Like any wave, light propagates outward from its source location at the same speed in all directions. The intensity of the propagating light is greatest closest to the source and gets weaker at further and further distances from its source. One can imagine that the rate at which energy lands upon a given surface as being inversely proportional to the square of the distance R (or d) from the source. As such, light intensity upon a surface follows the inverse square law. This intensity is also dependent upon the power of the light source - the rate at which it emits energy. This Concept Builder forcuses upon the relationship between light intensity, distance from the source, and power of the source.

The term illiuminance is used to describe the rate at which light lands upon a given area of a surface some distance d away. Illuminance is directly proportional to the power of the source and inveresely proportional to the distance between the source and the surface upon which illuminance is measured. Since the unit Watts (W) is the unit of power and meters (m) is the unit of distance, the unit W/m² is a common unit of illuminance. In this Concept Builder, the SI unit lux is used as the unit of illuminance.

This Concept Builder was intended as an in-class activity. After some early lab work, some discussion of the illuminance concept, and some discussion of the proportionalities involved in ithe concept, allow students an opportunity to interact with the questions. This Concept Builder consists of three separate activities, with each activity focusing on a unique aspect of the illuminance relationship. The first activity - Power Up - explores the relationship between the power of the light bulb and the illuminance. It includes three simple questions. The second activity - Distance Matters - explores the relationship between the illuminance and the separation distance between the source and the surface. This activity includes five questions that require the use of the inverse square law. The third activity - Putting It All Together - includes four questions that combine both variable relationships.Teachers using the Concept Builder with their classes should preview the activity (or view the Questions in the separate file) in order to judge which levels would be most appropriate for their students. There is no redundancy from one activity to another activity. That is, all the questions in the third activity are unique to that activity.  

In order to complete an activity, a student must correctly analyze each question in that activity. If a student's analysis is incorrect, then the student will have to correctly analyze the same or very similar question twice in order to successfully complete the activity. This approach provides the student extra practice on questions for which they exhibited difficulty. As a student progresses through an activity, a system of stars and other indicators are used to indicate progress on the level. A star is an indicator of correctly analyzing the question. Once a star is earned, that question is removed from the que of questions to be analyzed. Each situation is color-coded with either a yellow or a red box. A red box indicates that the student has incorrectly analyzed the question and will have to correctly analyze it twice before earning a star. A yellow box is an indicator that the question must be correctly analyzed one time in order to earn a star. Once every question in an activity has been analyzed, the student earns a trophy which is displayed on the Main Menu. This system of stars and trophies allows a teacher to easily check-off student progress or offer credit for completing assigned activities.

The most valuable (and most overlooked) aspect of this Concept Builder is the Help Me! feature. Each question group is accompanied by a Help page that discusses the specifics of the question. This Help feature transforms the activity from a question-answering activity into a concept-building activity. The student who takes the time to use the Help pages can be transformed from a guesser to a learner and from an unsure student to a confident student. The "meat and potatoes" of the Help pages are in the sections titled "How to Think About This Situation:" Students need to be encouraged by teachers to use the Help Me! button and to read this section of the page. A student that takes time to reflect upon how they are answering the question and how an expert would think about the situation can transform their naivete into expertise. 

 

 

There are only a few resources at The Physics Classroom website that serve as very complementary supports for the Light Intensity Concept Builder. These include:
 

Additional resources and ideas for incorporating this Light Brightness Concept Builder into an instructional unit on Light and Color can be found at the Teacher Toolkits section of The Physics Classroom website.  Visit Teacher Toolkits.