Apps-On-Physics/C3/Convex-Lenses/English-timed
From Script | Spoken-Tutorial
| 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. |
