Shadows (umbra and penumbra) (Item No.: P )

Transcription

1 Teacher's/Lecturer's Sheet Shadows (umbra and penumbra) (Item No.: P ) Curricular Relevance Area of Expertise: Physik Education Level: Klasse 7-10 Topic: Optik Subtopic: Lichtausbreitung Experiment: Schatten (Kern- und Halbschatten) Difficulty Preparation Time Execution Time Recommended Group Size Easy 10 Minutes 10 Minutes 2 Students Additional Requirements: Experiment Variations: Clear film (approx. 8 cm x 8 cm) White paper (DIN A4) Keywords: Task and equipment Information for teachers Additional Information In the first part of the experiment the students should observe the formation of shadow behind an illuminated object and make conclusions as to the conditions necessary for shadow formation and the properties of propagation of light. The students will become acquainted with umbra and penumbra in a second experiment. Both experiments can, however, also be performed independently. By using colour filters in a supplementary experiment, the students can observe an interesting and surprising phenomenon: the formation of coloured shadows. Suggestions Since the two light sources which are necessary to produce the umbra and penumbra of an object are not available, a simple procedure of light separation with a mirror will be used. To perform this, the mirror on the block is set up in its upright position. It should be as close as possible to the lateral opening so that a wide cone of light is produced. If the mirror holder ( ) is among your equipment, it should be used. In each case, be sure that the opaque object is placed in the mirror's cone of light. The procedure of light separation may, however, represent a barrier to the students' comprehension process. Another option is given by using a small candle as a second light source or by performing this experiment with two light boxes when two student groups work together.

2 Shadows (umbra and penumbra) (Item No.: P ) Task and equipment Task No light without shadow! In the first experiment the shadow formation behind illuminated objects is investigated. The second part of this experiment is about shadow formation when an object is illuminated by two light sources.

3 Equipment Position No. Material Order No. Quantity 1 Mirror on block, 50 mm x 20 mm Light box, halogen 12V/20 W Color filter set,add.color mixt PHYWE power supply DC: V, 2 A / AC: 6 V, 12 V, 5 A Additional material: Set-up and procedure Set-up Clear film (approx. 8 cm x 8 cm) White paper (DIN A4) 1 Put the light box with its lamp side on a white sheet of paper. Fold up the end of the paper so that it forms a screen. Put an opaque object (e.g. the light bulb package) in front of the lamp. Fig. 1

4 Procedure 1. Shadow forming with one light source Connect the light box to the power supply (12 V AC) and switch it on. Fig. 2 The opaque object should be about 10 cm away from the light box. Investigate the area behind it and on the paper screen. Fig. 3 Note your observations. Move the object closer to the light source, then move it further away. How does the shadow change each time? Note your results. Instead of the opaque body, now hold the piece of foil into the light path. What happens? Write down your observation. Place the opaque object again 10 cm in front of the lamp. Trace its outline. Mark both the boundaries of the cone of light and the shadowed area with crosses.

5 Fig. off the power supply and remove the light box from the paper. 2. Shadow formation with two light sources A second light source can be created by reflecting the light from the box with a mirror. Remove one of the lateral blinds from the light box. Place the mirror next to the light box so that the light is reflected forward. Reconnect the light box to the power supply (12 V AC). Place the opaque object so that it is hit by both cones of light. Fig. 5 Precisely observe what happens behind the object, then alternately cover each of the two light sources. What happens in each case? Note your observations in Table 1 in the report. Supplementary problem By using the same setup as in experiment 2, insert a red filter in the middle slot of the light box and a green one in the lateral slot. Observe the shadow formation behind the opaque body again. First, cover one and then the other of the two slots; record your observation in Table 2 in the report.

6 Report: Shadows (umbra and penumbra) Result - Observation 1 (10 Punkte) Note your observations for all parts of the experiment mentioned in the procedure. Result - Table 1 (3 Punkte) Note your observations. Illumination Observation behind the illuminated op with 2 light sources only with central light source (lateral light source well covered) only with lateral light source (central light source well covered)

7 Result - Table 2 (2 Punkte) Note your observations on the supplementary problem. Covered filter colour Colour of the shadowed area Red 1 Green 1 Result - Observation 2 (10 Punkte) Note your observation when interchanging two filters.

8 Evaluation - Question 1 (10 Punkte) Compare your observations for the first task. Under which conditions is a shadow formed? Evaluation - Question 2 (10 Punkte) Connect all the respective pairs of crosses on your sheet for the first task; hatch the shadowy area with your pencil. Which course do the margins of the shadow take? Try to explain your observations. What is the cause for this phenomenon?

9 Evaluation - Question 3 (10 Punkte) Where can shadow formation be applied? Give an example. Evaluation - Question 4 (10 Punkte) Compare your observations for the experiment with two light sources. Attempt to explain your results.

10 Evaluation - Supplementary problem 1 (10 Punkte) Attempt to explain your observations on the supplementary problem.