1. This work has been supported in part by the National Science Foundation Grant No. DUE-1022449 Contact information: pemigh@uw.edu Student Understanding of Blackbody Radiation and Its Application to Everyday Objects Paul J. Emigh Gina Passante Peter S. Shaffer Previous Research The explanation for blackbody radiation helped lay the foundations of quantum mechanics. However, it is often taught from a mathematical perspective that students find difficult to relate to the physical world. We are conducting research to identify student difficulties and develop interactive curriculum to address them. Introduction We asked students to describe the electromagnetic spectrum of an incandescent light bulb in the following contexts.  interviews (N=4) after a sophomore modern physics course  questions given to two upper-division populations of physics majors (N=40) We found significant difficulties across both populations, including  a tendency to confuse frequency and wavelength  an inability to locate different parts of the spectrum (e.g., visible and infrared)  a lack of ability to use blackbody radiation  struggles with the idea that temperature is the characteristic feature of this kind of radiation Future Research We are currently developing and testing curriculum aimed at extending the ideas in the online instructional homework to the classroom. This will take the form of an interactive lecture that will allow students to discuss their ideas with each other to form a more robust model of blackbody radiation as it relates to the real world. It will also help students bridge the gap between classical and quantum mechanics, which is the primary motivation for introducing this topic to modern or quantum physics courses. References 1. S.B. McKagan, K.K. Perkins, M. Dubson, C. Malley, S. Reid et al., Am. J. Phys. 76, 406 (2008). 2. http://phet.colorado.edu/en/simulation/blackbody-spectrum Online Instructional Homework We designed a one-hour online instructional homework for students (N=95) in the sophomore modern physics course. In this homework, students considered blackbody radiation from a phenomenological perspective. Some of the questions from this homework are included below. Question 2: (a) Estimate what percentage of the energy emitted by the light bulb is in the form of visible light. (b) Is a light bulb an efficient or an inefficient source of lighting? Explain your reasoning. Question 3: Some night-vision goggles can be used to distinguish objects in complete darkness (with no external sources of light). Using what you have learned, identify the conditions under which night-vision goggles can be used to distinguish between two different objects. Question 1: Which of the following have spectra that may be well-approximated by a blackbody distribution: fire, an incandescent light bulb, a star, a fluorescent light bulb, an LED, and a tree? