Full transcript
0:00it's worth mentioning that all of these
0:02electromagnetic waves
0:03whether it be light or radio waves or
0:06the other forms we're going to talk
0:08about they all travel at approximately
0:10300 million meters per second through
0:13space the speed does change when you go
0:16into different mediums like into water
0:18and to air but when we're talking about
0:21when they're traveling through space
0:23through the vacuum of space all the way
0:24to us they take 300 million meters per
0:27second is a true speed they're traveling
0:29at ok great well we're gonna look at
0:35waves now because we have these
0:37electromagnetic waves so which again
0:39we're saying is light but we must study
0:41some properties of wings and one thing
0:44to keep in mind and we're gonna use the
0:46idea of water waves to help to talk
0:48about some of these examples is that as
0:51it says here it's the wave that travels
0:53not the medium ok well what what does
0:57that even mean well let's say we have
0:59our little frog here who apparently just
1:01jumped on the water and we see the wave
1:02rippling out well some people might
1:05think that what he's done is hit the
1:07water and the water's spreading out from
1:09him no it's not quite true what happens
1:14is this frog depresses the water which
1:17causes that water to go down which
1:20causes the water beside it to go up and
1:22we get this traveling pattern this water
1:25going up and down spreading out the wave
1:28spreading out the water though isn't
1:30traveling the water is doing what we see
1:32right here the water is just going up
1:34and down the same water is passing the
1:37wave which causes the next part to start
1:39going up and down which causes the next
1:40part to start going up and down and so
1:42it's the wave it's the disturbance it's
1:45the energy traveling not this water and
1:49if you think about it if it was the
1:51water traveling out then we'd have this
1:53big hole you take a rock and throw it
1:55into a pond we don't see a big huge hole
1:57in the middle of the pond forming and
1:59spreading out because that's what would
2:01happen if the water was traveling but no
2:03that water gets pressed down the next
2:05part goes rising up as a result and then
2:08the next mark was traveling down and
2:10that wave travels out
2:12and that wave might hit this frog and
2:14disturb them and try passes that energy
2:17to them not the water not the medium and
2:20that's why electromagnetic waves are
2:22traveling because there's no medium to
2:25travel in the case anyway but it travels
2:27through space the disturbance travels
2:31not the medium but we also have to look
2:35at a few different terms because what we
2:37have when we have a wave is that it's a
2:39repeating pattern it's this repeating
2:41oscillating pattern you can see this
2:44shows all right here just a repeating
2:46shape over and over and over again
2:47that's spreading out into space we saw
2:49the e/m wave before where what we have
2:52is actually an electric component and a
2:54magnetic component that are on a tee to
2:57each other perpendicular at 90 degrees
2:59but it's just two of these in essence
3:01okay
3:02so let's define some terms well the
3:05highest point of the wave we call that
3:08the crest the lowest point called the
3:10trough and how far we are from the under
3:14stirrup state is the amplitude okay but
3:17was it undisturbed state well let's go
3:20back to thinking about water I take my
3:21rock I throw it into water
3:23it depresses the surface well the
3:25undisturbed state was the old level of
3:27the water before I threw the rock after
3:30I threw the rock we started getting
3:31waves Wow some of those waves will rise
3:34up to a certain crest and the distance
3:36from the crests to where the wave the
3:39water was before I threw the rock that's
3:41your amplitude how much it's been
3:43disturbed we also have the term
3:46wavelength and thankfully this is
3:48exactly like it sounds it's the length
3:50of one wave often that's measured from
3:54crest to crest or trough to trough but
3:57realistically as long as is one full
3:59wave we can measure the wavelength
4:02anywhere we can measure from here to
4:03here the only thing is that we have to
4:07be a little careful some people might be
4:08tempted to try and measure from here to
4:10here but the way I usually describe it
4:12is that you want the wave to be at the
4:14same point same height say above or
4:17below the zero point and moving the same
4:20direction so if I was following the wave
4:22I'd be going up here up here to be down
4:25so that's not a wavelength
4:26but here would be backup so if I skip
4:28around here and here that would also be
4:30a wavelength or literally the length of
4:32one wave okay I want to find a few more
4:38terms and that one is the idea of
4:40frequency frequency how frequent the
4:45waves are in essence how many waves or
4:48cycle is depending what you want to call
4:50you can call a waving called a cycle but
4:52how many waves would pass by a given
4:55point in one second so if you were
4:58watching your water and how many waves
5:02hit the shore every second that would be
5:04your frequency and that can be a decimal
5:07number it might be not a full wave hits
5:09every second it might be that many ways
5:11hit in a second and let's take a look
5:13right here at this figure that'll
5:15hopefully help demonstrate it because
5:17again frequency number of complete waves
5:20or cycles and what a per second and what
5:23we're gonna define is say that this
5:24block would represent the amount of time
5:27for a second so we're gonna find this
5:30arbitrary zone here and saying this is
5:33one second from this side to that side
5:34and so if we want to figure out the
5:37frequency given the symbol F F
5:39represents frequency it we can take the
5:43number of cycles in the total time and
5:45when we divide this out we end up
5:47getting it down to one we want this
5:50bottom number by dividing a total time
5:52we get to one in our case we are
5:54defining one second and one thing
5:57actually that's also worth mentioning by
5:58the way is frequency is measured in the
6:00units of Hertz which is one over second
6:05when you have one over second it comes
6:07up often enough that we give it a
6:09special trait Hertz a special unit Hertz
6:13now you might wonder why is it not
6:16cycles per second and often you will see
6:18frequency written as cycles per second
6:20but the cycles isn't really a proper
6:23unit so often it will get ignored
6:24it'll get omitted so you might see a
6:27formula starting off as the frequency is
6:29three cycles per second and then someone
6:31will just report it as Hertz the cycles
6:33will get dropped quick
6:35so hopefully is making sense but let's
6:38look at this example so again we're
6:39defining this region as
6:40second well then what would be the
6:43frequency here and actually we're kind
6:45of given the answer but Orion do is
6:47think well this is one wavelength
6:49because we can see it's from one trough
6:52to another trough one wavelength within
6:54that one second and we can think it's
6:57one cycle per second if we wish or one
7:00per second which is one Hertz and again
7:04this is a little bit of a constrained
7:06example because we've got a 2d on
7:09animated image here but if we were
7:11watching a certain point we'd see in
7:13that case one wave go by in one second
7:15is what that's telling us in this case
7:18we can actually see about a wave and a
7:21half from one trough ish over to the
7:24just past the crest of the other this is
7:27about a wave and a half and so our
7:31frequency is one point five Hertz one
7:34point five cycles per second
7:36finally this one's a little more
7:39confusing looking but it's the same idea
7:41how many waves are within this one
7:44second block would pass by in one second
7:46well we get three point three waves per
7:49second or cycles per second which is the
7:52same as three point three Hertz that's
7:54our frequency how frequent the waves are
7:57passing by how many waves pass by in one
8:00second
8:01yeah if you're looking at water lapping
8:03to a shore how many waves hit the shore
8:05every second