1
00:00:01,371 --> 00:00:03,589
- Double slits are cool because they show

2
00:00:03,589 --> 00:00:06,373
definitively that light can have

3
00:00:06,373 --> 00:00:08,725
wave like interference patterns

4
00:00:08,725 --> 00:00:11,318
and if you shine a
green laser through here

5
00:00:11,318 --> 00:00:13,337
what you'd see on the screen

6
00:00:13,337 --> 00:00:15,125
would be something like this.

7
00:00:15,125 --> 00:00:16,693
You'd have these bright spots

8
00:00:16,693 --> 00:00:18,816
but they'd kind of blend in to dark spots

9
00:00:18,816 --> 00:00:20,709
which blend into bright spots

10
00:00:20,709 --> 00:00:22,823
which is why when we draw a graphical

11
00:00:22,823 --> 00:00:25,917
representation of this, it kind of

12
00:00:25,917 --> 00:00:28,462
looks like this, where these spots

13
00:00:28,462 --> 00:00:30,590
are blending into each other

14
00:00:30,590 --> 00:00:33,293
which is cool but it also kind of sucks

15
00:00:33,293 --> 00:00:35,600
because if you were to go actually

16
00:00:35,600 --> 00:00:37,346
try to do this experiment, you'd want to

17
00:00:37,346 --> 00:00:39,421
measure some angles and that means

18
00:00:39,421 --> 00:00:40,909
that you'd have to measure some distances.

19
00:00:40,909 --> 00:00:43,613
I'd measure distance from
the screen to the wall.

20
00:00:43,613 --> 00:00:45,390
That would give me this
side of the triangle

21
00:00:45,390 --> 00:00:47,857
and then I would also
probably want to measure

22
00:00:47,857 --> 00:00:50,304
distance between two of these

23
00:00:50,304 --> 00:00:52,573
bright spots because that's what I can see

24
00:00:52,573 --> 00:00:55,504
but because they're smudgy, it's like

25
00:00:55,504 --> 00:00:57,246
is that the center?

26
00:00:57,246 --> 00:00:58,533
Is this the center?

27
00:00:58,533 --> 00:01:00,389
Sometimes the lights not so strong

28
00:01:00,389 --> 00:01:02,710
and it's hard to tell and what's worse

29
00:01:02,710 --> 00:01:04,503
is these kind of die off

30
00:01:04,503 --> 00:01:05,762
and so there's another problem.

31
00:01:05,762 --> 00:01:06,763
These die off pretty quick.

32
00:01:06,763 --> 00:01:08,850
Sometimes you're lucky to even see

33
00:01:08,850 --> 00:01:11,255
the 5th or 6th bright spot down the line.

34
00:01:11,255 --> 00:01:13,621
So, my question is, is there a better way?

35
00:01:13,621 --> 00:01:16,390
Is there a way to make these spots

36
00:01:16,390 --> 00:01:19,653
more defined and so you
can see more of them

37
00:01:19,653 --> 00:01:20,982
so they're brighter and the answer

38
00:01:20,982 --> 00:01:23,142
is definitively yes and we figured out

39
00:01:23,142 --> 00:01:25,816
how to do it and the way you do it,

40
00:01:25,816 --> 00:01:27,989
is you just make more holes.

41
00:01:27,989 --> 00:01:30,597
So you come over to here and if these

42
00:01:30,597 --> 00:01:32,639
are spaced to distance
D, I'm just going to

43
00:01:32,639 --> 00:01:36,238
make another hole at distance D

44
00:01:36,238 --> 00:01:37,535
and then I'm going to make another hole

45
00:01:37,535 --> 00:01:39,380
at distance D and then I'm going to make

46
00:01:39,380 --> 00:01:40,510
another hole at distance D.

47
00:01:40,510 --> 00:01:43,229
I'm going to make 1,000's of these holes

48
00:01:43,229 --> 00:01:45,360
extremely close together
as long as they're

49
00:01:45,360 --> 00:01:47,389
all at distance D apart,

50
00:01:47,389 --> 00:01:49,725
something magical happens.

