Please see the link in the description to download
a worksheet for this video. Experiments can be dangerous. If you are a child: never do
experiments unless your parent, guardian or adult educator says it's alright and is there with
you the whole time. Before watching this video, we suggest that you watch the video "Overview of
5 wave types in science." Waves are all around us and play a major part in our technology. For
this reason, we study wave details in our science courses. In this vid
eo, we'll discuss classes of
electromagnetic waves, mechanical wave groups, what a physical medium is, propagation and
oscillation, transverse and longitudinal waves, and the definition of a wave. We'll start by
discussing electromagnetic waves. We divide specific electromagnetic waves into one of seven
classes based on the length of their waves in a vacuum. In the prior video, we gave examples
of how we commonly experience radio waves, microwaves, infrared and visible light waves. The
thr
ee other classes of electromagnetic waves are: ultraviolet, X-rays and gamma rays. We commonly
use these as follows. We use ultraviolet waves inside our air ducts to kill harmful organisms
circulating in the air, such as viruses. We use x-rays in medical imaging such as getting an x-ray
to show the condition of our bones and teeth. And we produce gamma rays in nuclear power plants.
A key feature of all electromagnetic waves is that they can travel through a vacuum. A vacuum
refers to a volu
me of space that has no atoms in it. Outer space is a very good approximation
for a vacuum. For example, if the inside of this vehicle was like outer space, then there would be
only one atom inside it. But here on Earth, it's filled with air so there are about this number
of atoms inside it. Here are the classic examples of electromagnetic waves traveling in a vacuum.
Satellites send radio waves through outer space to our GPS enable devices. Astronomers study the
origins of the Universe by
detecting microwaves in outer space. The Sun generates infrared waves,
visible light waves and ultraviolet waves that travel across 93 million miles of outer space
to reach us. We experience the infrared waves as heat, the visible light waves as brightness and
the ultraviolet waves as sunburns. Black holes radiate X-rays and gamma rays as beams from their
North and South poles, which travel across outer space. Although all electromagnetic waves can
travel in a vacuum, this is not unique to
them. For example, gravitational waves can also travel
in a vacuum. These gravitational waves traveled across many trillions of miles of outer space to
reach our instruments on Earth. Next we'll discuss mechanical waves. Unlike electromagnetic waves
that are all similar to each other, mechanical waves are a very diverse group. There are many
ways to group these. We'll use four groups: string waves, surface waves, membrane waves and
acoustic waves. String waves include motion on strings, rop
es or wires that may look like
the wave is traveling. Or, the string may look like a wave is not traveling, but instead
it's just vibrating in place, as on this guitar. Water waves cover most of the Earth surface.
There is enormous variety among their size, speed and shape, but in physics we consider all
of these to be a type of surface wave. Membrane waves include motion on flags or cloths that move
freely, and it also includes motion on membranes that are held tight around all of their ed
ges
like on a drum. This bird is communicating with other birds by using sound waves that travel
through the air. Sound waves may also travel through liquids such as water, which is why we
can hear this diver. In physics, we consider sound waves to be a subgroup of a larger group
which are called acoustic waves. Other acoustic waves include seismic waves originating deep in
the Earth which we may feel as a shake during an earthquake. Many times these different wave groups
interact with eac
h other. For example, when string waves and membrane waves are part of musical
instruments, then they may create acoustic waves, which we then hear as sound waves. Although all of
these waves are very different, they all have one thing in common, which is that they can only exist
in a physical medium. Next, we'll discuss what a physical medium is. A physical medium is made of
many atoms that can transmit a wave. As we showed in the prior video, we can't hear the alarm clock
ringing when it'
s in a vacuum. But when we let the air back into the chamber, then we can hear the
alarm clock. This tells us two things. First that air is a physical medium. And second, that sound
waves are mechanical waves. What we commonly call air is a collection of atoms in the gas state. A
physical medium can also be in the liquid state, which is why milk waves only exist in milk, and
would be impossible if there were no atoms around, such as in a vacuum. And a physical medium
can also be in the soli
d state. Although electromagnetic waves can travel through a vacuum,
they can also travel through some physical mediums depending on the class of wave. For example,
visible light waves can travel through a gas medium like air. It can travel through a liquid
medium like water. And it can travel through a solid medium like transparent glass. But it can't
travel through a solid medium made of bricks, like the wall. Next, we'll discuss propagation and
oscillation. When we look at mechanical wav
es we can observe there are two types of motion. The
most obvious motion is that the waves appear to be traveling. We call this motion propagation. In
this case, the waves are propagating towards the shore. Here the waves are propagating away from
this athlete. In this animation of sound waves, we can see the waves are propagating away from the
speaker. The second motion is called oscillation. This refers to the relatively small distances that
atoms move within the physical medium as each w
ave propagates through them. With these water waves,
the oscillation is the up and down movement of the water that makes the boats bob, whereas the
propagation is the movement of the waves towards the shore. In this spring, the oscillations are
the up and down movement, whereas the propagation is towards the pole and away from the pole. In
this spring the oscillations are the small back and forth movements, whereas the propagation is
from one person to the other. In this animation of sound
waves, we can see the particles in
the air oscillating back and forth as each wave propagates past them. Please focus on one red
particle and you'll see that it only moves a short distance back and forth compared with the much
greater distance that the wave propagates. Next, we'll introduce transverse and longitudinal wave.
Transverse waves oscillate the physical medium in a direction that is perpendicular to the direction
of propagation. This flag is oscillating towards and away from us in
the y-axis, but the waves
are propagating in the x-axis towards the right, so these are transverse waves. These planes
are oscillating vertically, but the waves are propagating horizontally. So they're transverse
waves here. In contrast to transverse waves, longitudinal waves oscillate the physical
medium in a direction that's parallel to the wave propagation. Sound waves are the classic
example of longitudinal waves. We sometimes call these compression waves because we see areas of
compr
ession that are surrounded by areas that are spread out called "rarefaction." In this spring,
the oscillations are in the x-axis and the wave propagation is also in the x-axis. So these are
longitudinal waves. Some mechanical waves are made of both transverse and longitudinal waves.
The most common example is water. If we could look at some molecules of water, we would see a
motion very similar to this. Please focus on the top orange dot since it travels in a circle. That
means some of the
time it's going up and down, which means there is a transverse wave. But other
times it goes in a forwards and backwards motion, which means there's a longitudinal wave. Likewise,
in another surface wave such as those made by some earthquakes, we see circular motion, which tells
us there's both transverse and longitudinal wave motion. All electromagnetic waves are transverse
waves. The red waves represent the oscillating electric field, and the blue waves represent
the oscillating magnetic
field. That's why we call them electromagnetic waves. Now we're
ready to define a wave. A wave is a propagating disturbance. That disturbance could be in a
small, simple physical medium and caused by something we do, like mechanical waves in a
physical medium such as these surface waves being created by a child jumping into a swimming
pool. Or, that disturbance could be on a scale that may be beyond our imaginations, like
electromagnetic waves being created by a black hole and traveling for
billions of years across
the seemingly empty vacuum of outer space. We thank the following people and institutions for
sharing their amazing images. Here's a summary of this topic and some additional information.
Please pause the video if you wish to read this. Please subscribe if you'd like to be notified of future educational videos we
make. Thanks for your attention.
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