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

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

PoojaMoolya