BACKGROUND

The idea of this experiment is derived from Experiment 36 of Topic One in the Form 2 Science Syllabus, regarding the correction for short-sightedness and long-sightedness.
(Reference : Science Form 2, Penerbitan Pelangi Sdn. Bhd., pp 66-67)

Usually, teachers will conduct the experiment according to the method stipulated in the textbook. The set-up is as shown below:-

The above experiment needs 3 flat-bottomed flasks, 4 convex lenses of different thicknesses, 1 concave lens, fluorescent solution, a torchlight and some plasticine.

It is suggested in the textbook that this experiment be demonstrated instead of being done in groups. This is probably due to the lack of apparatus (especially the flat-bottomed flasks) and also the bulkiness and fragility of the apparatus. So, it is not feasible to let each group of young students carry out this experiment. Furthermore, students are not able to manipulate the apparatus with their hands in obtaining the images and so might not fully understand the concept involved.

OBJECTIVES

1. To provide opportunities to students to “play with lenses” and to solve problems relating to defective eyes.
2. To introduce the idea of simple method but yet effective in enhancing and deepening students’ interests.

BENEFITS FOR TEACHING-LEARNING PROCESS

1. This experiment serves as an effective activity through which students have a better conceptual understanding of long-sightedness and short-sightedness and their respective corrective measures.
2. In the process of carrying out this experiment, students generate questions and develop better thinking processes at application level.

APPARATUS/ MATERIALS

3 pieces of manila cards per group, 1 concave lens (f = 10 cm) with stand to represent the eye-lens, 1 concave lens with stand for the correction of near point, 1 convex lens with stand for the correction of far point, 2 white screens, 1 illuminated object and screen (combined) (e.g. cross-wires)

CONSTRUCTION OF TEACHING AIDS

1. The teacher makes preliminary preparations before teaching the topic, that is, choosing suitable focal lengths for the concave and convex lenses which are to be used.

2. The teacher also draws beforehand simple diagrams of the cross-section of the eye onto the manila cards (the total number of diagrams drawn will depend on the number of groups). A diagram representing a normal eye will be drawn on 1 manila card whereas the other 2 manila cards will show eyes which suffer defects.

3. The “illuminated object” is prepared using the following method :

The object (e.g. a pair of cross-wires) is fixed onto a white cardboard and is brightly illuminated by a bulb. The image (real) formed by the lens is focussed onto a white screen which is placed on the manila card at the “retina” of the eye diagram.

4.    Students are asked to set up the apparatus as shown by these pictures taken during the demonstration :

a) First, the set-up for a normal eye is used. (Picture not taken)
b) Please see 4-1P
c) Please see 4-2P
d) Please see 4-3P
e) Please see 4-4P
f) Please see 4-5P

IMPLEMENTATION

1. Teacher poses some general questions to provoke students’ thinking.

Why do some people wear spectacles? What problem causes them to wear spectacles?
What do you understand about short-sightedness and long-sightedness?
What happens to the image which is formed at the retina of a person who has eye defects?
Would you like to see what actually happens by doing an experiment?

2. Teacher divides the students into 6 to 8 groups depending on the number of students in the class (2 to 4 students per group is ideal).

3. Teacher then distributes the manila cards on which diagrams of the eye have been drawn and the position of the illuminated object have been marked to each group.

4. Students are given opportunities to focus the sharpest image formed onto the screen by moving it on the ‘eye’ which has been drawn on the manila card.

5 After the experiment, students are guided by the teacher to give conclusions about what kinds of lenses are suitable for correcting certain eye defects. So, students are trained directly through their own investigations to really voice out as well as write down their findings.

6. Teacher makes an overall conclusion by drawing diagrams on the board to show how short-sightedness and long-sightedness happens and also how they can be corrected.

SUGGESTIONS FOR MODIFICATION

In order to invoke a better thinking in the children, it is suggested that teachers write down all the questions prior to the lesson. Questions that raise the level of dialogue and thinking are those that require students to reason, synthesize and evaluate.