Full transcript
0:06there's one last idea we have to talk
0:09about this week and that is the Doppler
0:11effect basically opto now we've more or
0:14less treated everything as if it's kind
0:16of just standing still we're looking at
0:18a star we get the light from it we study
0:21it great but the truth of the matter is
0:25it's a little more complicated because
0:26that star is moving relative to us I
0:29mean we are moving that star is probably
0:32moving as well and so there's relative
0:34motion either away or towards us in most
0:37cases between us and this object we're
0:40looking at whether it be a star or
0:41something else and that actually
0:43complicates what we see because when you
0:47have this kind of relative motion it
0:49does actually impact what we see and
0:51this diagram actually demonstrates it
0:54but I want to start off with a more
0:55familiar example because this is
0:57something you've probably experienced
0:59many many times in your life and that is
1:03the example of a car driving by or some
1:08kind of car with an siren say an
1:12ambulance firetruck police car any of
1:14those obvious going by you've probably
1:16experienced the Doppler effect with the
1:18sound in tomates because as that siren
1:22comes at you well you notice is it's a
1:25very high frequency high pitch and then
1:28as it passes you it changes and you're
1:31actually gonna hear it differently and
1:33it's even more different if you're
1:35standing beside the ant let's say if
1:38you're in your car at a light and it
1:40just starts up that noise is different
1:42than if it's coming towards you versus
1:44if it's actually passing by you and
1:48that's the Doppler effect in action it's
1:51actually the motion between you and the
1:54object emitting those sound waves is
1:56changing how you hear it
1:58well the same thing happens with
2:00electromagnetic waves as well or light
2:04what we see is impacted by the motion
2:07again up till now we've basically
2:09assuming there's some objects spitting
2:11out light
2:12and we all would see the same thing no
2:16matter where the observer is at it's the
2:19sphere around this object or in this
2:22case the two-dimensional surface we're
2:24looking at the circle around this object
2:25we'd all see the same thing it spits out
2:28regular waves we think each of these
2:30circles is a wave and great or a photon
2:34and we see the same thing no matter
2:36where we are problem is again likely a
2:40lot of these things are moving relative
2:41to us and that impacts what we see and
2:45we can see that from this image here if
2:47we have a source spitting out light
2:51spitting out electromagnetic radiation
2:53at regular photons of light as it moves
2:58what we're going to see is the those
3:00lights those waves bunch up if you will
3:04in the direction it's moving because
3:07well first it spit out one wave here
3:10then the next one it had moved in that
3:13time and so we end up finding it's a
3:15little shifted and the next one a little
3:17shifted and the next one little shifted
3:19so we actually get the waves kind of
3:21bunching up in the direction of motion
3:23and spreading out in the direction away
3:25from motion so if I'm observer a I'm not
3:31seeing what this observer would have saw
3:33what I'm actually seeing is this waves
3:36shifted to a higher frequency I would
3:40actually see a higher frequency of
3:43electromagnetic radiation then what this
3:45object is actually spitting out whereas
3:48if I'm observers see where the objects
3:50are moving away from me I'm gonna see a
3:52lower frequency a longer wavelength
3:56we're actually gonna observe that a
3:59longer wavelength than what this object
4:01is spitting out now if I'm observer B
4:06and the motion is perpendicular to me so
4:09not towards me not away just moving to
4:10the side then I'm not gonna actually
4:12notice a difference I will see the
4:15actual frequency that this source is
4:17spitting out it's only if it's mu has
4:19relative motion towards or away from me
4:22that it matters so if I'm in this
4:24position this position or anywhere not
4:26acting perpendicular anywhere along this
4:28angle I'm gonna see some combination of
4:30since it has some general motion towards
4:32me as well but the fact that if it's
4:35moving towards me or away from me will
4:38actually impact what I see now the
4:44terminology we use for this is if it's
4:47moving towards us and we get a higher
4:49frequency we call this a blue shift and
4:53this is because blue light has a higher
4:55frequency than red so if it's moving
4:57away from us and we get a lower
4:59frequency it's called a red shift and
5:01this applies even if we're not talking
5:03about visible light even if we're
5:04looking at different parts of the
5:05spectrum these terms cross over just
5:08because it's convenient for us also with
5:12noting we use the term blue shift even
5:14though technically violate s-- more
5:16shifted on higher frequency but blue
5:18shift when we're shifting towards higher
5:21frequency and the red shift when we're
5:25moving towards lower frequency or larger
5:32wavelengths of red shift and smaller
5:36wavelengths would be the blue shift so
5:39important terms to know and that's just
5:42because we got to keep in mind then that
5:44pollution is higher frequency than red
5:46blue shift means moving towards higher
5:48frequency smaller wavelength