Apps-On-Physics/C2/Reflection-and-Refraction/English-timed

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Time Narration
00:01 Welcome to the Spoken Tutorial on Reflection and Refraction.
00:06 In this tutorial, we will learn to,

Simulate reflection and refraction of a light ray.

00:13 Calculate the angles of reflection and refraction.
00:17 Change the medium and angle of incidence to verify Snell's Law.
00:22 Calculate the value of critical angle.
00:26 Verify Huygens' principle.
00:29 Here I am using,

Ubuntu Linux OS version 16.04

00:35 Firefox web browser version 62.0.3
00:41 To follow this tutorial, learner should be familiar with Apps on Physics.
00:47 For pre-requisites tutorials please visit this site.
00:52 Use the given link to download the Apps.
00:56 I have already downloaded the Apps on Physics to my Downloads folder.
01:01 In this tutorial, we will use,

Refraction of Light and Reflection and Refraction of Light Waves Apps.

01:11 Right-click on refraction_en.htm file.
01:16 Select the option Open With Firefox web Browser.
01:21 Reflection and Refraction of Light App opens in the browser.
01:26 The App shows reflection and refraction of light through a given medium.
01:31 The default media are air and water.
01:36 Note that the medium with lesser refractive index is shown in white background.
01:42 The medium with greater refractive index is shown in blue background.
01:47 Light from the top left corner, strikes the boundary surface of the two media.
01:52 It shows reflection and refraction.
01:56 Reflection is shown by the blue coloured angle.
02:01 Observe that the angle of incidence and reflection are the same.
02:06 Here we see the refraction of light.
02:09 When light travels from rarer medium to denser medium, it bends towards normal.
02:16 On the green panel we have a choice to change a few parameters.
02:21 Let us reverse the two media using the drop downs.
02:26 Select water as the upper medium.
02:29 Select air as the lower medium.
02:33 Here the ray of light travels from denser to rarer medium.
02:38 The ray bends away from the normal.
02:42 Note that both the drop downs show the same material media.
02:48 Below the drop down, we see two text fields.

These are provided to enter the values of refractive indices.

02:48 Here we can also change the values manually between the range of 1 to 5.
03:05 Press F5 key on the keyboard to refresh the App.
03:10 Next Angle of incidence can be changed from 0.1 degrees to 90 degrees.
03:18 Press F5 key to see the default value.

It shows 30 degrees.

