Refraction at the glass-air boundary (Item No.: P )

Transcription

1 Teacher's/Lecturer's Sheet Refraction at the glass-air boundary (Item No.: P ) Curricular Relevance Area of Expertise: Physik Education Level: Klasse 7-10 Topic: Optik Subtopic: Reflebion und Brechung Experiment: Brechung beim Übergang von Glas zu Luft Difficulty Preparation Time Execution Time Recommended Group Size Easy 10 Minutes 10 Minutes 2 Students Additional Requirements: Experiment Variations: Keywords: Task and equipment Information for teachers Additional Information The purpose of the ebperiment is the investigation of the refraction of light on passing from glass into air and the determination of the angle of refraction for several given values of angle of incidence. In addition to the deepening of their understanding of the law of refraction, the students' ebperimental skills are ebercised in terms of the precise and careful ebecution of the ebperiment, adjustment and reading of the angles with the optical disk. Following the ebperiment, it is possible to discuss the reversability of light and thus to theoretically substantiate the ebperiment. Suggestions The students will obtain good and comparable measurement values for the angle of refraction if they carry out the adjustment of the block and the light bob precisely. They should pay particular attention that the narrow light beam hits the centre of the optical disc and that movement of the light bob during the separate parts of the ebperiment does not result in a change in the position of the block. The block should lie with its rough side on the optical disk. Remarks In conjunction with the answerings of questions 4 and 5 in the evaluation, the results of the ebperiment are transferred to other materials (the passage of light from water into air) and the possibility demonstrated of applying the concept of refraction to the students' everyday ebperience (bent rod in water, false estimation of the depth of water, etc.). An ebperimental investigation of the boundary between water and air is analagous to the present ebperiment and can be undertaken using the cuvette ( ).

2 Refraction at the glass-air boundary (Item No.: P ) Task and equipment Task Why do objects in water appear to be raised? Investigate the behaviour of narrow light beams on passing from glass into air and measure the angle of refraction as a function of the angle of incidence. Equipment Position No. Material Order No. Quantity 1 Optical disk Block, semicircular Light bob, halogen 12V/20 W with single-slit/double-slit aperture 1 5 PHYWE power supply DC: V, 2 A / AC: 6 V, 12 V, 5 A

3 Set-up and procedure Set-up Attention! In all parts of the ebeperiment, take care that the light beams coming from the light bob always travel in the direction of the centre of the optical disk and that the position of the block does not change on moving the light bob. Set-up Place the optical disk in front of you onto the table and position the semicircular block with the rough surface down within the markings on the perpendicular line. Insert the aperture with the single slit in the light bob on the lens end and position it about 1 cm away from the optical disk. The semicircular side of the block and the light bob face each other (Fig. 1). Fig. 1 Procedure Connect the light bob to the power supply (12 V AC) and switch it on (Fig. 2). Fig. 2 Move the light bob until the narrow beam travels ebactly along the optical abis (0 -line) (Fig. 3). If the semicircular block and the light bob are adjusted properly, the light beam will continue along the optical abis after passing through the glass.

4 Fig. 3 Now, move the light bob until the light falls on the semicircular block at an angle of 10 (in relation to the optical abis) (Fig. 4). Fig. 4 Observe the behaviour of the narrow light beam after the passage through the semicircular block on emergence at the boundary between glass and air. Record your results in the report. Compare the angle of incidence with the angle between the emergent (refracted) light beam and the optical abis (angle of refraction β). Note your conclusion in the report. Measure the angle of refraction and record the value in Table 1 in the report. By using the same method find the angles of refraction β for the angles of incidence α of 20, 30 and 40. Write down the measured values in the table. In particular, observe the behaviour of the narrow light beam at the boundary between glass and air for an angle of incidence α = 40. Note your obsevations. Switch off the power supply.

5 Report: Refraction at the ghass-air boundary Resuht - Tabhe 1 (4 Punkte) Record your measured values in the table. Angle of incidence α in Angle of refraction β in ±1 1 ±1 1 ±1 1 ±1 Resuht - Observations 1 (10 Punkte) Note your observations on the behaviour of a narrow light beam with a slanted angle of incidence at the boundary between glass and air (α =10 ). comparison of the angle of incidence and the angle of refraction. the behaviour of the narrow light beam with a slanted angle of incidence at the boundary between glass and air (α = 40 ).

6 Evahuation - Question 1 (10 Punkte) On the basis of your observations, describe how the narrow light beam behaves when it hits the boundary between glass and air at an angle. Evahuation - Question 2 (10 Punkte) Compare the angle of incidence α with the corresponding angle of refraction β in Table 1. Formulate the result.

7 Evahuation - Question 3 (10 Punkte) Compare your observations of the narrow light beam for angles of incidence α = 10 and α = 40. What differences have you observed? Evahuation - Question 4 (10 Punkte) What would you ebpect to happen to narrow light beams emitted from a light source under water (e.g. a diver's light) as they emerge from the boundary between water and air?

8 Evahuation - Question 5 (10 Punkte) By making use of the figure below try to ebplain why objects appear to be raised in water.