Functioning of the human eye (normal vision)

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1 Teacher's/Lecturer's Sheet Functioning of the human eye (normal vision) (Item No.: P ) Curricular Relevance Area of Expertise: Physik Education Level: Klasse 7-10 Topic: Optik Subtopic: Das Auge Experiment: Funktionsweise des menschlichen Auges (Normalsichtigkeit) Difficulty Preparation Time Execution Time Recommended Group Size Intermediate 10 Minutes 10 Minutes 2 Students Additional Requirements: Experiment Variations: White paper (DIN A4) Compasses Ruler (approx. 30 cm) Beaker, approx. 100 ml Keywords: Task and equipment Information for teachers Additional Information The experiments on accommodation in the human eye and on the defects of vision and their correction are appropriate for showing the application and validity of the laws of physics in a biological context. In this experiment, the students should apply their knowledge about the light path in convex lenses and the changes in focal length for lenses differing in thickness (combinations of lenses) to the human eye. In the first part of the experiment, image formation on the retina is considered using parallel incident light and in this way the position of objects at infinity is simulated. Starting from observations of light incident to the optical axis at a slight angle, image formation on the retina can be explained. By changing the lens in the second part of the experiment, a sharp image on the retina is also observed with divergent incident light (simulated position of the object near the lens). The experiment is demanding in terms of abilities and experimental skills required of the students. On the other hand, due to its motivational value in the consideration of questions related to physics, it has a highly significant worth. Suggestion With a careful adherence to the suggestions concerning preparation (position of the arc and cuvette adjustment), an optimal experimental result is to be expected. The upward folded edge of the paper serves to prevent an extension of the light path beyond the focal point, which is uninteresting in terms of the objective of the experiment. To further improve the model eye, a narrow strip of paper can be glued on the arc of the circle perpendicular to the sheet, to represent the retina. The angle of incidence of the light rays should not be greater than 10 as, otherwise, the focal point no longer lies on the circle arc. If a syringe (syringe, 20 ml, from ) is available, then the procedure for reducing eye curvature and thereby the eye s accommodation for distant objects can be simulated by siphoning off water. Remark

2 Teacher's/Lecturer's Sheet The advantage of the described experimental setup, in contrast to that with an optical bench, lies in the better approximation to the actual processes in the eye, which should be subject of the lesson before experiment. The elastic change in the curvature of the crystal lens and the associated sharp focusing of the image on the retina is brought about by the ciliary muscles (circular muscles). The average arc radius varies between 10.4 mm with the non-contracted ciliary muscle and 5.7 mm with complete accommodation to object located close to the eye.

Functioning of the human eye (normal vision) (Item No.: P1066700) Task and equipment Task How are we able to see both near and far objects with our eyes?

3 Functioning of the human eye (normal vision) (Item No.: P ) Task and equipment Task How are we able to see both near and far objects with our eyes? Investigate the path of light in the human eye with the help of a model when: 1. Focusing the eye (accommodation) on a far away object. 2. Focusing the eye on a near object.

Equipment Position No. Material Order No. Quantity 1 Cuvette, double semicircular 09810-06 1 2, 3 Light box, halogen 12V/20 W 09801-00 1 Additional material PHYWE power supply DC: 0.

4 Equipment Position No. Material Order No. Quantity 1 Cuvette, double semicircular , 3 Light box, halogen 12V/20 W Additional material PHYWE power supply DC: V, 2 A / AC: 6 V, 12 V, 5 A White paper (DIN A4) 1 Compasses Set-up and procedure Set-up Attention! Ruler (approx. 30 cm) 1 Beaker, approx. 100 ml Take care that the position of the cuvette is not changed on moving the light box. Set-up Prepare your sheet of paper as shown in the picture. Draw crossed lines at right angles (intersection point M) at a distance of 8.5 cm from the right edge and make two marks, each at a distance of 3 cm from M, on the perpendicular line. Draw a semicircle from M with a radius 7.5 cm. The intersection point with the optical axis is F. The semicircle represents the retina in your model eye.

Fig. 1 Fold the paper 1 cm from the right edge to form a screen. The point F lies in the fold.

The dividing wall within the cuvette must be placed at right angles to the optical axis, i.e. on the perpendicular line.