51
00:01:49,725 --> 00:01:52,863
So if these are all D
apart, what happens is

52
00:01:52,863 --> 00:01:54,206
on the wall over here, instead of getting

53
00:01:54,206 --> 00:01:58,415
this smudgy pattern,
you'll get, you really will

54
00:01:58,415 --> 00:02:02,780
just get, a dot right
there and then darkness

55
00:02:02,780 --> 00:02:04,773
and then another dot and then darkness

56
00:02:04,773 --> 00:02:07,662
and another dot and you'll see this

57
00:02:07,662 --> 00:02:11,540
continue out much further
than you could previously.

58
00:02:11,540 --> 00:02:12,639
Why?

59
00:02:12,639 --> 00:02:14,382
Well let's talk about why.

60
00:02:14,382 --> 00:02:15,375
So let's talk about this.

61
00:02:15,375 --> 00:02:16,525
How can you see this pattern

62
00:02:16,525 --> 00:02:18,239
over here like this?

63
00:02:18,239 --> 00:02:21,679
So, the 1st wave from this 1st hole.

64
00:02:21,679 --> 00:02:22,863
Let's imagine this 1st wave

65
00:02:22,863 --> 00:02:24,958
from this 1st hole, it's going to travel

66
00:02:24,958 --> 00:02:26,910
a certain distance to the wall.

67
00:02:26,910 --> 00:02:28,262
Let's say we look at a point over here

68
00:02:28,262 --> 00:02:29,766
where it is constructive.

69
00:02:29,766 --> 00:02:30,983
Let's say we just had these two holes

70
00:02:30,983 --> 00:02:32,678
to start off with right?

71
00:02:32,678 --> 00:02:34,482
Two holes, double slit, disregard

72
00:02:34,482 --> 00:02:36,940
all this stuff for a minute.

73
00:02:36,940 --> 00:02:37,980
Two waves coming in from two holes

74
00:02:37,980 --> 00:02:39,700
get over to here.

75
00:02:39,700 --> 00:02:40,176
Let's say this is a bright spot

76
00:02:40,176 --> 00:02:41,376
but say it's the bright spot

77
00:02:41,376 --> 00:02:46,376
that corresponds to, delta
x equals one wavelength.

78
00:02:47,561 --> 00:02:49,375
In other words, this would be

79
00:02:49,375 --> 00:02:52,551
the constructive point where the 2nd wave

80
00:02:52,551 --> 00:02:55,846
from the 2nd hole travels
one wavelength further

81
00:02:55,846 --> 00:02:57,894
than the wave from the 1st hole

82
00:02:57,894 --> 00:03:00,246
and again what that means is if I were

83
00:03:00,246 --> 00:03:02,166
to carefully draw a line from here

84
00:03:02,166 --> 00:03:04,331
at a right angle right there, that means

85
00:03:04,331 --> 00:03:06,183
that this wave, from the 2nd hole,

86
00:03:06,183 --> 00:03:09,654
this is the extra part so that would be

87
00:03:09,654 --> 00:03:11,670
one extra wavelength and because this

88
00:03:11,670 --> 00:03:14,503
2nd wave is traveling
one extra wavelength,

89
00:03:14,503 --> 00:03:15,686
it's going to be constructive

90
00:03:15,686 --> 00:03:17,820
because if I draw my
wave, they're going to

91
00:03:17,820 --> 00:03:21,477
match up perfectly there
so if I draw my wave,

92
00:03:21,477 --> 00:03:25,113
let's say the wave from the 1st hole

93
00:03:25,113 --> 00:03:26,927
happened to be at this

94
00:03:26,927 --> 00:03:29,678
particular point on it's cycle.

95
00:03:29,678 --> 00:03:30,958
It doesn't have to be but let's

96
00:03:30,958 --> 00:03:32,222
just say it was there.

