Set-up Directions:
Set up demos for teacher motivation: a 500 mL beaker 1/2 filled with water. Insert a pencil in the water to demonstrate refraction. Put this in front of the class so that everyone has a clear view.
Also for teacher motivation: set up the light box. Be sure to have the convex and concave lenses ready to show differences of light behavior. Every lightbox comes with diffration gratings that makes 3-4 light beams from the box. This will make the pathway of light clear when you use the lenses. Play around with the diffration grating to see which one gives the best effect.
To prepare for the lab activity, be sure to have all the candle and lens supplies laid out in one section of the room. Supplies should be labeled. This will help with clean-up later.
Prepare a board diagram that depicts focal distance and focal point. It would be easier if you copy the experiment set up detailed on Handout #2. Be sure to label clearly.
Make copies of the Experiment Set Up sheet (Handout #2), enough for each team. Be sure there is enough copies of Handout #3 for all students.
Teacher Presentation & Motivation:
As a DO NOW activity, instruct the students to explain the phenomenon that you have set up in the front of the classroom. Ask: "Explain why the pencil seems bent from the side of the beaker". Students should write in their notebooks silently. As a class share, students contribute their responses. Answers include refraction, or "light bends when it hits from air to water."
Bring the students up to the lightbox demo. Tell students to bring their notebooks for observations. Show them the two different lenses. Turn on the lightbox to show the the beams of light. Ask:"What do you think will happen to the light when I place the CONVEX lens in front of it? What about the CONCAVE?" Be sure to distinctly show the students the names of the shapes of the lenses. After students share their predictions, demonstrate the results. Have the students themselves describe the results to you: concave lenses diverge light, whereas convex lenses converge light to a single point.
Ask students: "What kind of lens do you think our eyes have?" Concur with students who guess that eye lenses are convex. Our eyes need to focus light images onto one point on the retina in order to sense the light signals and bring them to the brain. Introduce today's activity: "Today, we will conduct an experiment to see how our lenses help us see far and near objects; we will see how our lenses ACCOMMODATE to objects at different distances".
Put students in their groups. Distribute and review Handout #4.
Note: Depending upon your pre-assessment of student's knowledge about refraction, you may want to begin the presentation/motivation with a quick review of refraction.
Activity 1 - Wacky lenses or just the way it works?
Distribute Handout #2 and Handout #3 (if they are not placed at experiment stations). Instruct students to refer to Handout #2. Point out the basic layout of the experiment, and the glossary terms. They can set their candle by mounting it on a small piece of clay. Compare and contrast how this experiment setup is a model of the eye. Then, instruct the following:
- "In Experiment #1: You will place the candle (I will light it up!) at a fixed distance from the focal point, which is on the white paper. The candle is the image that you will project onto the focal point. Then, you will move the magnifying glass, which has a convex lens, back and forth to determine the focal length at which the image of the candle appears clearest on the white cardboard. Be sure to level all three objects so that the magnifiying lens can capture the light from the candle so that it can be projected onto the cardboard!"
- "In Experiment #2: You will move the candle at three different distances from the cardboard. At each position, you have to find out which focal distance best projects the candle onto the white cardboard."
Engage students in the experiment. Most will be amazed that the image projected onto the white cardboard is of an upside down candle! Throw in a "think about side question" for teams to ponder as they continue with the experiment. When teams complete the experiment, they must state 2-3 sentences on the conclusion of the experiment, and how the data prove/disprove their hypothesis. Emphasize that while each team might be the same conclusion, individual students have different hypthesis. Therefore all conclusions must be in their own WORDS!
Have students complete Handout #3.
Activity 2 - What do we see?
After the experiment, bring the class back together. Team leaders from each team share their results, while the whole class either agrees or disagrees with their responses. Most results should show that as objects became further away, the focal distance decreases to help with clarifying the image.
Ask students to connect the results with their eyes: "How might the same ideas apply to our eyes?" Students might offer this response: "We move closer to images to make them clearer to us." Ask them: "While this may be true, what is happening to our lenses and focal distance if we don't move? Do our lenses move back and forth so that we don't have to move everytime we need to see something far away?"
Explain to students that our lenses change in shape: from being fat to skinny (dense, less dense) in order to change the focal distance to help us see objects no matter where it is. You can demonstrate this by replacing the magnifying lens with a water balloon that is stretchable. Pull both ends of the balloon to demonstrate decrease in balloon density and squish the ballon to make it "fat" or more dense.
Wrap Up:
Students refine their conclusions by applying this new knowledge. For homework (or if there's enough time) students should revise their Activity Sheets (Handout #3). Be sure to emphasize the rubric.