03:25 Below the text fields App shows the Angle of reflection and refraction.
03:31 Graph shows the angle of refraction with angle of incidence.
03:36 We can also change the angle of incidence by dragging this red coloured ray.
03:42 Notice the change in angle of reflection and refraction.
03:48 Simultaneously observe the graph.
03:51 Now from the graph we will learn the two cases of Snell’s law.
03:57 Before that let us state Snell’s law of refraction.
04:02 Ratio of sine of angle of incidence to sine of angle of refraction is a constant.
04:09 n21 is the refractive index of second medium with respect to first medium.
04:16 Here are the 2 cases of Snell’s law.
04:19 If n21 is greater than 1, angle of refraction is less than angle of incidence.
04:26 If n21 is less than 1, angle of refraction is greater than angle of incidence.
04:34 Change the Angle of incidence to 20 degrees.
04:38 Observe that the angle of refraction has changed to 14.9.
04:44 This graph shows the first case of Snell’s law.
04:48 Here angle of incidence is greater than the angle of refraction.
04:53 Observe that the light ray bends towards the normal.
04:58 Let us see what happens, when incident ray passes from denser to rarer medium.
05:04 From the first drop down change the material medium to diamond.
05:09 Notice that the light ray has bent away from the normal.
05:14 This graph shows the second case of Snell’s law.
05:18 Here the angle of refraction is greater than the angle of incidence.
05:23 Increase the Angle of incidence to 30 degrees.
05:27 The refracted ray has bent still more further away from the normal.
05:32 Again increase the angle of incidence to 35 degrees.
05:37 In this case observe that the refraction is not possible.
05:42 Here the incident ray is totally reflected.
05:46 This phenomenon is known as total internal reflection.
05:51 Here the critical angle is formed.
05:55 The critical angle for diamond and water is 33.3 degrees.
06:01 Now we will calculate the critical angle using the formula.
06:06 Let us make a tabular column to calculate critical angle for two different media.
06:13 Here I have calculated the critical angle for diamond and water.
06:18 The calculated value is comparable to the value shown in the App.
06:24 Now enter these values in the table.
06:28 Next change the upper medium to water and lower medium to air.
06:34 Note the refractive indices for both the media.
06:39 Then calculate the critical angle using the above formula.
06:44 Observe that the values are comparable.
06:48 Note these values in the table.
06:52 As an assignment
Note the values of refractive indices for the following media from the App.
07:00 Calculate the critical angle for the two media.
Compare the values with the ones shown in the App.
07:09 Now we will move on to the next App.
07:12 Right click on refractionhuygens_en.htm file.
07:18 Open With Firefox Web Browser.
07:22 The App opens in the browser.
07:25 App shows Reflection and Refraction of light waves using Huygen's principle.
07:31 Click on the Restart button.
07:34 Here the plane wavefront is incident diagonally on the boundary of the media.
07:40 Click on the Next step button.
07:44 Explanation of each step is provided in this text box.
07:49 Note the change in media, when the wavefront is incident on the boundary.
07:55 Click on the Pause button.
07:58 Observe the pink points on the boundary between the media.
08:03 Each pink point is the source of spherical wavefront.
08:08 These generating waves in the Medium 1 and Medium 2 are the wavelets.
08:14 Click on the Resume button.
08:17 Note that the waves in Medium 2 move with less velocity as compared to Medium 1.
08:25 This is because, the medium 2 has higher refractive index.

So here the waves move with less velocity.

08:35 Click on the Next step button.
08:39 Click on the Pause button.
08:42 Observe the tangent drawn to all these spherical waves.
08:47 This line, here is the source of the secondary wavefront.
08:52 So, the points on every wavelet results in the formation of secondary wavefront.
08:59 Here the values of Angle of incidence, reflection and refraction are given.
09:06 Click on the Resume button.
09:09 Direction of propagation changes when waves move from medium 1 to medium 2.
09:16 Again click on the Next step button.
09:20 Here the direction of the propagation of waves is shown.
09:25 Click on the Pause button.
09:28 Observe that these lines of propagation are perpendicular to the wavefronts.
09:35 Click on the Resume button.
09:38 Change the Angle of incidence to 60 degrees.
09:42 Click on the Next step button.
09:45 Here we can see a series of wavefronts that are incident on the boundary surface.
09:52 Observe the speed and wavelength of wavefronts in both the media.
09:58 The wavelength and speed of the wavefront decreases in the denser medium.
10:04 But the frequency of the plane wavefronts remains the same.
10:09 Let us reverse the refractive indices and observe the formation of wavefronts.
10:15 Here the speed of the wavefront decreases, as it moves from denser medium.
10:22 This shows the total internal reflection.
10:26 Here the incident wavefront is completely reflected and not refracted.

This results in the formation of critical angle.

10:37 As an assignment

Change the refractive index values of both media as given in Refraction of Light App.

10:46 Observe the formation of wavefront and give an explanation.
10:51 Let us summarize
10:53 Using these Apps, we have Simulated reflection and refraction of a light ray.
11:00 Calculated the angles of reflection and refraction.
11:05 Changed the medium and angle of incidence to verify Snell's Law.
11:10 Calculated the value of critical angle.

Verified Huygens' principle.

11:18 These Apps were created by Walter-fendt and his team.
11:23 The video at the following link summarizes the Spoken Tutorial project.

Please download and watch it.

11:31 The Spoken Tutorial Projectteam, conducts workshops and gives certificates.

For more details, please write to us.

11:41 Please post your timed queries in this forum.
11:46 Spoken Tutorial Project is funded by MHRD Government of India.
11:52 This is Himanshi Karwanje from IIT Bombay

Thank you for joining.

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