what is the wave going through? So in this example, the rope
would be our medium. But the reason why I don't want
to use that definition of a wave is because in future
videos, we'll learn about electromagnetic waves and those
don't propagate through any medium. They propagate through
a vacuum. So to keep things as general as
possible, we'll just call it a disturbance that propagates
through space. And it usually transfers
energy. What do I mean by transferring
energy? On this left hand part
of the rope, I gave a little energy the rope. I moved it up, down, and
then back again. And then after I did that, that
up, down, back again is happening successively
to every point to the right on the rope. So if I waited long enough, at
this point on the rope right here, it's going to move up,
down, and then back again. Exactly what I did over here
is going to happen to this point on the rope. And then later on, it's going to
happen to some other future point on the rope. So that energy that I originally
put on the left-hand side of the
rope is being transferred down the rope. If I had some type of object
here sitting on the rope, maybe when the wave-- when the
disturbance- passes by it, this thing could get flipped
into the air; it might get pushed into the air,
and go into a higher potential energy. So this disturbance
is transferring energy in this case. Now, what I've drawn here isn't
the only type of wave you can have. I mean my
definition is fairly general. But the definition is more
general than just what I've drawn here. For example, you could have a
sound wave. If you just look at all of the molecules of the
air, they have some density that looks something
like that. And now let's say I had some
type of a membrane-- maybe it's a speaker--
that jolts this left-hand side of the air. So it just pushes-- so let me
see if I can draw this. Let's say I had some type of
surface here that just really quickly jolts-- that just moves
it in that direction, and then just comes back. So similar to what I did
here, I go up and down. But instead of doing that, it
just pushes the air and then pushes back. So what's going to happen? Right after it pushes it, the
air molecules that it pushes up against are going
to jam together. They're going to
get compressed. Right here, all these air
molecules that were right on the surface, they're going to
get pushed next to all these air molecules that
are right there. And then when it pushes back,
or when the membrane goes back, you're going to have
fewer air molecules here, because you're going to have
a low density here. And then these guys, that are
all scrunched up together, they're going to want to get
away from each other. They might even run
into each other. And so these guys are going to
run into those guys, who are going to run into
the next guys. And so on and so forth. And after these guys bump into
those guys, those guys are going to go back to
where they were. So essentially, you're going
to have this disturbance that's going to be a set of
molecules compressing, or bumping into, its neighboring
molecules. So if you look at this at some
future period in time, all of a sudden, this area
might look normal. Let me clear it and draw it
just the way it I started. So this area might
look normal. But that compression of the
particles might have reached right over here. And not only that, we saw that
right after the compression, you usually have this area
of low pressure. So if I were to really draw this
wave, and actually, if this membrane were to keep doing
it over, and over, and over again-- so it kept going
forward and back, forward and back, or right and left, right
and left-- what you would have is a series of compressions. The air would just have a
series of compressions. So that's one compression. You'd have another compression
right there. Another compression
right there. And then, in between
the compressions the air is less dense. The air is less dense
like this. And what we've essentially just
generated is a sound wave travelling through air. So this right here is a sound
wave. And this type of wave, where the direction of the
disturbance is the same, or along the same axis as the
direction in which the wave is travelling-- the wave is
travelling in that direction-- this is call a longitudinal
wave. So sound waves sound through air, they're
longitudinal waves. Sometimes called a compression
wave. Same thing. Compression wave. Because it's
caused by compression. Our example of the string, this
is called a transverse wave. Because the disturbance,
the movement of the medium, is going in a direction transverse
to-- or at an axis that's transverse to-- the
direction of our movement. We're moving in that direction,
to the right. Actually, our wave is moving to
the right, but the actual medium is moving up and down. Our medium is moving
up and down. That's why this is called
transverse. While here the medium is moving
left and right while the wave moves to the right. So it's along the same axis. So we're dealing with the
compressional or longitudinal. Now in this first example,
I just did one cycle. I just jerked up, down,
and back again. And I created this
one disturbance. And we can call this, when you
just do it once, you can view this as a wave pulse. If I kept doing that-- if I
just went up, down, back again, up, down, back again,
and I kept doing it periodically, over and over
again, then I would generate a periodic wave. And my
string would look something like this. Well actually, it would look
something that right there, where that's the disturbance
generated from our first time that we moved this left-hand
part of our string. So this right here is
a periodic wave. In the next video, we're going
to talk about a lot of the properties of a periodic wave.
How the wavelength, and its frequency, and its period relate
to its velocity, and all of that. But I'll leave that alone
in this video. But I just wanted you to
appreciate what I think is a concept that we use
in everyday life. So it's a wave, it's a sound
wave, and all that. But it's a fairly abstract
notion where we talk about a wave, we're really just pointing
to a disturbance that's moving, usually along
a medium, at least when we visualize it, but not always. But we're just pointing
to this disturbance. And this disturbance could take
many forms. It could be a transverse disturbance if we're
dealing with a string. It could be a disturbance in
terms of the density of air molecules in terms of a sound
wave. And there is a relation. So if you wanted to just plot
the density here by position-- if I were to mathematically
represent this compression wave right here-- let's say that
this line represents just resting before the sound
wave hits, that's just your normal density. If we were to plot the
density it might look something like this. Over here, we have very
high density. Over here, we have
very low density. Over here, we have very
high density. And if you were to plot it, it
would look a lot like that transverse wave that I did with
the rope at the beginning of this video. And that's why they're even
grouped together. Because mathematically, even
though a compression wave looks very different, or
you might visualize or conceptualize it very different
than a transverse wave, mathematically, they're
essentially the same thing. You have this quantity. In this case, it's the density
of the air varying over time. In this case, it's the height,
or the position, or how much your displacement from your
resting position-- that's the quantity varying through time,
that disruption is travelling over the course of the medium. That's why we call both
of these things waves. Anyway, I'll let you go here. And in the next video, we'll
talk a little bit more of the properties of periodic waves.