L&C Activity III - D3:

How do Lenses Form Images?





Class Period:




Introduction: In this on-line sample activity, clicking on the link to the “simulators” will open snapshots of the simulator setups in a separate window. Some of the buttons and tools in these snapshots “work.” For example, in both simulator examples, the buttons Run and Stop allow you to “turn on” and “turn off” the light rays.

Act III-D3 Sim 1, the Paste button allows you to paste the Screen View AFTER you render the view (see below).

In Act III-D3 Sim 2, the tools displayed to the right allow you to add a screen, add a spray, or delete one spray and/or screen, respectively.

Screen Spray Delete


To open the Screen View, click on the “eye” next to the screen, as indicated to the right. In the Screen View, you can Render the screen image, Copy the rendered image to clipboard (Sim 1 only), and Close the window.


Finally, in Sim 2, you can make the lens thicker, thinner, or return the thickness to the original value by clicking in the areas indicated.



In the Elicitation activity you saw that a lens could form a sharp image, but the screen had to be at special location. You also found that covering part of the lens only made the sharp image dimmer, but the entire image remained. How can these observations be explained? What happens to the light rays as they pass through a lens?




Materials: mini-maglite and maglite holder; small glass lens (converging) with stand to hold it; clear torpedo-shaped bulb in socket, white screen




1.      Remove the top from one of the maglites and place it in hole #2. Mount the small lens about 15 cm from the maglite tip. Make sure the middle of the lens is at the same height above the table as the tip of the maglite. Move the screen back and forth until you observe a sharp point of light. This is the image of the point source.




Note that if you start with the screen positioned where the image is sharpest, and you move it either closer to the lens or further from the lens, the image blurs considerably.



2.      It is often convenient to draw a ray diagram to represent how you think light behaves when it travels from the source, through the lens, and to the screen. You probably realize from the previous activities that light actually bends both when it enters the front surface of the lens and also when it leaves the back surface of the lens. However, to make the ray diagram simple to draw, you need only show the light rays bending at the middle of the lens. In the sample drawing below, we show only one light ray going from the source, through the lens, and to the screen, just to give you an idea of how this is drawn. The screen view of the sharp point image is also shown. Your task is to draw at least two more light rays leaving the source, going through the lens, and then hitting the screen to form the image.






3.      You can compare your diagram with one constructed by a simulator. When you open the simulator, drag out a spray of light rays, then turn on the simulator. Take a snapshot and paste it below. Discuss how your diagram compares with the one provided by the simulator. Click here to get to the simulator: Act III-D3 Sim1.





4.      Returning to your actual apparatus, place a small pencil mark on your screen where the sharp image is formed. Imagine raising the maglite from hole #2 to hole #3. Predict what will happen to the image of the dot on the screen: Will it move upwards on the screen, move downwards on the screen, or remain where it is at? In the picture below, draw where you think the image will appear on the screen, and draw a ray diagram (with at least three rays) to support your prediction.




5.      Test your prediction with your apparatus. Move the maglite from hole #2 to hole #3. What actually happens?






6.      Return to the simulator to get feedback on your ray diagram. With the spray of rays attached, drag the source upwards about one centimeter, then turn on the simulator. You may need to rotate the spray so that all rays pass through the lens. Take a snapshot and paste it below.





7.      Compare your ray diagram with the one drawn by the simulator. What differences are there, if any?







8.      Although we use the tip of the maglite as a point source, in fact most light sources in everyday life are extended sources, that is they have a definite shape to them. Remove the maglite and holder, place the clear torpedo bulb in its place, and turn it on. Find the position of the screen where the inverted image is sharp. Notice that if you then move the screen closer to the lens, or further from it, the image blurs.



9.      Below is a set-up picture from the simulator, in which we represent the extended source as a “lollipop” shaped source. The screen view shows the sharp inverted image. Some rays are shown leaving the top and bottom of the source. Use these rays to draw a ray diagram showing how the image of both the top and bottom of the source are formed on the screen.






10.  You can use a new simulator set-up to compare with your diagram. When you open the simulator, turn it on and the light rays should spread out and then pass through the lens. Place the screen in its proper position. Then take a snapshot of the diagram and paste it below. Click here to get to the simulator: Act III-D3 Sim2.





11.  In Cycle II you did an experiment with a concave mirror in which you changed the distance between the bulb and mirror and observed what happened to the sharp image. Do lenses behave in a similar manner? Consider the following questions:

a.      If you were to move the source either closer to the lens, or further from it, would the image remain at the same place, or would it be formed at a different place?

b.      Would the size of the image always be the same?

c.      Are there positions where you can place the source so that no image is formed on the screen?


Explore the answers to these questions, using both your apparatus and the simulator. Summarize your findings below, including relevant snapshots from the simulator.





1.      The lens you have been using in this activity bulges outward, that is, it is thicker in the middle than at the edges. (Because it brings light together it is called a “converging lens.”)  Does the amount of “bulginess” make a difference in where the lens forms an image? Below are pictures of a source placed the same distance in front of three lenses that differ in bulginess. A ray diagram shows where the lens in the left picture forms an image. (To simplify the diagram we only show a spray of rays coming from the top of the source.) The lens in the middle is thinner, and the one on the right is thicker. Predict whether the thinner lens will form its image closer to the lens, further from the lens, or at the same distance as the lens on the left. Also, predict what the thicker lens will do.







How did you decide?





2.      You can test your predictions using the simulator. To make the results easier to interpret we suggest you delete the rays from the bottom of the source, keeping only the spray of rays from the top. After making sure the pointer arrow is pressed in, select the lens. Then you can click-drag its hotspot to make the lens fatter or thinner. (See picture to the right.)  Try it for a thinner lens, then for a thicker lens, and paste snapshots in the spaces below. Click here to return to the simulator: Act III-D3 Sim2.








3.      How did the simulator results compare with your predictions?




4.      Go to your Cycle III Idea Journal. Based on the evidence you gathered in this experiment you may wish to modify (or add evidence to) an existing idea in response to the following question.


Question #4: In terms of the behavior of light, how is an image formed with a lens?


Your idea should include a complete ray diagram, and also a description how the position of the image depends both on how far the source is from the lens and on the “bulginess” of the lens.