Teacher Demo: Covered Lens

Transcription

1 SNC2D/2P Light and Geometric Optics/Light and Applications of Optics Teacher Demo: Covered Lens Topics geometric optics and image formation Timing preparation: 20 min demonstration: 10 min Specific Expectations SNC2D A1.1 formulate scientific questions about observed relationships, ideas, problems, and/or issues, make predictions, and/or formulate hypotheses to focus inquiries or research A1.8 analyse and interpret qualitative and/or quantitative data to determine whether the evidence supports or refutes the initial prediction or hypothesis, identifying possible sources of error, bias, or uncertainty A1.10 draw conclusions based on inquiry results and research findings, and justify their conclusions E2.5 predict, using ray diagrams and algebraic equations, the position and characteristics of an image produced by a converging lens, and test their predictions through inquiry [PR, AI, C] E3.4 explain the conditions required for partial reflection/refraction and for total internal reflection in lenses, and describe the reflection/refraction using labelled ray diagrams E3.5 describe the characteristics and positions of images formed by converging lenses (e.g. orientation, size, type) with the aid of ray diagrams SNC2P A1.1 formulate scientific questions about observed relationships, ideas, problems, and/or issues, make predictions, and/or formulate hypotheses to focus inquiries or research A1.8 analyse and interpret qualitative and/or quantitative data to determine whether the evidence supports or refutes the initial prediction or hypothesis, identifying possible sources of error, bias, or uncertainty A1.10 draw conclusions based on inquiry results and research findings, and justify their conclusions E3.8 explain how the properties of light or colour are applied in the operation of an optical device (e.g., a reflecting telescope, stop lights, stage lights) Introduction This demonstration will help students understand what is required for an image to be formed by a lens. As well, it will help to diagnose possible misconceptions about image formation. Materials 9-LED flashlight electrical tape converging lens (e.g., magnifying glass or Fresnel lens) and stand large projector screen on opposite wall cardboard or opaque tape

2 Safety Considerations None Procedure Wear appropriate PPE: safety goggles. Prepare the equipment before the class begins. 1. Use small pieces of electrical tape to block out four of the flashlight s LEDs as shown in Fig.1. Fold back the edges of the tape. (a) (b) (c) Fig.1 Modifying the flashlight to form an arrow-shaped object 2. Set up the equipment as shown in Fig. 2, holding the flashlight in one hand. 3. Adjust the distance between the flashlight and the lens (close to the focal length, f) until there is a focused image (upside down and real) of the flashlight arrow on the screen. Fig.2 Set-up of flashlight, lens, and screen Organize your class into groups of 2 to 3 students. 4. Predict/Explain Ask students to predict what will happen to the image when the lens is partially covered by the cardboard or opaque tape. Encourage all groups to provide a rationale for their prediction. 5. Observe Cover half of the lens with cardboard. Again shine the flashlight at the lens and provide time for students to record their observations. 6. Explain

3 Ask the small groups to reconvene and revise their explanations, if necessary. Challenge them to suggest a tool or strategy to help explain their observations. Disposal No special concerns What happens? Initially, the light from the flashlight forms an image on the screen as shown in Fig.3. Fig.3 Ray diagram of initial set-up When half of the lens is covered up with the cardboard there is still a complete inverted real image but the brightness of the image is about half that of the original image. Note that it does not matter which half is covered up. How does it work? Students are exposed to drawing ray diagrams in order to predict where images (real or virtual) will form. These ray diagrams are simplified and they typically use two or three strategic rays of light. Students often think that if one of these rays is blocked then only a part of the object s light will reach the screen and only part of an image will appear. In reality, the flashlight (object) is emitting a huge number of light rays that leave every point of the flashlight in all directions (Fig.4). Fig.4 Ray diagram of initial set-up showing wasted rays

4 Any rays that pass through the lens help to form the image. When the lens is partially covered, the amount of light passing through the lens is less and therefore the brightness of the image, in turn, is less. This is part of the reason why astronomical telescopes have large objective (light collecting) lenses. Teaching Suggestions/Hints 1. This demonstration can be used as a powerful diagnostic tool for misconceptions about image formation. It should be used with a Predict, Explain, Observe and Explain (PEOE) framework. 2. A 9-LED flashlight can be purchased from many hardware and dollar stores for under $10. It makes an excellent luminous object for Optics demonstrations. The adaptation shown in Fig.1 produces a very bright object shaped like an arrow that can be used to demonstrate the properties of images formed by a curved mirror or lens. The fact that it looks like an arrow makes vertical inversion of many images obvious. To demonstrate horizontal inversion, simply rotate the flashlight 90. The orientation of the object (the flashlight) can be easily demonstrated by shining it toward the students. The light is bright but not dangerous. This source is much brighter and safer than a candle. This results in an image, which is much easier to see. 3. Fresnel lenses can be purchased or salvaged from an old overhead projector (Fig.5). Split the overhead projector lens in two by sanding the edges off and then carefully separating them. These lenses are about 30 cm 30 cm. The larger the lens, the more light is collected and focused, which results in a brighter image that is easier for the class to see. Fig.5 The formation of a Fresnel lens from a regular converging lens 4. In Step 4, students will typically predict that only half of an image will form or that the image formed will be upright instead of inverted. In Step 5 students should observe that the entire image is still present but that the image is about half as bright. 5. In Step 6, a suitable tool or strategy would be a ray diagram.

5 Next Steps As a follow-up, ask students to repeat this activity and this time cover up half of the light emitted from the flashlight. In this case, they will observe a partial image. As well, you could ask the students what would happen if the lens were removed, or if a larger Fresnel lens or smaller magnifying glass were used Additional Resources 1. A more detailed explanation of the demonstration and related concepts - Five Easy Lessons: Strategies for Successful Physics Teaching by Randall D. Knight. 2. More details on this demonstration - Goldberg, F.M. and McDermott, L. C. An investigation of student understanding of the real image formed by a converging lens or concave mirror. Am. J. Phys. 55, (1987). 3. Online details on this demonstration: Optics Re-education -