Task 2: The law of reflection

It is not possible to use mechanical analogy as such and work with the terms like perfectly elastic collision, kinetic energy and momentum. It is necessary to rely on empirical experience of the pupils. Eventually an improvised billiard table can be assembled – horizontal desk with a cushion on one side – and pupils try to bounce a flexible ball against the cushion under various angles. It can be considered that the experience pupils gain by doing this experiment, they have already experienced before. Based on this preparatory experiment from mechanics or based on previous experience, pupils fill in the first column of the table with the assumed course of the experiment.

After that pupils start the experiment with the light source and a mirror. We put a white piece of paper on the desk and place a mirror at the right angle. By using the laser pointer, pupils project the light ray on the paper and observe where it reflects. The laser ray is obviously narrow even in the perpendicular direction to the sheet of paper so even when placing the laser pointer literally on the level of the paper, only a part of the ray is projected. By a slight tilt or by dangling the pointer in the vertical level pupils project the closer or further part of the ray on the sheet of paper. They draw the trace of the ray on the sheet of paper with a pencil so that they gradually get the course of the whole ray both incident and reflected. (When tilting the pointer it is important not to change the point of impact of the ray on the mirror nor the placement of the pointer itself.) After pupils have marked the whole course of the ray, they draw it in the table. (Instead of a laser pointer, a light source from the school set for optics can be used, ideally laser one as well.)

They repeat the procedure for all four reciprocal positions of a light source and a mirror indicated in the assignment. It is not completely determinative if the angle in the experiment corresponds precisely to the angle in the assignment. However, what is determinative for pupils to carry out the experiment for these situations are the following: 1) the incident ray is almost parallel with the mirror ( the angle towards the level of the mirror is about 10°); 2) an incident ray makes the angle of about 45° towards the level of the mirror (so that the incident and reflected ray is more or less perpendicular); 3) the ray strikes the mirror under the angle about 70–80° (almost but not completely perpendicular); 4)the ray falls perpendicularly to the mirror. The given angles are set only for the teacher´s orientation, as pupils do not know the angles measurement yet. In addition, it is not suitable to set pupils the position of the ray from the level of mirror in numbers as in the quantitative formulation of the law of reflection is worked with the angle between the ray and a vertical.

Based on the experimental findings of courses of incident and reflected rays, pupils deduce the law of reflection in the sense that the incident and reflected rays are the same angle with the mirror. Here you can still tolerate that pupils apply an angle to the plane of the mirror, although due to the above it is preferable to try to compare the angles from the perpendicular. Since the whole experiment takes place in the plane of the sturdy paper, it is not necessary now to introduce the second law of reflection that the reflected ray remains in the plane of impact.

Notes for SEN pupils:

• Pupils with learning difficulties – completing two rows of the table is satisfactory (for example the first and the third). The conclusion about the same angle of impact and reflection should be always inferred if abilities allow.
• Gifted pupils – By using a logical deduction and by generalization of the experiment into space (= 3rd dimension), it is possible to come to the second law of reflection that the reflected ray remains in the plane of impact.