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

Transcription

1 Teacher's/Lecturer's Sheet Refraction at the air-water boundary (Item No.: P064500) Curricular Relevance Area of Expertise: Physik Education Level: Klasse 7-0 Topic: Optik Subtopic: Reflexion und Brechung Experiment: Brechung beim Übergang von Luft zu Wasser Difficulty Preparation Time Execution Time Recommended Group Size Easy 0 Minutes 0 Minutes 2 Students Additional Requirements: Experiment Variations: Beaker, approx. 00 ml Keywords: Task and equipment Information for teachers Additional Information In this experiment, the students should investigate the refraction of light when it passes from air into water and measure the angle of refraction β for given angles of incidence α. In addition to developing the students' experimental skills in undertaking precise and careful experiments, making careful adjustments and measuring the angles at the optical disk, they also learn to formulate another application of the law of refraction. Investigation of the special case of α = 0 leads to the recognition that laws of physics are only valid under specific conditions. The second part of the experiment is more demanding in terms of the experimental skills of the students. Both experiments can be seen as a unit, but it is nevertheless possible to carry out the two experiments separately. Suggestions for preparation and execution In order to obtain clear and comparable results for the angle of refraction, it should be emphasised that the students carefully carry out the adjustment of the cuvette and the light box. It is particularly important that the narrow light beam always hits the centre of the optical disc and the movement of the light box during the experiment does not result in a change in the position of the cuvette. The concurrent appearance of reflection at the boundary between air and water is, indeed, of secondary importance, but the students' attention should be drawn to their (unconscious) observations of the less intense reflection from large water surfaces.

2 Refraction at the air-water boundary (Item No.: P064500) Task and equipment Task Why are reflections of objects on large water surfaces darker than the originals?. Investigate the behaviour of narrow light beams on passing from air into water. 2. Measure the angle of refraction as a function of the angle of incidence when light passes from air into water.

3 Equipment Position No. Material Order No. Quantity Optical disk Cuvette, double semicircular Light box, halogen 2V/20 W with single-slit/double-slit aperture 5 PHYWE power supply DC: V, 2 A / AC: 6 V, 2 V, 5 A Additional material Beaker, approx. 00 ml (360-0)

4 Set-up and procedure Set-up Attention In all parts of the experiment take care that the light beams emerging from the light box always travel towards the centre of the optical disk and that the position of the cuvette does not change when moving the light box. Set-up Place the optical disk in front of you onto the table and position the cuvette precisely within the markings at the intersection. The partition inside the cuvette must lie exactly on the perpendicular line. Insert the single-slit aperture in the light box on the lens side and place it at a distance of about cm away from the optical disk (Fig. ). Fig. Fill that half of the cuvette carefully with about 20 ml of water which faces away from the light box (Fig. 2). Fig. 2 Procedure. Behaviour of a narrow light beam at the boundary between air and water Connect the light box to the power supply (2 V AC) and switch it on (Fig. 3).

5 Fig. 3 Move the light box until the narrow light beam travels along the optical axis (0 -line) (Fig. 4). If the cuvette and the light box are adjusted properly, the narrow light beam will continue along the optical axis after passing through the water. Fig. 4 Now move the light box until the light beam falls on the cuvette with an angle of incidence of 40 (Fig. 5). Describe the behaviour of the narrow light beam on hitting the boundary between air and water (report). Compare the angle of incidence α with the angle between the optical axis and the refracted light beam (angle of refraction β). Fig Determination of the angle of refraction β in relation to the angle of incidence α

6 Now adjust the light box until the incident light beam is exactly at an angle of 0 to the perpendicular line of incidence (0 -line) (Fig. 6). Read the corresponding angle of refraction β and write it down in the table in the report. This is the angle between the refracted light beam and the perpendicular axis of emergence (optical axis). Fig. 6 Repeat the procedure for angles of incidence α of 0, 20, 30, 45, 60 and 75 and write down the corresponding angles of refraction β. Switch off the power supply.

7 Report: Refraction at the air-water boundary Result - Observations (0 Punkte) Note your observations on the behaviour of a narrow light beam with a slanted angle of incidence (non-perpendicular angle of incidence). comparison of the angle of incidence with the angle of refraction. Result - Table (7 Punkte) Record your measured values from the second part of the experiment. Angle of incidence α in B Angle of refraction β in B ± ± ± ± ± ± ±

8 Evaluation - Question (0 Punkte) On the basis of your observations, describe how a narrow light beam with a slanted angle of incidence behaves at the boundary between air and water. Evaluation - Question 2 (0 Punkte) Why are the reflections from the surface of larger bodies of water darker than the original?

9 Evaluation - Question 3 (0 Punkte) Compare the angle of incidence α with the corresponding value of the angle of refraction β in Table with one another. Formulate the result. Evaluation - Question 4 (0 Punkte) What result was obtained with an angle of incidence of 0B? Try to give an explanation for this result.

10 Evaluation - Question 5 (0 Punkte) On the basis of your measurements and by taking account of the experimental setup, try to formulate a principle describing the passage of light from air into water.