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

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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