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

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1 Teacher's/Lecturer's Sheet Long-sightedness and its correction (Item No.: P ) Curricular Relevance Area of Expertise: Physik Education Level: Klasse 7-10 Topic: Optik Subtopic: Das Auge Experiment: Weitsichtigkeit und ihre Korrektur Difficulty Preparation Time Execution Time Recommended Group Size Easy 10 Minutes 10 Minutes 2 Students Additional Requirements: Experiment Variations: White paper (DIN A4) Compasses :uler (approb. 30 cm) Keywords: Task and equipment Information for teachers Additional Information On a model of the long-sighted eye (shorter distance from the retina to the eye lens), the students should ebperimentally investigate the effect of long sightedness on seeing distant and near objects. In this way, not only is their understanding of the light path through a conveb lens and lens combinations (in particular, with reference to the change in focal length) refreshed, but also possibilities are demonstrated for correcting this defect. The basis for the ebperiment was already laid down in the ebperiment on short-sightedness. Here, also, the ebtrapolation from the level of the model to the construction of the real eye should be made easier with the aid of the teacher. Both ebperiments are also appropriate for investigation in separate groups (two work groups each investigating a different type of visual defect; at the end, the results are presented and discussed together). Suggestion Although the preparation of the sheet for the ebperiment is not simple (amongst other things, the circular arc from S as the retina of the short-sighted eye), with conscientious observation of the instructions optimal ebperimental results can be ebpected. For determining the position of all the focal points on the optical abis, ebact adjustment of the semicircular conveb lens which serves as the eye lens, but also the symmetrical positioning of the light bob and the supplementary lens in relation to the optical abis is essential. With small, vertical movements of the light bob or the lenses, this can easily be achieved. The marking of the path of the light beams serves the subsequent discussion of the light path for distant objects following correction of the farsighted eye. The observation of divergent light is made using the three-slit held in front of the eye lens, in order that a definite intersection point for the light beams can be observed. The position of the aperture also has an influence on the location of the intersection point as thick lenses have different focal lengths for light beams close to and far away from the optical abis. The prescribed distance for the eye lens should therefore be observed. Because of the short distance from the retina to the eye lens, a planoconveb lens must be placed directly in front of the eye lens to achieve accommodation. Remark As the eye of far-sighted individuals is already accommodated for seeing distant objects, the ability to accommodate to near objects is inadequate. Objects at a distance of 30 cm and also up to a distance of 1 m can no longer be sharply focused. If the ability of the eye to accommodate is absent (accommodation defect associated with old age) then spectacles with divided lenses are necessary; very strong conveb, domed for the near region, less strongly curved for the far region. With this knowledge of the causes of defective vision, a contribution to the understanding of the problems of these strongly handicapped people in everyday life is made possible.

2 Teacher's/Lecturer's Sheet

Long-sightedness and its correction (Item No.: P1066900) Task and equipment Task What is long-sightedness and how can it be corrected?

3 Long-sightedness and its correction (Item No.: P ) Task and equipment Task What is long-sightedness and how can it be corrected? Investigate with the model eye what effect long-sightedness can have on seeing far away objects and by what means it can be corrected.

4 Equipment Position No. Material Order No. Quantity 1, 2 Block,planoconveb lens,fl+100mm Block, semicircular , 5 Light bob, 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 :uler (approb. 30 cm) 1 Take care that the semicircular eye lens always lies with its plane surface on the perpendicular of the crossed lines and that its adjusted position does not change on moving the light bob. Setup Prepare a sheet of paper as shown in the picture. Draw perpendicular lines at distances of 10 cm and 23 cm from the righthanded edge of the paper (the intersection points of the lines are M and P).

Fig. 1 Draw a semicircle from M with a radius of 3 cm. The intercept with the optical abis is S. Hatch this semicircular area. Fig. 2 Make two marks, 3 cm above and below P, on the perpendicular line.

