PhET-Simulations-for-Physics/C2/Bending-Light/English-timed

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Time Narration
00:01 Welcome to this tutorial on Bending Light, an interactive PhET simulation.
00:07 In this tutorial, we will learn how to use Bending Light, interactive PhET simulation.
00:14 To follow this tutorial, learners should be familiar with topics in high school science.
00:21 Here I am using -

Ubuntu Linux OS version 14.04

Java version 1.7.0

Firefox Web Browser version 53.02.2.

00:37 Using this simulation, we will learn:

1. Reflection of light

2. Refraction of light

3. Dispersion of light

4. How reflection and refraction vary with different material media.

00:52 We will also learn to,

1. Use optical instruments like prism and lens

2. Measure intensity of light

01:02 3. Measure velocity of light and view different wave patterns.
01:08 Let’s define reflection, refraction and dispersion.
01:14 Reflection, is bouncing back of a light ray after hitting any surface.

For example- A mirror reflects the image of the observer.

01:26 Refraction, is bending of a light ray when it enters a medium.
01:32 Light changes its speed and angle of propagation as it enters a different medium.

For Example: Bending image of a pencil in a beaker of water.

01:45 Dispersion, is a process of splitting of white light into seven colours.
01:51 For Example, formation of a rainbow on a cloudy day.
01:57 So, let's begin.
01:59 Use the given link to download the simulation.

http://phet.colorado.edu

02:03 I have already downloaded Bending light simulation to my Downloads folder.
02:09 To open the simulation, right-click on bending-light_en.html file.
02:14 Select Open With Firefox Web Browser option.

File opens in the browser.

02:23 This is the interface for Bending Light simulation.
02:28 The interface has three screens- Intro, Prisms and More Tools.
02:35 Let us begin our simulation with Intro screen.

Click on Intro screen to open it.

02:42 This screen has Ray and Wave radio buttons on the top left corner,

a movable laser pointer to turn on the laser light.

02:53 We can see green colored arrows when mouse pointer is placed on the laser.

These arrows show the direction in which the laser moves.

03:06 Click and drag the laser to move it from zero to 90 degrees.

Let us bring the laser to its default position.

03:16 Now let’s move onto the tools at the bottom left.
03:21 Here we see a tool box.

It consists of a protractor to measure angles, Intensity tool to measure intensity of light.

03:32 Below the tool box, we have a check-box to show Normal.

A Reset button is provided at the right hand side corner, to reset the simulation.

03:44 Let us begin the simulation.
03:47 We can focus light in the form of a Ray , or a Wave depending on what we choose.
03:54 By default Ray is selected.

To turn on the laser, click the red button on the laser pointer.

04:03 Notice that laser light gets reflected and refracted.
04:09 Observe that as we drag the laser pointer, angle of reflection and refraction changes.
04:17 Let’s use the protractor tool to measure angle of incidence refraction and reflection.
04:25 Place the protractor at the intersection point of incident ray, normal, refracted and reflected rays.
04:35 Observe that angle of incidence is equal to angle of reflection.

Drag the protractor back to its place.

04:44 Next we will move on to right hand side of the Intro screen.
04:50 Here, we have controls to measure refractive index in both the media.
04:56 There is a slider to change the material medium, or buttons to click. Initially, the slider is near Air.
05:07 As we drag the slider, medium changes from Air to Water and then to Glass.
05:15 When medium is changed, the angle of refraction changes. Index of Refraction(n) also changes.
05:24 We can see the changing value in the box. Drag the slider back to Air in the upper medium.
05:32 By default, slider is on Water in the lower medium.

Drag the slider and place it between Water and Glass in the lower medium.

05:44 Now let's measure the intensity of light using Intensity tool.
05:49 Click and drag the Intensity tool to the medium.
05:53 Intensity tool consists of a magnifier and a meter to measure intensity of light.
06:00 I will place the magnifier on the incident light. Meter shows 100% intensity.
06:08 Now, drag and place the magnifier on refracted light. Meter shows value as 86.08%.

Intensity decreases because it depends on the medium.

06:24 As we drag the medium slider, value on the intensity meter changes.

This is because angle of refraction changes.