97
00:03:32,222 --> 00:03:33,872
The 1st wave hit that point

98
00:03:33,872 --> 00:03:35,903
at this point in it's cycle.

99
00:03:35,903 --> 00:03:38,380
Well, the 2nd wave, since it's traveling

100
00:03:38,380 --> 00:03:40,704
one wavelength further, is going to hit

101
00:03:40,704 --> 00:03:44,544
at this point in it's
cycle so it would be here.

102
00:03:44,544 --> 00:03:45,598
Now these are both hitting there

103
00:03:45,598 --> 00:03:47,774
at the same point so the 1st wave

104
00:03:47,774 --> 00:03:49,662
gets there hitting right here.

105
00:03:49,662 --> 00:03:51,700
The 2nd wave get there hitting right here.

106
00:03:51,700 --> 00:03:54,143
These overlap because these are

107
00:03:54,143 --> 00:03:55,839
two different waves
overlapping at this point

108
00:03:55,839 --> 00:03:57,119
it's going to be constructive because

109
00:03:57,119 --> 00:03:59,175
if a peak matches a peak, constructive.

110
00:03:59,175 --> 00:04:02,503
If a valley matches a
valley, constructive.

111
00:04:02,503 --> 00:04:03,958
How about the 3rd hole?

112
00:04:03,958 --> 00:04:05,894
Here's where it gets interesting.

113
00:04:05,894 --> 00:04:08,614
This 3rd hole, the wave from the 3rd hole

114
00:04:08,614 --> 00:04:11,770
is going to have to travel
this far to get there.

115
00:04:11,770 --> 00:04:13,959
Well, let's see.

116
00:04:13,959 --> 00:04:16,214
From the 2nd, how much
further does it travel

117
00:04:16,214 --> 00:04:18,334
compared to the 2nd hole?

118
00:04:18,334 --> 00:04:19,254
I'm going to do the same game

119
00:04:19,255 --> 00:04:20,646
I did just a minute ago.

120
00:04:20,646 --> 00:04:23,369
It travels this much further which again,

121
00:04:23,369 --> 00:04:26,646
since these are the same angle,

122
00:04:26,646 --> 00:04:28,505
this is going to be
the same distance here.

123
00:04:28,505 --> 00:04:31,995
This is a also going to be one wavelength.

124
00:04:32,223 --> 00:04:33,487
Remember we derived at this.

125
00:04:33,487 --> 00:04:37,807
That d sin theta, is the
path length difference

126
00:04:37,807 --> 00:04:39,182
and the theta is the same for all of them

127
00:04:39,182 --> 00:04:40,613
and so I could just look at these two

128
00:04:40,613 --> 00:04:42,382
consider these two as the double slit.

129
00:04:42,382 --> 00:04:44,218
This one travels one wavelength

130
00:04:44,218 --> 00:04:46,259
further than the 2nd.

131
00:04:46,259 --> 00:04:48,860
How much further does
it travel than the 1st?

132
00:04:48,860 --> 00:04:50,908
Well I'm going to continue that line down.

133
00:04:50,908 --> 00:04:53,213
It's just going to travel two wavelengths

134
00:04:53,213 --> 00:04:55,577
farther than the 1st.

135
00:04:55,577 --> 00:04:57,929
So the wave coming out of the 3rd hole

136
00:04:57,929 --> 00:04:59,373
travels two wavelengths farther

137
00:04:59,373 --> 00:05:01,610
than the 1st hole.

138
00:05:01,610 --> 00:05:02,825
The 2nd and the 3rd are going to be

139
00:05:02,825 --> 00:05:04,697
constructive because
they're one wavelength apart

140
00:05:04,697 --> 00:05:06,506
and the 3rd and the 1st are going to be

141
00:05:06,506 --> 00:05:08,939
constructive because they're
two wavelengths apart.

142
00:05:08,939 --> 00:05:10,681
That's okay, two
wavelengths doesn't matter.

