1
00:00:00,090 --> 00:00:02,490
The following content is
provided under a Creative

2
00:00:02,490 --> 00:00:04,030
Commons license.

3
00:00:04,030 --> 00:00:06,840
Your support will help in
MIT OpenCourseWare continue

4
00:00:06,840 --> 00:00:10,690
to offer high quality
educational resources for free.

5
00:00:10,690 --> 00:00:13,320
To make a donation, or
view additional materials

6
00:00:13,320 --> 00:00:17,270
from hundreds of MIT courses,
visit MIT OpenCourseWare

7
00:00:17,270 --> 00:00:18,180
at ocw.mit.edu.

8
00:00:20,613 --> 00:00:22,530
SHAOUL EZEKIEL: In our
previous demonstrations

9
00:00:22,530 --> 00:00:26,580
in two-beam interferometry, we
showed some very good contrast

10
00:00:26,580 --> 00:00:30,090
in the fringes of the
interference path.

11
00:00:30,090 --> 00:00:31,700
In this demonstration,
we're going

12
00:00:31,700 --> 00:00:36,020
to show how easy it is to ruin
the contrast in the fringes.

13
00:00:36,020 --> 00:00:40,450
The set up we'll use is, again,
a Michaelson interferometer.

14
00:00:40,450 --> 00:00:41,620
And here it is.

15
00:00:41,620 --> 00:00:42,930
There's the laser.

16
00:00:42,930 --> 00:00:44,655
Here's the beam from the laser.

17
00:00:44,655 --> 00:00:47,820
Now reflect off
this mirror here.

18
00:00:47,820 --> 00:00:50,800
And then reflect it,
again, by this mirror here,

19
00:00:50,800 --> 00:00:53,300
into the Michaelson
interferometer.

20
00:00:53,300 --> 00:00:54,510
Here is one arm.

21
00:00:54,510 --> 00:00:57,240
Here's the other arm.

22
00:00:57,240 --> 00:00:59,610
The output of the
interferometer gets

23
00:00:59,610 --> 00:01:05,680
reflected off this mirror, into
this lens, onto the screen.

24
00:01:05,680 --> 00:01:09,090
So now let's take a
close look at the screen,

25
00:01:09,090 --> 00:01:11,270
and look at the fringes.

26
00:01:11,270 --> 00:01:16,710
Now as you can see, the
contrast between bright and dark

27
00:01:16,710 --> 00:01:18,990
is pretty good which, means
the dark is pretty dark,

28
00:01:18,990 --> 00:01:21,690
and the bright is as
bright as we can--

29
00:01:21,690 --> 00:01:22,440
as we can get it.

30
00:01:25,530 --> 00:01:28,200
Now I'm going to ruin the
contrast between the fringes

31
00:01:28,200 --> 00:01:34,490
by simply shaking
this mirror here.

32
00:01:34,490 --> 00:01:38,890
So you can see that the
contrast is washed out,

33
00:01:38,890 --> 00:01:42,280
due to the fact that
the fringes are moving.

34
00:01:42,280 --> 00:01:45,370
If I take my hand away,
the fringes come back.

35
00:01:45,370 --> 00:01:49,180
I can also tap the
beam splitter here.

36
00:01:49,180 --> 00:01:53,170
And, again, looks as if
there's no interference at all,

37
00:01:53,170 --> 00:01:56,110
because I'm shaking
one of the mounts.

38
00:01:56,110 --> 00:02:01,540
I could also tap the table.

39
00:02:01,540 --> 00:02:05,710
And, again, I can
wash the fringes out.

40
00:02:05,710 --> 00:02:08,979
So the conclusion
here is that, if you

41
00:02:08,979 --> 00:02:11,470
want to get good contrast
between the fringes,

42
00:02:11,470 --> 00:02:13,990
make sure that
the optical mounts

43
00:02:13,990 --> 00:02:16,330
within the interferometer
are stable.

44
00:02:16,330 --> 00:02:18,940
Because if they shake, then
the fringes would shake,

45
00:02:18,940 --> 00:02:21,580
and you get very
poor contrast or--

46
00:02:21,580 --> 00:02:23,830
you don't even see
any fringes at all.

47
00:02:23,830 --> 00:02:26,410
So, then, be careful
how you set up

48
00:02:26,410 --> 00:02:28,510
an interferometer
so that you can

49
00:02:28,510 --> 00:02:31,140
get the best contrast possible.