Apps-On-Physics/C3/Convex-Lenses/English
| Visual Cue | Narration |
| Slide Number 1
Title Slide |
Welcome to the Spoken Tutorial on Convex Lenses. |
| Slide Number 2
Learning objective |
At the end of this tutorial you will be able to,
Change the focal length and see the kind of image formed. Change the object distance and object height and see the kind of image formed. Calculate the magnification and length of the telescope tube. |
| Slide Number 3
System Requirements |
Here I am using,
Ubuntu Linux OS version 16.04 Firefox Web Browser version 62.0.3 |
| Slide Number 4
Pre-requisites |
To follow this tutorial, learner should be familiar with Apps on Physics.
For the Pre-requisites tutorials please visit this site. |
| Point to the file in the downloads folder | I have already downloaded Apps on Physics to my Downloads folder. |
| Slide Number 5
Apps on Physics |
In this tutorial we will use,
Image Formation by Converging Lenses and Refracting Astronomical Telescope Apps. |
| Right click on imageconverginglens_en.htm file.
Select the Open With Firefox web Browser option. Cursor on the App. |
Right click on imageconverginglens_en.htm file.
Select the Open With Firefox web Browser option. Image Formation by Converging Lenses App opens in the browser. |
| Point to the convex lens in the App. | The App shows a ray diagram of the convex lens. |
| Slide Number 6
Converging Lens Ray Diagram Show the ray diagram with all the positions. |
Before moving to the App let us first be familiar with a ray diagrams.
Let us define principal axis. It is an imaginary line passing through the optical center. A vertical axis divides the lens into two equal halves. There are four positions on the principal axis, these positions are 2F , F , F’ and 2F’. F is the focal length and 2F is the twice the distance of focal length. F’ and 2F’ are on the opposite side of the lens with the same distance as F and 2F’. |
| Open App. | Now let us open the App. |
| Point to the scale at the bottom of the yellow panel. | Let us use the scale to spot the positions of focal length F and 2F. |
| Point to show the Zero on the scale.
Point to Object distance. Move the screen using the pressed mouse. |
Initially the object is placed at the zero position of the scale.
The distance of the object from the lens is 50 cm. The vertical black line beyond the lens is a screen. This screen can be moved back and forth. |
| Point to the blue arrow. | Blue arrow indicates the height of the object.
It is placed beyond 2F. |
| Point the cursor on 2F position. | 2F is twice the distance of the focal length F. |
| Show it on the scale. | From the App, the focal length is 10 cm, so position of 2F has to be at 20 cm. |
| Point to the green arrow. | Green arrow indicates the image formed by the convex lens. |
| In the green control panel point to Focal length,
Object distance, and Object height. |
In the green control panel we can edit the values of the following parameters. |
| Edit the Focal length to 20 cm and press Enter. | Change the value of Focal length to 20 cm and press Enter. |
| Point to the radio buttons Principle light rays and Bundle of light rays. | At the bottom of the green panel there are two radio buttons.
Principal light rays and Bundle of light rays. |
| Point to Principal light rays. | By default Principal light rays option is selected. |
| Click on the drop down list. | A drop down is provided to Emphasize different parameters. |
| Click on the drop down and Select Object distance. | From the drop down list, select Object distance. |
| Point to the object distance.
Point to blinking line. |
Observe that the App emphasizes the object distance using a blinking line.
The blinking line disappears after sometime. |
| Drag the object(blue arrow) to show the changes in object distance. | We can also change the object distance by dragging the object.
As we drag the value in the text-box changes accordingly. |
| Press F5 key on the keyboard | Press F5 key on the keyboard to refresh the App. |
| Edit the Object height to 15 cm and press Enter. | Now change the value of Object height to 15 cm.
Change the Focal length to 20 cm. |
| Cursor on the ray diagram. | Let us learn about the ray diagram. |
| Point to the ray which is parallel to the optical axis.
Point to the ray. |
The ray emerging from the object is parallel to the principal axis of the lens.
This ray after refraction passes through the second principal focus F’. |
| Point to the second ray of light which passes without any deviation. | A second ray of light passes through the optical center of the lens.
This ray after refraction emerges without any deviation. |
| Point to the last ray which parallel to the optical axis after refraction. | A third ray passes through the first principal focus F.
This ray, after refraction is parallel to the principal axis. |
| Point to show the meeting point of the rays. | The image is formed, at point of intersection of the three rays. |
| Cursor on the interface. | Let us change the position of the object and see where the image appears.
|
| Edit the value of Object distance to 35 cm and Object height to 10 cm. | Change the Object distance to 40 cm and Object height to 10 cm. |
| In the control panel point to the Kind of image formed.
real, inverted and equal dimension. |
The Kind of image is real, inverted and equal dimension. |
| Point to show the values of following.
Object distance Object height Image distance Image height. |
This is the condition for 2F, when object is at 2F the image will appear at 2F’.
Here the object distance and height will be equal to image distance and image height. |
| Drag the object between 2F and F.
Drag the object to 10 cm. |
Drag the object between the 2F and F.