143
00:05:10,681 --> 00:05:13,497
Look at, if the 1st wavelength hits here,

144
00:05:13,497 --> 00:05:15,225
the wave from the 2nd one hits here,

145
00:05:15,225 --> 00:05:16,986
the wave from the 3rd hole travels

146
00:05:16,986 --> 00:05:19,152
two extra wavelengths.

147
00:05:19,152 --> 00:05:21,353
When that 3rd wave gets to this point

148
00:05:21,353 --> 00:05:23,652
it will be at this point on it's cycle

149
00:05:23,652 --> 00:05:25,470
there all going to by overlapping

150
00:05:25,470 --> 00:05:27,500
at the same point on their cycle,

151
00:05:27,500 --> 00:05:28,142
it's going to be constructive.

152
00:05:28,142 --> 00:05:29,789
You can keep doing this.

153
00:05:29,789 --> 00:05:31,212
You can come down to this hole

154
00:05:31,212 --> 00:05:34,844
and it will also be
constructive interference.

155
00:05:34,844 --> 00:05:36,516
In other words, this 4th hole travels

156
00:05:36,516 --> 00:05:38,740
one wavelength further than the 3rd,

157
00:05:38,740 --> 00:05:42,116
two wavelengths further than the 2nd hole,

158
00:05:42,116 --> 00:05:44,910
three wavelengths
further than the 1st hole

159
00:05:44,910 --> 00:05:46,283
but there still all going
to overlap perfectly.

160
00:05:46,283 --> 00:05:48,398
You will get an extremely bright spot here

161
00:05:48,398 --> 00:05:50,809
because you have even
more light overlapping

162
00:05:50,809 --> 00:05:54,121
and it's all perfectly
constructive bright spot.

163
00:05:54,121 --> 00:05:56,696
Now here's where it gets strange

164
00:05:56,696 --> 00:05:57,832
so you've got to be careful.

165
00:05:57,832 --> 00:05:59,800
This is the part of the explanation

166
00:05:59,800 --> 00:06:00,969
I hated as a student.

167
00:06:00,969 --> 00:06:03,116
I thought this made no sense whatsoever

168
00:06:03,116 --> 00:06:05,768
so pay close attention at this point.

169
00:06:05,768 --> 00:06:07,320
Here's the weird thing.

170
00:06:07,320 --> 00:06:10,953
If you deviate slightly
from this constructive point

171
00:06:10,953 --> 00:06:13,768
if I go up just a little bit over here

172
00:06:13,768 --> 00:06:15,591
to some point right here.

173
00:06:15,591 --> 00:06:18,105
Let's just see what happens.

174
00:06:18,105 --> 00:06:20,953
This wave from the 1st hole would travel

175
00:06:20,953 --> 00:06:24,139
that far to get there okay and the wave

176
00:06:24,139 --> 00:06:28,107
from the 2nd hole travels
that far to get there.

177
00:06:28,107 --> 00:06:29,586
The path length difference isn't

178
00:06:29,586 --> 00:06:31,178
going to be one wavelength.

179
00:06:31,178 --> 00:06:34,800
Let's say the path length difference

180
00:06:34,800 --> 00:06:39,260
happens to be 1.1 wavelengths.

181
00:06:39,260 --> 00:06:42,104
So let's say the wave from the 2nd hole

182
00:06:42,104 --> 00:06:44,590
happened to be traveling
not one wavelength

183
00:06:44,590 --> 00:06:46,754
further anymore, it's not traveling

184
00:06:46,754 --> 00:06:48,530
one wavelength further, it's going to be

185
00:06:48,530 --> 00:06:53,490
traveling 1 and .1 wavelengths further.

186
00:06:53,490 --> 00:06:55,986
So if the 1st wave happened to hit

187
00:06:55,986 --> 00:06:59,675
at this point, the 2nd
wave would be hitting

188
00:06:59,675 --> 00:07:03,690
at not exactly one wavelength but 1.1.

