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Asia-Pacific Forum on Science Learning and Teaching, Volume 6, Issue 1, Article 1 (June, 2005) Peter HUBBER Explorations of Year 10 students’ conceptual change during instruction
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Asia-Pacific Forum on Science Learning and Teaching, Volume 6, Issue 1, Article 1 (June, 2005) Peter HUBBER Explorations of Year 10 students’ conceptual change during instruction
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Implications for teaching geometrical optics
The existence of an alternative framework, such as that described above, has implications for the teaching and learning of the scientific concepts related to the framework. The coherence with which a student holds several alternative conceptions may make it difficult to change any one of the alternative conceptions. However, a change in thinking about any one of the alternative conceptions will bring into conceptual conflict the understanding of the other related conceptions in the mind of the student. This study found that the addressing of the students' alternative conception related to vision was profitable in terms of changing the students' views related to other areas in respect of their alternative framework. There was evidence of the students using their newly constructed conception of vision in arguing for the scientific concept of light propagation. For example, the students argued that light from a dim light source travels away from the source as it can be seen from some distance away. The students' newly constructed understanding of vision also brought into conflict their views about the colour of objects. However, instead of accommodating the scientific concept the students assimilated their understanding of vision into their existing understanding of coloured objects. This produced a hybrid alternative conception where the reflected light that enters the observer's eyes is coloured through interacting with the observed object.
In terms of the teaching of geometrical optics at Middle School an initial approach would be to elicit and address any alternative conceptions that relate to vision. A scientific understanding of the vision process allows for the use of the eye as a light detector in discussions and activities involving other concepts. The idea that to see an object implies light is entering the eye gives plausibility to concepts involving rectilinear propagation, diffuse reflection, colour and image formation in mirrors and lenses. For example, a plausible argument can be made whereby the ability to see a point on an object from many directions implies that light is reflecting in many directions from the point. As light enters the eye from an observed object it seems plausible that information about the colour of the object is related to the colour of the reflected light. Similarly, the observed image in a plane mirror has a relationship to the reflected light that enters the eye. The exploration of the wider applications of the vision concept enhances the construction of a framework of scientific ideas and at the same time addresses contextualized thinking. The exploration of the wider applications of other concepts, such as the reflection rule, also proves successful in consolidating scientific ideas.
The difficulties in changing the students’ views related to the emission of light from luminous objects, refraction, colour of objects and image formation in plane mirrors and convex lenses has implications for the teaching of these areas, particularly to addressing the possible factors that give rise to persistent difficulties.
An implication for the teaching of the key concept of isotropic emission of light from each point on a luminous object is for this concept to be reinterpreted to a more fundamental concept that luminous objects are composites of point sources of light. The teaching and learning strategies developed by the teacher in the area of emission of light from luminous objects should then focus on this key concept. In addition, students’ consolidation of this concept needs to precede the introduction of ideas about image formation in lenses and mirrors.
An implication for the teaching and learning of refraction may be that students require a range of situations to predict the light path through transparent materials. These situations should include: (a) the passage of light as it enters and leaves transparent materials, (b) different angles of incidence, (c) curved interfaces, and (d) different combinations of transparent materials. Given the multitude of different situations, students may find that the application of a teaching model, or their own model, provides a more useful mechanism for predicting the light path than remembering a number of rules, each one applicable to a specific situation. The model emphasises that a change in speed occurs, which defines the phenomenon of refraction, rather than a rule that emphasises a change in direction.
Teaching and learning strategies that consolidate the understanding the concept light from luminous objects such as the sun is composed of different coloured light would make it a key concept necessary to be understood before addressing the classroom discussions about the perceived colour of objects. From a teaching and learning perspective care needs to be taken in developing scientific ideas about colour. The teacher needs to look continually for evidence of students harbouring the alternative conception that colour is an intrinsic property of an object. Such monitoring is particularly important where students are required to use a simple colour theory to predict the apparent colour of objects in different lighting conditions.
From a curriculum perspective the scientific model of a ray is important to the explanations of a whole range of optical phenomena within the topic of geometrical optics. However, this study found the students' conceptual model of a ray as a physical entity of light presented a barrier to a conceptual understanding of some key concepts. Given the central place ray diagrams play in the explanation of a whole range of optical phenomena, then a focus on the ray scientific model within the classroom needs to be given in conjunction with ideas about the nature of light. To not attend to ideas about the nature of light at all, a common occurrence in many secondary school geometrical optics courses, leaves the student with little option but to construct a view of light as consisting of rays. That is of course if the student has not already constructed this view before instruction, given the everyday use of the term 'ray'. For many students a course in geometrical optics may reinforce pre-instructional conceptual models about the ray as a constituent of light. Further study in optics following a geometrical optics course leads to ideas about the nature of light in courses described as physical optics and quantum ideas. Students entering such courses may then have well entrenched non-scientific conceptual models of the nature of light. Curriculum planners and teachers need to be well aware of the possibility of this situation arising.
The change in thinking from a ray as a physical entity to a ray as a graphical representation represents a conceptual change across ontological categories and may be difficult to achieve (Chi, 1992). In describing the historical change in thinking about the ray, Galili and Lavrik (1998) suggest, "Light rays complete a dramatic metamorphosis from the concept of central importance, the essence of light, to an auxiliary tool of semantic nature, a subordinate notion" (p. 603). Reinterpreting the ray concept for the students leaves them with a void in terms of perceptions about the nature of light. One may then argue that a teaching sequence that focuses on a scientific understanding of a ray needs to go hand in hand with discussions on the development of ideas about the nature of light, namely the scientific models of the nature of light. For example, light spreading from a point source can be represented by water waves spreading from a single disturbance. The waves have an associated ray that gives the direction of propagation of the waves. Light reflection can be represented by a ball bouncing off a wall. The ray gives the direction of propagation of the ball.
Classroom discussion about the scientific understanding of the term 'ray' needs to be contrasted with the everyday usage of the term and inappropriate usage in some textbooks. Students need to appreciate that there are times when scientific meanings to terms differ from their everyday usage and that it is quite appropriate to attach different meanings of such terms depending on the context of the terms' use. While Galili and Lavrik (1998) argue for the replacement of the classroom teaching of a ray with a flux idea to offset misinterpretations of the ray the use of the ray scientific model allows the use of ray diagrams to explain a whole range of optical phenomenon. The success of the ray model to explain and make predictions in geometrical optics is very compelling as distinct from other areas of physics. For example, Newton's Laws do not readily explain everyday observations relating to forces and motion unless frictional effects are taken into account.
If ideas about the nature of light through the scientific models of light are incorporated into the Middle School optics curriculum then as each new phenomenon of light is explored an appropriate scientific model is used to explain the observations made. A curriculum emphasis during the teaching period of geometrical optics on the nature of light and its accompanying scientific models may make it a matter of course for students to think about refraction in terms of the application of a model.
From a teaching and learning perspective students' alternative conceptions associated with the emission of light from luminous objects and understandings of the ray need to be addressed before the teaching of image formation in mirrors and lenses. In addition, rays diagrams to explain image formation should be considered incomplete if they do not include multiple rays from points on the object. This reinforces the concept that a flux of light emanating from each object point contributes to the production of the image. The inclusion of an eye in the diagram is also important, particularly in cases where a virtual image is formed.
Copyright (C) 2005 HKIEd APFSLT. Volume 6, Issue 1, Article 1 (June, 2005). All Rights Reserved.