Drag the object to 10 cm. Here we can use the scale to take the measurement. |
| Point to the image formed. | Observe that the image is formed beyond 2F’.
The image formed is real, inverted and magnified. |
| Drag the object between F and optic center.
Drag it to 30 cm. |
Drag the object between F and optic center.
Drag the object to 30 cm. |
| Point to the image (green arrow). | Observe that image is formed at the first principal focus behind the object.
Here the image formed is virtual, upright, and magnified. |
| Slide Number 7
Assignment |
As an assignment
Change the focal length of a convex lens to 10 cm and its object distance to 15 cm. What characteristics of the image do you observe? |
| Cursor on the interface. | Let us move on to next App. |
| To open the App right click on refractor_en.htm.
Select option Open with Firefox Web Browser. |
To open the App right click on refractor_en.htm file.
Select the option Open with Firefox Web Browser. |
| Point to the App. | The App opens with a Refracting Astronomical Telescope. |
| Point the cursor to show the information. | Before moving to the simulation please read the information given on the screen. |
| Scroll down. | Scroll down the screen. |
| Point to the bigger lens. | In the yellow panel the bigger lens is the objective.
The objective has a large focal length. |
| Point to the smaller lens. | Here the smaller lens is an Eyepiece. |
| Point to the red coloured rays. | The red coloured rays indicate the light from a distant object. |
| Move the cursor to show the path of the rays. | Light rays from a distant object enter the objective lens.
After refraction a real image is formed at the second focal point. |
| Point to the eyepiece. | Then the eyepiece magnifies the image.
The image formed is enlarged and inverted. |
| Point to the black circle which is at the bottom right corner. | The magnified image of six brightest star of the pleiades is seen in the black circle. |
| In the green control panel point to Focal length of Objective and Eyepiece. | In the green panel, Focal lengths of Objective and Eyepiece can be edited. |
| Highlight this sentence from the App(2nd para 1st line). | Here we can vary the Focal lengths of Objective and Eyepiece from 0.05 m to 0.5 m. |
| Point to the Angles
Objective and Eyepiece. Point to Magnification. |
As per the changes in the focal lengths,
App calculates Angles and Magnification. |
| Scroll down to see the formula for magnification. | At the bottom of the screen App has given the formula for magnification. |
| Point to the formula. | That is:
v= - f1/ f2 Here v is the magnification f1 is the focal length of Objective and f2 is the focal length of Eyepiece. |
| Let us calculate the magnification using the data from the App. | |
| Edit the value of Focal length of the Objective to 0.45 m
and Eyepiece to 0.1 m. |
Change the Focal length of Objective to 0.45 m and Eyepiece to 0.1 m. |
| Point to the observed value. | Observe that App has calculated the value for magnification. |
| Point to the black circle. | Notice the changes in the black circle.
If we increase the focal length of the Objective image will be more magnified. |
| Let us now calculate the length of the telescope tube. | |
| Press F5 key on the keyboard. | Change the Focal lengths of the Objective and Eyepiece to their default values.
Press F5 key on the keyboard to restart the App. |
| Slide Number 8
Telescope Tube Length of the tube is sum of the focal lengths of objective and eyepiece. L= f1 + f2 = 0.50 + 0.10 = 0.6 m |
Formula to calculate the length of the telescope tube is sum of the focal lengths of objective and eyepiece.
That is: L= f1 + f2. Here f1 is focal length of Objective and f2 is focal length of Eyepiece. Substitute the focal lengths and calculate the length of the telescope tube. Observe that the length of the telescope is 0.6 m. |
| Edit the values of Focal length of Objective to 0.1 m
and Eyepiece to 0.5 m. |
Now reverse the Focal lengths of the Objective and Eyepiece. |
| Point to the single dot in the second black circle. | Observe that the six brightest stars of pleiades appear to be a single point. |
| This is because the focal length of the Objective is smaller than that of the Eyepiece. | |
| Slide Number 9
Assignment An astronomical telescope has an objective of focal length 0.45 m and an eyepiece of focal length 0.25 m. Find the magnification produced by the telescope. Verify the calculated value with the ones shown in the App. |
As an assignment solve this numerical. |
| Let us summarise | |
| Slide Number 10
Summary |
Using these Apps we have,
Changed the focal length and saw the kind of image formed. Changed the object distance and object height and saw the kind of image formed. Calculated the magnification and length of the telescope tube. |
| Slide Number 11
Acknowledgement These Apps are created by Walter-fendt and his team. |
These Apps are created by Walter-fendt and his team. |
| Slide Number 12
About Spoken Tutorial project. |
The video at the following link summarizes the Spoken Tutorial project.
Please download and watch it. |
| Slide Number 13
Spoken Tutorial workshops. |
The Spoken Tutorial Project team,
conducts workshops and gives certificates. For more details, please write to us. |
| Slide Number 14
Forum for specific questions: |
Please post your timed queries in this forum. |
| Slide Number 15
Acknowledgement |
The Spoken Tutorial Project is funded by MHRD, Government of India. |
| This is Himanshi Karwanje from IIT-Bombay.
Thank you for joining. |