189
00:07:03,690 --> 00:07:04,850
Now if I keep drawing them over here

190
00:07:04,850 --> 00:07:06,232
I'm going to run out of room.

191
00:07:06,232 --> 00:07:07,795
So since they're all the same points

192
00:07:07,795 --> 00:07:09,187
on the cycle, the cycle's

193
00:07:09,187 --> 00:07:11,418
the same over and over.

194
00:07:11,418 --> 00:07:15,400
One wavelength .1, I'm just going

195
00:07:15,400 --> 00:07:16,330
to draw that right here.

196
00:07:16,330 --> 00:07:17,610
So the 2nd wave would hit right there.

197
00:07:17,610 --> 00:07:19,322
Those would overlap you know,

198
00:07:19,322 --> 00:07:21,690
that's partially constructive.

199
00:07:21,690 --> 00:07:24,170
I mean, just looking at these, you'd think

200
00:07:24,170 --> 00:07:26,971
you'd get a bright spot
but if you keep going

201
00:07:26,971 --> 00:07:28,670
let's see what happens.

202
00:07:28,670 --> 00:07:30,890
The wave from the 3rd hole

203
00:07:30,890 --> 00:07:33,300
also travels to get there.

204
00:07:33,300 --> 00:07:34,986
How much farther does it travel?

205
00:07:34,986 --> 00:07:38,522
Well, it travels 1.1 wavelengths further

206
00:07:38,522 --> 00:07:42,814
than the 2nd hole but it travels 1.1

207
00:07:42,814 --> 00:07:45,557
plus 1.1 wavelengths further than

208
00:07:45,557 --> 00:07:48,691
the 1st hole, so it
travels 2.2 wavelengths

209
00:07:48,691 --> 00:07:51,347
further than the 1st hole.

210
00:07:51,347 --> 00:07:54,555
So to make this clear,
let me just be clear here.

211
00:07:54,555 --> 00:07:56,604
So if I do the same trick right?

212
00:07:56,604 --> 00:07:59,323
I draw this down to a right angle.

213
00:07:59,323 --> 00:08:00,651
It's a little farther so I have to draw

214
00:08:00,651 --> 00:08:02,409
the line a little further out.

215
00:08:02,409 --> 00:08:04,650
1st wave travels one wavelength

216
00:08:04,650 --> 00:08:07,146
and .1 wavelengths further.

217
00:08:07,146 --> 00:08:08,954
How about this one here?

218
00:08:08,954 --> 00:08:12,750
Well, the wave from the 3rd hole

219
00:08:12,750 --> 00:08:15,827
travels one wavelength .1,

220
00:08:15,827 --> 00:08:17,731
1.1 wavelengths further than the wave

221
00:08:17,731 --> 00:08:22,180
from the 2nd hole but it's going to travel

222
00:08:22,275 --> 00:08:27,275
1.1 plus 1.1 wavelengths
further than the 1st hole.

223
00:08:27,407 --> 00:08:29,518
So it's going to be 2.2.

224
00:08:29,518 --> 00:08:33,261
This wave from the 3rd
hole is going to travel

225
00:08:33,261 --> 00:08:36,413
2.2 wavelengths further than the wave

226
00:08:36,414 --> 00:08:37,672
from the 1st hole.

227
00:08:37,672 --> 00:08:39,336
Where would that be?

228
00:08:39,336 --> 00:08:42,198
So, one, two wavelengths .2 would be

229
00:08:42,198 --> 00:08:44,408
even further down the line here.

230
00:08:44,408 --> 00:08:46,111
Not completely at the bottom but

231
00:08:46,111 --> 00:08:47,589
further down the line.

232
00:08:47,589 --> 00:08:51,931
So it would be, at this point over here

233
00:08:52,337 --> 00:08:54,127
and you can keep doing this and let's

234
00:08:54,127 --> 00:08:55,167
just see what happens.