5 Fig. 1 Draw a semicircle from M with a radius of 3 cm. The intercept with the optical abis is S. Hatch this semicircular area. Fig. 2 Make two marks, 3 cm above and below P, on the perpendicular line. Furthermore, draw a second arc, but from S and with a radius of 4.5 cm. The intercept with the optical abis is F. This arc represents the retina of the long-sighted eye in the model. Fig. 3 Insert the three-slit in the light bob on the lens side and position the light bob as shown.

Fig. 4 Procedure 1. Seeing distant objects.

$optical abis and passes unrefracted through the lens.$ If neccessary, carefully move the lens slightly along the perpendicular

6 Fig. 4 Procedure 1. Seeing distant objects. Connect the light bob to the power supply (12 V AC) and switch it on. Fig. 5 Move the light bob until the middle light beam falls precisely along the optical abis and passes unrefracted through the lens. If neccessary, carefully move the lens slightly along the perpendicular line. Fig. 6 Observe the path of the parallel light rays following passage through the semicircular conveb lens, in particular the

position of the focal point in relation to F. Note your observations in the table in the report. Mark the focal point of the parallel incident light with F1.

$7 Mark the outline of the lens and the incident and refracted light beams in front of, between and behind the lenses with two crosses each. Fig. 8 :emove the planoconveb lens from the paper. 2.$

7 position of the focal point in relation to F. Note your observations in the table in the report. Mark the focal point of the parallel incident light with F1. Place the narrow planoconveb lens in front of the semicircular lens. Again observe the and describe the path of the light, in particular the position of the focal point. Move the thin planoconveb lens a little. To which point can you shift the convergence point of the incident light? Note your observations. Fig. 7 Mark the outline of the lens and the incident and refracted light beams in front of, between and behind the lenses with two crosses each. Fig. 8 :emove the planoconveb lens from the paper. 2. Seeing near objects :otate the light bob through 180 and remove the aperture so that the divergent light falls on the domed side of the lens. Move the lightbob to the perpendicular line (point P) so that it is with in the markings.

Fig. 9 Hold the three-slit approbimately 5 cm in front of the eye lens so that the

Observe and describe the path of the light behind the lens.

8 Fig. 9 Hold the three-slit approbimately 5 cm in front of the eye lens so that the middle light beam is incident along the optical abis. Observe and describe the path of the light behind the lens. Place the thin planoconveb lens directly in front of the eye lens and note your observations again in the report. Fig. 10 Place the second planoconveb lens at a position marked in the first part of the ebperiment and note your observations. Fig. 11 Switch off the power supply and remove the light bob and the lenses from the paper.

9 Report: Long-sightedness and its correction Result - Table 1 (6 Punkte) Note your observations in the table. Ebperimental conditions Observations 1. Parallel light along the optical abis 1 Parallel light, supplementary lens between the light bob and eye lens 1 Setup as before, shifting the supplementary lens 1 2. Divergent light, position of the light bob at P 1 Setup as before, supplementary lens directly in front of the eye lens (accomodation) Setup as before, supplementary lens in front of the eye lens (spectacles) 1 1 Evaluation - Question 1 (10 Punkte) Compare the position of F (intersection point of the arc form S with the optical abis) with the position of focal point F 1.

10 Evaluation - Question 2 (10 Punkte) Object points far away from the eye give out virtually parallel light. Formulate a statement about the course of the light in the long-sighted eye for distant objects. Evaluation - Question 3 (10 Punkte) Connect the crosses which belong together on your sheet of paper so that the path of the light beams through the lens are visible. What changes when a planoconveb lens is placed in the light path?

11 Evaluation - Question 4 (10 Punkte) Divergent light reaches the eye from nearby objects. Can the far-sighted eye form a sharp image of near objects? Evaluation - Question 5 (10 Punkte) Which possibility is there for the far-sighted person to remedy his eye defect and allow him to see both distant and nearly objects?

Functioning of the human eye (normal vision)

Teacher's/Lecturer's Sheet Functioning of the human eye (normal vision) (Item No.: P1066700) Curricular Relevance Area of Expertise: Physik Education Level: Klasse 7-10 Topic: Optik Subtopic: Das Auge