06:34 Similarly, place the magnifier on reflected light. Meter shows value as 13.55%.
06:44 You can drag the slider to see a different value of intensity.
06:51 Now, change Ray to Wave and observe the bending of light.
06:56 Now we will move on to Prisms screen.
07:00 Click on Prisms screen at the bottom of the interface.
07:06 In this screen, we have the following tools-

Slider to change the environment,

Monochromatic ray,

07:15 Monochromatic beam, white light, slider to change wavelength of light.
07:22 At the bottom of the screen, we have-

prisms and lenses,

07:29 slider to change the Objects,

check boxes for Reflections, Normal and Protractor.

Reset button is at the right hand-side bottom corner.

07:43 laser has a screw provided at its end. Direction of laser can be changed using this screw.
07:52 Here, arrows indicate direction of motion. laser can be moved up and down.
08:00 Now let’s begin the simulation.
08:03 Click on monochromatic beam. Click on the laser light.
08:09 Click on Reflections check box to see multiple reflections.
08:14 Place a prism in the path of light. You can see bending of light.
08:21 Move the wavelength slider gradually from red to violet.

Notice the bending of light as wavelength changes.

08:32 Prism is provided with a screw at its corner.
08:38 Drag the screw to change the direction of the prism.

As we change the direction, notice the bending of light.

08:47 As an assignment, change prism to glass slabs and observe the bending of light.
08:54 Now will see dispersion of light using white light.
08:59 Click on white light tool. Observe the dispersion of light.
09:05 Drag the Objects medium slider and observe the dispersion of light.
09:11 Place a lens in the path of dispersed beam of light.

The dispersed beam of light shows multiple reflections.

09:26 As an assignment:

Use various lenses and observe the dispersion.

09:32 Change various Environments and observe the dispersion.
09:37 Now we move on to More Tools screen.
09:41 Click on More Tools screen.

More Tools screen has a combination of tools from Intro and Prisms screens.

09:51 Tool box at the bottom has two additional tools Speed and Time.

At the bottom we also have Angles check box.

Click on the Angles check box.

10:04 Now we will change the laser light to Wave.

Additionally radio buttons- Normal and Slow Motion. Also, Play/Pause and Forward buttons appear.

10:18 Let’s begin the simulation.
10:21 Turn on the laser light.
10:23 Click on red button to turn on the laser pointer. Let’s use the Time tool.
10:31 Drag and place the Time tool in the medium. Time tool has two magnifiers and a Meter.
10:39 Place one of the magnifiers on incident wave and observe the wave pattern.

Now place the other magnifier on the refracted wave.

10:51 Observe the wave pattern. It shows a phase shift.
10:56 Place the magnifier on the reflected wave.

Observe that the new wave pattern is with reduced amplitude.

11:06 This is because, the reflected wave has much lower intensity.
11:12 Drag and place the Speed tool on the refracted wave.

The speed is 0.67 c. Here c is velocity of light equal to 3x10^8 m/s. Value of speed decreases.

11:33 This is because, velocity of light decreases in a material medium.
11:39 As an assignment,

Change material medium and notice the change in speed and wave pattern.

11:48 In this tutorial, we have learnt how to use Bending Light, interactive PhET simulation.
11:57 Using this simulation, we have learnt:

1. Reflection of light

2. Refraction of light

3. Dispersion of light

4. How reflection and refraction varies with different material media.

12:12 We have also learnt to:

1. Use optical instruments like prism and lens

12:19 2. Measure intensity of light, measure velocity of light and view different wave patterns.
12:29 The video at the following link summarizes the Spoken Tutorial project. Please download and watch it.
12:36 The Spoken Tutorial Project team conducts workshops using spoken tutorials and gives certificates on passing online tests.
12:46 For more details, please write to us. Please post your timed queries on this forum.
12:54 This project is partially funded by Pandit Madan Mohan Malaviya National Mission on Teachers and Teaching.
13:03 Spoken Tutorial Project is funded by NMEICT, MHRD, Government of India. More information on this mission is available at this link.
13:17 This is Madhuri Ganapathi from IIT-Bombay, signing off. Thank you for joining

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