235
00:08:55,167 --> 00:08:56,687
Let's just do a couple more.

236
00:08:56,687 --> 00:08:59,182
This wavelength has to travel this far.

237
00:08:59,182 --> 00:09:00,861
Now you can probably see the pattern.

238
00:09:00,861 --> 00:09:04,904
This wave through the
4th hole has to travel

239
00:09:04,904 --> 00:09:09,671
1.1 wavelengths further
than the wave from the 3rd.

240
00:09:09,671 --> 00:09:11,751
It travels 2.2 wavelengths further

241
00:09:11,751 --> 00:09:13,687
than the wave from the 2nd.

242
00:09:13,687 --> 00:09:16,443
It travels 3.3 wavelengths further

243
00:09:16,443 --> 00:09:18,247
than the wave from the 1st.

244
00:09:18,247 --> 00:09:20,727
So if I compare this wavelength

245
00:09:20,727 --> 00:09:22,895
where this wave is on it's cycle

246
00:09:22,895 --> 00:09:25,895
compared to the 1st, then I'm going

247
00:09:25,895 --> 00:09:27,831
to be 3.3 wavelengths.

248
00:09:27,831 --> 00:09:29,718
So if I go one, two, three and then

249
00:09:29,718 --> 00:09:33,478
.3 would be down here somewhere.

250
00:09:33,478 --> 00:09:35,102
I'd get one that's at .4, I'd get one

251
00:09:35,102 --> 00:09:37,241
that at .5, I'd get one that's at .6,

252
00:09:37,241 --> 00:09:41,751
one at .7, one at .8, one at .9,

253
00:09:41,751 --> 00:09:44,255
one at, well .10, which is back to

254
00:09:44,255 --> 00:09:47,295
a whole wavelength again,

255
00:09:47,295 --> 00:09:48,760
a whole wavelengths difference.

256
00:09:48,760 --> 00:09:49,679
What is this going to be?

257
00:09:49,679 --> 00:09:50,942
If all of these waves are overlapping

258
00:09:50,942 --> 00:09:52,734
like this at this point that's only

259
00:09:52,734 --> 00:09:56,255
slightly deviated from this other point.

260
00:09:56,255 --> 00:09:58,394
What am I going to see there?

261
00:09:58,394 --> 00:10:01,312
Well, look at you could pair these off.

262
00:10:01,312 --> 00:10:03,103
This point and this point are going

263
00:10:03,103 --> 00:10:06,339
to interfere completely destructively.

264
00:10:06,339 --> 00:10:08,579
One's at a maximum, one's at a minimum,

265
00:10:08,579 --> 00:10:10,531
one's at the peak, one's at the trough,

266
00:10:10,531 --> 00:10:13,795
so you get zero and you
can keep pairing these off.

267
00:10:13,795 --> 00:10:16,692
This one here and that one there

268
00:10:16,692 --> 00:10:20,662
completely annihilate each
other, completely destructive.

269
00:10:20,982 --> 00:10:23,944
You keep doing this, this
one here and this one there

270
00:10:23,989 --> 00:10:25,886
completely destructive and you can

271
00:10:25,886 --> 00:10:27,588
keep finding points that completely

272
00:10:27,588 --> 00:10:30,350
destruct each other and that means

273
00:10:30,350 --> 00:10:32,504
that you're going to get no intensity

274
00:10:32,504 --> 00:10:35,126
at all, even slightly off from

275
00:10:35,126 --> 00:10:37,142
this magical point, this magical

276
00:10:37,142 --> 00:10:40,346
integer wavelength point.

277
00:10:40,346 --> 00:10:43,174
What that means is
instead of getting a blur,

278
00:10:43,174 --> 00:10:44,903
instead of getting this, instead of

279
00:10:44,903 --> 00:10:47,414
getting this smudgy pattern.

280
00:10:47,414 --> 00:10:49,409
You're going to get, at the bright spots,

281
00:10:49,409 --> 00:10:52,442
a dot and then another dot at

282
00:10:52,442 --> 00:10:53,967
the bright spot and in between

283
00:10:53,967 --> 00:10:57,343
these bright spots, you will get darkness.

284
00:10:57,343 --> 00:10:59,477
Which is great, it's great because

285
00:10:59,477 --> 00:11:01,911
it's easier to measure
so that's one good thing

286
00:11:01,911 --> 00:11:04,296
and it relies on this fact that

287
00:11:04,296 --> 00:11:05,735
if you've got multiple holes,

288
00:11:05,735 --> 00:11:08,123
100's even 1,000's of holes,

289
00:11:08,123 --> 00:11:10,519
if you go off even slightly

290
00:11:10,519 --> 00:11:12,313
because this isn't going to always

291
00:11:12,313 --> 00:11:15,663
match up perfectly, you
keep going down the line

292
00:11:15,663 --> 00:11:17,153
one of these are going to interfere

293
00:11:17,153 --> 00:11:18,684
constructively with
another and you can keep

294
00:11:18,684 --> 00:11:20,172
pairing these off and then the 2nd

295
00:11:20,172 --> 00:11:22,006
would interfere destructively with

296
00:11:22,006 --> 00:11:24,133
a different one and the 3rd with

297
00:11:24,133 --> 00:11:25,780
a different one and the 4th with

298
00:11:25,780 --> 00:11:26,523
a different one and they're always

299
00:11:26,523 --> 00:11:29,563
going to match up so
you can destroy them all

300
00:11:29,563 --> 00:11:31,132
and you get destructive point in between.

301
00:11:31,132 --> 00:11:32,740
That's why it's dark.

302
00:11:32,740 --> 00:11:34,276
Now at first I didn't like that argument.

303
00:11:34,276 --> 00:11:37,060
I had to go back over that and make sure

304
00:11:37,060 --> 00:11:38,180
that made sense to me.

305
00:11:38,180 --> 00:11:40,164
If it doesn't make sense to you

306
00:11:40,164 --> 00:11:42,061
go through, you've got to hammer it out,

307
00:11:42,061 --> 00:11:43,809
try it out, draw it out.

308
00:11:43,809 --> 00:11:45,908
I've tried my best to explain it here

309
00:11:45,908 --> 00:11:48,228
but I have to admit, it's a difficult

310
00:11:48,228 --> 00:11:52,012
one to comprehend but that's the idea.

311
00:11:52,012 --> 00:11:54,300
So this is great actually.

312
00:11:54,300 --> 00:11:55,964
These multiple holes here are giving us

313
00:11:55,964 --> 00:11:57,909
points that are points that are

314
00:11:57,909 --> 00:12:02,162
spaced out and clearly delineated.

315
00:12:02,162 --> 00:12:03,952
Now I can clearly see, okay if I wanted

316
00:12:03,952 --> 00:12:06,044
to measure this distance
I don't have to guess.

317
00:12:06,044 --> 00:12:07,708
There's no guesswork, one dot's here

318
00:12:07,708 --> 00:12:11,733
one dot's there, bright,
bright, no smudginess.

319
00:12:11,733 --> 00:12:13,276
That's one great thing about this.

320
00:12:13,276 --> 00:12:15,822
The other great thing is
because I've got more holes,

321
00:12:16,084 --> 00:12:17,932
this brightness lasts longer.

322
00:12:17,932 --> 00:12:20,408
These bright spots will
keep going further,

323
00:12:20,408 --> 00:12:23,144
I can see this travel further

324
00:12:23,144 --> 00:12:25,508
down the line then I
would with the smudginess

325
00:12:25,508 --> 00:12:26,778
because I've got multiple holes

326
00:12:26,778 --> 00:12:27,855
interacting and I'll have more

327
00:12:27,855 --> 00:12:29,473
intense dots, it's brighter.

