Full transcript
0:00we can also define the period of a wave
0:02and this is simply how long it takes for
0:06one wave to pass so again if we were
0:09able to stand at a certain point and see
0:13how long it takes for one full wave once
0:16the crest to crest or one trough to
0:18trough however long it takes for that to
0:20pass by that is the period of the wave
0:23and so a mean in this case if we just
0:26watched this boy and saw from here to
0:30the next time it was fully at the top
0:32for a crest if we had recorded that time
0:35to some high degree of accuracy that
0:37would be the period of the wave so
0:39another trait of the way we can define
0:42frequency and the period because often
0:45it is kind of tricky to see how many
0:48waves will pass by in a certain second
0:51because well if it's say 2.4 well that's
0:54hard to estimate whereas the period
0:58again if we're watching say this and
1:00seeing how long it takes to rise and
1:02then to rise fully again that's a little
1:06easier to measure so often we will
1:07measure the period of a wave now what we
1:11find is frequency and period and period
1:14by the way is represented by the letter
1:15T which I know is a little weird but it
1:18just is period is represented by T and
1:21what we find is frequency and period are
1:23inversely related to each other so if
1:27you have one you can solve for the other
1:29using this formula it's take the number
1:321 and divide the value by it so if you
1:36have the period to take 1 divided by
1:37period that gives you frequency or vice
1:39versa and when you think about it that
1:42kind of makes sense frequency how many
1:44waves per second period how many or how
1:47long for a wave but let's also
1:50demonstrate this is an example so we're
1:54looking at someone out in a boat and
1:56they are really into measuring the waves
2:02and so they look and they see it takes
2:05five waves about 10 seconds to pass
2:08their boat or assuming they're pretty
2:11good with their measurements
2:12now from that information we want to
2:14figure out the frequency of the waves
2:15and appeared in waves so let's start off
2:18with the frequency and if we think back
2:20to that previous formula we had it was
2:23really the cycles per time or the number
2:26of waves per time so in this case we've
2:29got five waves five cycles in ten
2:32seconds so when what we do is 5/10 or
2:360.5 which romera that's measured in
2:40hertz so this tells us that the
2:43frequency of 0.5 Hertz half a wave
2:47passes in a second so now even a full
2:50wave passes in one second how are we
2:52gonna get the period well we know it's
2:56the it would be one over frequency so
2:58basically we take this formula and flip
3:00it period is time over number of cycles
3:04and we still have the same numbers ten
3:07seconds over five waves so what we get
3:10is two seconds so it takes a wave two
3:14seconds to pass by but the thing is I
3:18didn't need to do all this over here I
3:19just did it to show you how it works out
3:22but if I had taken one divided by
3:25frequency one divided by 0.5 is two and
3:28if you don't believe me check it on your
3:29calculator or if you think 1 divided by
3:32the period 1/2 1/2 is also equal to 5 or
3:370.5 sorry so these are inversely related
3:39if you have one you get the other and
3:42again let's think about this for a sec
3:44frequency how many waves per second half
3:47a wave per second and we're told a
3:50period is how long for one wave
3:52well if half a wave takes a second a
3:56full wave is gonna take two seconds kind
3:59of all makes sense as it should so these
4:02two ideas do heavily relate to each
4:04other but now that we've defined enough
4:06terms we can actually look at the
4:08velocity of a wave as well velocity
4:11velocity is how fast something is
4:15traveling and the direction it's
4:16traveling in so velocity is the speed
4:19and direction and what we have to do is
4:23define lambda lambda is our wave length
4:26which remember is the length of a wave
4:29so our velocity is defined as the
4:32frequency times the wavelength inversely
4:37we I mean conversely sorry we could
4:39think of this replace this F with the
4:41period one over period and then what
4:43we'd have is distance over time the
4:45distance is the wave length the length
4:47of a wave and a period time per one wave
4:50so if we replace this F with one over T
4:53we could think of this in a more logical
4:55way for some people but you can also
4:57just think of s lambda frequency times
5:00the wavelength now we have to think
5:03about there's a little more detail
5:04because what we're talking about
5:06especially in our case is we're looking
5:08at the velocity of a certain type of
5:10wave that is electromagnetic radiation
5:13light and the various forms of like
5:15electromagnetic radiation we're used to
5:17visible light or we see but it's all
5:21electromagnetic radiation and remember
5:23we said that that travels at a constant
5:25speed of approximately 300 million
5:28meters per second and so I'm bringing in
5:30this previous image we looked at because
5:32if the velocity is constant velocity
5:36can't change well what happens if you
5:38change the frequency the only thing that
5:42can change to make this still a constant
5:43is the wavelength or vice versa so when
5:47you're defining a wave if you define the
5:49frequency that also defines the
5:52wavelength and we can see that here for
5:55this frequency we saw it's 1 Hertz
5:58well the wavelength is quite big as we
6:03get a larger frequency the wavelength is
6:06getting smaller look much smaller waves
6:10in that same second to make it more
6:13frequent which makes some sense
6:14I mean smaller waves are going to be
6:17able to be more frequent will pass ba
6:19will be able to pass by quicker so as
6:23frequency increases wavelength decreases
6:25or vice versa you can define a wave
6:28either way in terms of its frequency or
6:30wavelength because the velocity is a
6:33fixed quantity so when we talk about the
6:36different types of waves we're gonna see
6:38we use the idea of the frequency of that
6:40wave or the wavelength interchangeably