1 00:00:00,500 --> 00:00:06,032 [SQUEAKING] [RUSTLING] [CLICKING] 2 00:00:06,032 --> 00:00:07,490 VIVIAN SONG: In this video, we will 3 00:00:07,490 --> 00:00:09,680 be going over a Goodie Bag number two which is 4 00:00:09,680 --> 00:00:11,870 about electronic transitions. 5 00:00:11,870 --> 00:00:14,180 What you'll need are four LEDs-- 6 00:00:14,180 --> 00:00:16,480 white, blue, green, and red-- 7 00:00:16,480 --> 00:00:18,130 and one spectrometer. 8 00:00:18,130 --> 00:00:21,920 The objectives in this video are to understand photon absorption 9 00:00:21,920 --> 00:00:23,990 and emission, apply the Bohr Model, 10 00:00:23,990 --> 00:00:26,970 and use the spectrometer to see sample spectra. 11 00:00:26,970 --> 00:00:29,450 The conceptual questions you should keep in mind 12 00:00:29,450 --> 00:00:32,990 are, how do electrons transition to different energy levels? 13 00:00:32,990 --> 00:00:37,160 And why is spectroscopy a method of material characterization? 14 00:00:37,160 --> 00:00:38,780 As a reminder, the Bohr Model can 15 00:00:38,780 --> 00:00:41,750 be used to model these electronic transitions, 16 00:00:41,750 --> 00:00:45,030 assuming that there's only one electron in the atom. 17 00:00:45,030 --> 00:00:52,690 So right now, I'm going to draw my energy axis and three 18 00:00:52,690 --> 00:00:57,640 different energy levels that my electron can hop into. 19 00:00:57,640 --> 00:01:02,290 So this is one, and two, and three. 20 00:01:02,290 --> 00:01:06,310 So let's say that our electron starts in the ground state. 21 00:01:06,310 --> 00:01:09,790 And it absorbs some energy and can 22 00:01:09,790 --> 00:01:12,580 hop to a higher energy level. 23 00:01:12,580 --> 00:01:15,610 But in doing so, at this higher energy level, 24 00:01:15,610 --> 00:01:17,580 it is in a more unstable state. 25 00:01:20,250 --> 00:01:24,330 Eventually, it'll want to return to that more stable state. 26 00:01:24,330 --> 00:01:30,150 And when it does so, it emits energy in the form of a photon. 27 00:01:30,150 --> 00:01:36,950 So emit photon and absorb energy. 28 00:01:44,060 --> 00:01:45,980 These different electronic transitions 29 00:01:45,980 --> 00:01:49,290 can be tracked with this equation. 30 00:01:49,290 --> 00:01:57,590 So the change in energy is equal to minus 13.6z squared times 1 31 00:01:57,590 --> 00:02:04,330 over nf squared minus 1 over ni squared. 32 00:02:04,330 --> 00:02:08,060 Where z is the atomic number, and f 33 00:02:08,060 --> 00:02:09,800 is the final state of the electron. 34 00:02:09,800 --> 00:02:12,350 And i is the initial state of the electron. 35 00:02:12,350 --> 00:02:16,840 And this change in energy is in electron force. 36 00:02:16,840 --> 00:02:19,110 So remember that these electronic transitions 37 00:02:19,110 --> 00:02:22,710 are quantized since these energy levels are not continuous, 38 00:02:22,710 --> 00:02:24,540 but they're integers. 39 00:02:24,540 --> 00:02:27,240 This means that the photon that the electron emits 40 00:02:27,240 --> 00:02:33,370 is going to have a certain set of different wavelengths. 41 00:02:33,370 --> 00:02:37,125 And we can see that with this equation energy equals 42 00:02:37,125 --> 00:02:43,810 hc over lambda, where energy is equal to this change in energy. 43 00:02:43,810 --> 00:02:45,870 h is Planck's constant. 44 00:02:45,870 --> 00:02:47,180 c is the speed of light. 45 00:02:47,180 --> 00:02:50,940 And lambda is the wavelength of our emitted photon. 46 00:02:50,940 --> 00:02:53,520 After looking at the red and white LEDs 47 00:02:53,520 --> 00:02:56,230 through the spectrometer, you should see something like this. 48 00:02:56,230 --> 00:02:59,730 Notice how for the red LED, mostly the red and orange bands 49 00:02:59,730 --> 00:03:00,410 are visible. 50 00:03:00,410 --> 00:03:03,000 Whereas for the white LED, almost all the colors 51 00:03:03,000 --> 00:03:04,170 are visible. 52 00:03:04,170 --> 00:03:05,760 Another thing that you could explore 53 00:03:05,760 --> 00:03:09,240 is using your spectrometer to look at the ceiling lights 54 00:03:09,240 --> 00:03:12,330 and seeing how that spectra would differ from the spectra 55 00:03:12,330 --> 00:03:15,090 that you observed from your LEDs. 56 00:03:15,090 --> 00:03:17,460 In summary, electrons absorb and emit 57 00:03:17,460 --> 00:03:21,030 photons of different wavelengths because electronic transitions 58 00:03:21,030 --> 00:03:21,980 are quantized. 59 00:03:21,980 --> 00:03:24,360 By capturing this information, the spectrometer 60 00:03:24,360 --> 00:03:29,210 becomes a very useful tool for characterizing our materials.