328
00:12:29,473 --> 00:12:31,360
The dots are more delineated

329
00:12:31,360 --> 00:12:32,552
and they're typically brighter,

330
00:12:32,552 --> 00:12:33,971
they're more intense.

331
00:12:33,971 --> 00:12:35,200
So we give this a special name

332
00:12:35,200 --> 00:12:38,191
because this is so useful,
these multiple holes

333
00:12:38,191 --> 00:12:39,615
instead of a double slit,

334
00:12:39,615 --> 00:12:41,394
these multiple holes are more useful.

335
00:12:41,394 --> 00:12:44,479
We call this a diffraction grading.

336
00:12:44,479 --> 00:12:48,905
So this is a diffraction grading

337
00:12:48,905 --> 00:12:52,432
and it's more useful than a double slit

338
00:12:52,432 --> 00:12:54,451
in many ways because it gives you

339
00:12:54,451 --> 00:12:56,838
clearly delineated dots and it let's

340
00:12:56,838 --> 00:12:58,547
you see them more clearly.

341
00:12:58,547 --> 00:13:00,764
How many holes are there
in a diffraction grading?

342
00:13:00,764 --> 00:13:02,699
Well typically, these are rated

343
00:13:02,699 --> 00:13:05,305
in lines per centimeter.

344
00:13:05,305 --> 00:13:07,006
In other words, if you
took a diffraction grading

345
00:13:07,006 --> 00:13:09,387
and you asked, how many holes are there?

346
00:13:09,387 --> 00:13:10,411
The lines or the holes.

347
00:13:10,411 --> 00:13:13,404
How many holes are there per centimeter?

348
00:13:13,404 --> 00:13:16,123
Well, the lines could be the blocked parts

349
00:13:16,123 --> 00:13:17,579
and the holes could be the parts

350
00:13:17,579 --> 00:13:18,891
where there's no blocked part

351
00:13:18,891 --> 00:13:20,266
but regardless, there will be as many

352
00:13:20,266 --> 00:13:21,546
lines as there are holes.

353
00:13:21,546 --> 00:13:23,881
How many holes are there per centimeter?

354
00:13:23,881 --> 00:13:26,300
There's typically 1,000's of lines

355
00:13:26,300 --> 00:13:28,091
per centimeter in a diffraction grading.

356
00:13:28,091 --> 00:13:32,157
That's how small these
are spaced together.

357
00:13:32,157 --> 00:13:33,908
You might worry though, isn't the math

358
00:13:33,908 --> 00:13:35,843
going to be complicated here?

359
00:13:35,843 --> 00:13:37,662
We've got all these holes here.

360
00:13:37,662 --> 00:13:38,852
It turns out no.

361
00:13:38,852 --> 00:13:40,082
This is the best part.

362
00:13:40,082 --> 00:13:41,378
By far the best part.

363
00:13:41,378 --> 00:13:42,656
This relationship still holds.

364
00:13:42,656 --> 00:13:45,483
All that math we did, still fine

365
00:13:45,483 --> 00:13:47,562
and because these all line up

366
00:13:47,562 --> 00:13:50,282
for the good points, the magic points.

367
00:13:50,282 --> 00:13:52,827
These points where they all overlap

368
00:13:52,827 --> 00:13:55,307
in such a way that they're
one wavelength further.

369
00:13:55,307 --> 00:13:57,555
Those line up perfectly so that

370
00:13:57,555 --> 00:13:59,747
no matter how many holes
you go down the line,

371
00:13:59,747 --> 00:14:03,091
they're all perfectly
constructively interfering

372
00:14:03,091 --> 00:14:04,393
and so you get this equation.

373
00:14:04,393 --> 00:14:06,914
It's the same equation that we had before

374
00:14:06,914 --> 00:14:11,914
where d sin theta equals m lambda

375
00:14:13,833 --> 00:14:17,155
gives you the constructive points

376
00:14:17,155 --> 00:14:19,426
for a diffraction grading interference

377
00:14:19,426 --> 00:00:00,000
pattern on the wall.