Reversing prisms (Item No.: P )

Transcription

1 Teacher's/Lecturer's Sheet Reversing prisms (Item No.: P ) Curricular Relevance Area of Expertise: Physik Education Level: Klasse 7-10 Topic: Optik Subtopic: Reflexion und Brechung Experiment: Umkehrprisma Difficulty Preparation Time Execution Time Recommended Group Size Intermediate 10 Minutes 10 Minutes 2 Students Additional Requirements: Experiment Variations: White paper (DIN A4) Ruler (approx. 30 cm) Keywords: Task and equipment Information for teachers Additional Information In this experiment, the students get to know another special case of refraction of light at a prism which has great importance for technological applications (the construction of optical instruments). The students should recognise and understand the possibility of using a right-angled prism for reversing the path of light. Through the investigation and explanation of the light path, the understanding of the law of refraction as well as of the total internal reflection is strengthened. As the experiment entails the careful observation and marking of the light path of two crossing light beams, the experiment is more demanding in terms of the abilities and experimental skills required of the students. Suggestions A careful preparation of the experiment (light incidence parallel to the optical axis) and a conscientious adherence to the suggestions for execution ensure an optimal result. In particular, one should take care that the rough side of the prism is underneath in order to make sure that the light path inside the prism is visible. The marking of the outline of the prism in the separate parts of the experiment enables the complete representation of the light path. At the same time, difficulties can arise for the students as a precise marking within the prism is not possible. A marking of the position on the hypotenuse of the prism at which total internal reflection takes place is therefore absolutely essential. In this experiment, the three-slit aperture is intentionally used in order to gain a good comparison between the refracted and the totally reflected light beams and the non-refracted light beams. These non-refracted light beams can also be faded out. Comment If the accessory for colour mixing is available, then colour filters ( ) contained in it can be used for colouring the light paths of the two incident light beams falling on the prism. The reversal of the light can be well observed under this condition. Remark Reversing prisms are, for example, used in binoculars in order to make sure that the reversed and inverted image produced by the objective appears upright and the correct way round. Incidentally, two reversing prisms, one behind the other, are required.

2 Teacher's/Lecturer's Sheet

3 Reversing prisms (Item No.: P ) Task and equipment Task A bewildering prism? Investigate the course of two light beams at a parallel incidence to the hypotenuse of a right-angled prism.

4 Equipment Position No. Material Order No. Quantity 1 Block, rectangular triangle Light box, halogen 12V/20 W with triple-slit/five-slit aperture Additional material PHYWE power supply DC: V, 2 A / AC: 6 V, 12 V, 5 A White paper (DIN A4) 1 Ruler (approx. 30 cm) 1

5 Set-up and procedure Set-up Draw in the middle of your sheet of paper crossed lines at exactly right angles to each other. Lay the prepared sheet crosswise on the table in front of you. Place the right angled block (rough side down) with the hypotenuse on the horizontal line of the ruled, crossed lines (Fig. 1). Carefully mark the outline of the prism with fine pencil lines. Fig. 1 Insert the three-slit aperture in the light box on the lens side and position it about 5 cm away from the cathetus of the prism at the edge of the sheet of paper (Fig. 2). Fig. 2 Procedure Connect the light box to the power supply (12 V AC) and switch it on.

6 Fig. 3 Move the light box until the middle beam is about 3 mm away from and parallel to the optical axis. The lower light beam travels unrefracted below the hypotenuse of the prism (Fig. 4). Observe the course of the two narrow light beams incident to the prism inside and outside the prism. Note your observation for each case in the report. Fig. 4 Now fade out the middle light beam in front of the prism (Fig.5). Observe and note the result (report). Fig. 5 Mark the course of the upper light beam in front of and behind the prism with two crosses. Also mark the point at which this light beam hits the hypotenuse. Now fade out the upper light beam in front of the prism (Fig. 6). Observe and note the results in the report. Mark again the ligth beam as above, but use another colour.

7 Fig. 6 Switch off the power supply.

8 Report: Reversing prisms Result - Table 1 (4 Punkte) Note your observations in the table. Observation of the light path Light path in front of and behind the prism Light path inside the prism Fading out the middle light beam Fading out the upper light beam Evaluation - Question 1 (10 Punkte) Connect the crosses which are related to one another so that the course of the light beam outside and also, after making the appropriate connections, inside the prism gets visible. Compare this again with your observations on the course of the light beams (Table 1).

9 Evaluation - Question 2 (10 Punkte) What statements can be made about the position of the light beams in relation to the optical axis in front of and behind the prism? Evaluation - Question 3 (10 Punkte) Formulate a principle about the result of this experiment.

10 Evaluation - Question 4 (10 Punkte) What phenomenon appears inside the prism? Try to give an explanation for the path of the narrow light beam. Evaluation - Question 5 (10 Punkte) A prism such as that used in this experiment is called a reversing prism. Why?