Apps-On-Physics/C3/Convex-Lenses/English-timed

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Time Narration
00:01 Welcome to the Spoken Tutorial on Convex Lenses.
00:05 At the end of this tutorial you will be able to,

Change the focal length and see the kind of image formed.

00:15 Change the object distance and object height and see the kind of image formed.
00:22 Calculate the magnification and length of the telescope tube.
00:27 Here I am using,

Ubuntu Linux OS version 16.04, Firefox Web Browser version 62.0.3

00:39 To follow this tutorial, learner should be familiar with Apps on Physics.
00:45 For prerequisite tutorials please visit this site.
00:50 I have already downloaded Apps on Physics to my Downloads folder.
00:56 In this tutorial we will use,

Image Formation by Converging Lenses and Refracting Astronomical Telescope Apps.

01:07 Right-click on imageconverginglens_en.htm file.
01:14 Select Open With Firefox Web Browser option.
01:19 Image Formation by Converging Lenses App opens in the browser.
01:25 The App shows a ray diagram of the convex lens.
01:30 Before moving to the App let us first be familiar with a ray diagram.
01:37 Let us define principal axis.
01:40 It is an imaginary line passing through the optical center.
01:46 A vertical axis divides the lens into two equal halves.
01:51 There are four positions on the principal axis.
01:55 These positions are 2F, F , F prime and 2 F prime
02:03 F is the focal length and 2F is twice the distance of focal length.
02:10 F prime and 2F prime are on the opposite side of the lens with the same distance as F and 2F.
02:19 Now let us open the App.
02:22 Let us use the scale to spot the positions of focal length F and 2F.
02:28 Initially the object is placed at the zero position of the scale.
02:34 The distance of the object from the lens is 50 cm.
02:40 The vertical black line beyond the lens is a screen.
02:45 This screen can be moved back and forth.
02:50 Blue arrow indicates the height of the object.

It is placed beyond 2F.

02:58 2F is twice the distance of the focal length F.
03:03 From the App, the focal length is 10 cm, so position of 2F has to be at 20 cm.
03:13 Green arrow indicates the image formed by the convex lens.
03:18 In the green control panel we can edit the values of the following parameters.
03:24 Change the value of Focal length to 20 cm and press Enter.
03:31 At the bottom of the green panel, there are two radio buttons.
03:36 Principal light rays and Bundle of light rays.
03:41 By default Principal light rays option is selected.
03:47 A drop-down is provided to Emphasize different parameters.
03:52 From the drop-down list, select Object distance.
03:57 Observe that the App emphasizes the object distance using a blinking line.
04:04 The blinking line disappears after sometime.
04:09 We can also change the object distance by dragging the object.
04:15 As we drag, the value in the text-box changes accordingly.
04:22 Press F5 key on the keyboard to refresh the App.
04:27 Now change the value of Object height to 15 cm.
04:33 Change the Focal length to 20 cm.
04:37 Let us learn about the ray diagram.
04:41 The ray emerging from the object is parallel to the principal axis of the lens.
04:48 This ray after refraction passes through the second principal focus F’.
04:55 A second ray of light passes through the optical center of the lens.
05:02 This ray after refraction emerges without any deviation.
05:08 A third ray passes through the first principal focus.
05:13 This ray, after refraction, is parallel to the principal axis.
05:19 The image is formed at point of intersection of the three rays.
05:25 Let us change the position of the object and see where the image appears.
05:31 Change the Object distance to 40 cm and Object height to 10 cm.
05:39 The Kind of image is real, inverted and equal dimension.
05:45 This is the condition for 2F.
05:49 When object is at 2F the image will appear at 2F’.
05:55 Here the object distance and height will be equal to image distance and image height.
06:03 Drag the object between the 2F and F.
06:08 Drag the object to 10 cm.
06:12 Here we can use the scale to take the measurement.
06:16 Observe that the image is formed beyond 2F’.
06:22 The image formed is real, inverted and magnified.
06:28 Drag the object between F and optic center.
06:34 Drag the object to 30 cm.
06:38 Observe that image is formed at the first principal focus behind the object.
06:45 Here the image formed is virtual, upright, and magnified.
06:52 As an assignment

Change the focal length of a convex lens to 10 cm and its object distance to 15 cm.

07:04 What characteristics of the image do you observe?
07:09 Let us move to next App.
07:13 To open the App right-click on refractor_en.htm file.
07:21 Select the option Open with Firefox Web Browser.
07:26 The App opens with Refracting Astronomical Telescope.
07:31 Before moving to the simulation, please read the information given on the screen.
07:38 Scroll down the screen.
07:41 In the yellow panel, the bigger lens is the objective.
07:46 The objective has a large focal length.
07:50 Here the smaller lens is an Eyepiece.
07:54 The red coloured rays indicate the light from a distant object.
08:00 Light rays from a distant object enter the objective lens.
08:06 After refraction a real image is formed at the second focal point.
08:12 Then the eyepiece magnifies the image. The image formed is enlarged and inverted.
08:21 The magnified image of six brightest star of the pleiades is seen in the black circle.
08:29 In the green panel, Focal lengths of Objective and Eyepiece can be edited.
08:36 Here we can vary the Focal lengths of Objective and Eyepiece from 0.05 m to 0.5 m.
08:46 As per the changes in the Focal lengths, App calculates Angles and Magnification.
08:53 At the bottom of the screen, App has given the formula for magnification.
08:59 That is: v= - f1/ f2
09:05 Here v is the Magnification
09:08 f1 is the focal length of Objective and f2 is the focal length of Eyepiece.
09:16 Let us calculate the magnification using the formula.
09:21 Change the Focal length of Objective to 0.45 m and Eyepiece to 0.1 m.
09:29 Observe that App has calculated the value for Magnification.
09:34 Notice the changes in the black circle.
09:37 If we increase the focal length of the Objective, image will be more magnified.
09:44 Let us now calculate the length of the telescope tube.
09:49 Change the Focal lengths of the Objective and Eyepiece to their default values.
09:56 Press F5 key on the keyboard to restart the App.
10:01 Formula to calculate the length of the telescope tube is sum of the Focal lengths of Objective and Eyepiece.
10:12 That is: L= f1 + f2.
10:16 Here f1 is focal length of Objective and f2 is focal length of Eyepiece.
10:23 Substitute the Focal lengths and calculate the length of the telescope tube.
10:29 Observe that the length of the telescope is 0.6 m.
10:35 Now reverse the Focal lengths of the Objective and Eyepiece.
10:40 Observe that the six brightest stars of pleiades appears to be a single point.
10:47 This is because the focal length of the Objective is smaller than that of the Eyepiece.
10:54 As an assignment solve this numerical.
10:59 Let us summarise.
11:01 Using these Apps we have,

Changed the focal length and seen the kind of image formed.

11:09 Changed the object distance and object height and seen the kind of image formed.
11:16 Calculated the magnification and length of the telescope tube.
11:21 These Apps are created by Walter-fendt and his team.
11:26 The video at the following link summarizes the Spoken Tutorial project.

Please download and watch it.

11:34 The Spoken Tutorial Project team conducts workshops and gives certificates.

For more details, please write to us.

11:44 Please post your timed queries in this forum.
11:48 The Spoken Tutorial Project is funded by MHRD, Government of India.
11:54 This is Himanshi Karwanje from IIT-Bombay.

Thank you for joining.

Contributors and Content Editors

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