2 Insert the three-slit in the light box on the lens side and position the light box about 1 cm

5 Fig. 1 Fold the paper 1 cm from the right edge to form a screen. The point F lies in the fold. Position the cuvette precisely within the markings on the crossed lines. The dividing wall within the cuvette must be placed at right angles to the optical axis, i.e. on the perpendicular line. The cuvette represents the lens of the eye in your model. Fig. 2 Insert the three-slit in the light box on the lens side and position the light box about 1 cm from the left-hand edge of the sheet. Fill the half of the cuvette facing the light box carefully with approximately 20 ml of water. Fig. 3 Procedure

1. Focusing the eye on a far-away object.

$unrefracted through the cuvette. If neccessary, move the cuvette slightly along the perpendicular line.$ Fig. 5 With thin pencil lines mark the outline of the cuvette carefully without moving it. Fig.

Fig. 5 With thin pencil lines mark the outline of the cuvette carefully without moving it. Fig.

6 1. Focusing the eye on a far-away object. Connect the lightbox to the power supply (12 V AC) and switch it on. Fig. 4 Adjust the light box until the middle light ray falls precisely along the optical axis and travels unrefracted through the cuvette. If neccessary, move the cuvette slightly along the perpendicular line. Fig. 5 With thin pencil lines mark the outline of the cuvette carefully without moving it. Fig. 6 Observe the path of the light on passing through the cuvette and on arrival at the screen. Note your observations in the table in the report. Mark the incident and refracted light beams with two crosses each.

Fig. 7 Now move the light box until it falls at a slight angle to the optical axis, downward on the cuvette.

lines M. Again, observe the path of the light, in particular the position of the focal point in relation to F.

Note your observations. Fig. 9 2. Focusing the eye on nearby objects.

7 Fig. 7 Now move the light box until it falls at a slight angle to the optical axis, downward on the cuvette. The middle light beam must, at the same time, travel further exactly in the direction of the intersection of the crossed lines M. Again, observe the path of the light, in particular the position of the focal point in relation to F. Note your observations again in the report. Fig. 8 Repeat the last step with the light box inclined slightly upwards. Note your observations. Fig Focusing the eye on nearby objects. Rotate the light box by 180 and remove the aperture so that now the diverging light falls on the cuvette. The position of the light box is identical to that in the first part of the experiment.

Observe the path of light behind the cuvette, in particular on striking the screen. Note your observations. Fig.

Observe the change in the light course and note the result in the table in the report. Fig.

direction of the optical axis on the completely filled cuvette.

8 Observe the path of light behind the cuvette, in particular on striking the screen. Note your observations. Fig. 10 Now carefully fill the other half of the cuvette also with approximately 20 ml of water. Observe the change in the light course and note the result in the table in the report. Fig. 11 Again rotate the light box through 180 so that the parallel light (with the three-slit) again falls in the direction of the optical axis on the completely filled cuvette. Observe the course of the light, in particular the position of the focal point and the image on the screen. Note your results. Fig. 12 Switch off the power supply and remove the light box and the cuvette from the paper.

9 Report: Functioning of the human eye (normal vision) Result - Table 1 (6 Punkte) Note your observations in the table. Experimental conditions Observations Parallel light along the optical axis Parallel light, light box in position 2 Parallel light, light box in position 3 Divergent light Filling water in the second half of the cuvette Parallel light along the optical axis and on the completely filled cuvette Evaluation - Question 1 (10 Punkte) Join the crosses which belong together on your sheet of paper so that the path of the light rays outside and, after making the corresponding connections, also within the cuvette are visible. Compare the course of parallel light on passing through the left-half side of the cuvette filled with water with that of the light path through the convex glass lenses.

10 Evaluation - Question 2 (10 Punkte) Compare your observations on the course of the parallel light on striking at the side of the cuvette filled with water in the table. What have you found in common? Evaluation - Question 3 (10 Punkte) Virtually parallel light is emitted by object points far away from the eye. Formulate a statement about the course of light in the human eye from far away objects.

11 Evaluation - Question 4 (10 Punkte) What is changed when the cuvette illuminated by divergent light is completely filled with water? Evaluation - Question 5 (10 Punkte) Divergent light reaches the eye from nearby objects. How does the eye adapt to this object position?

12 Evaluation - Question 6 (10 Punkte) What conclusion can be drawn from results of the last experiment (parallel light falling on the completely full cuvette) if the result is carried over from the model to the eye? Evaluation - Supplementary problem 1 (10 Punkte) What conclusion is suggested about the image on the retina when you take into account the result of the experiment in which the light falls at an angle from above or below the cuvette?

Long-sightedness and its correction (Item No.: P )

Teacher's/Lecturer's Sheet Long-sightedness and its correction (Item No.: P1066900) Curricular Relevance Area of Expertise: Physik Education Level: Klasse 7-10 Topic: Optik Subtopic: Das Auge Experiment: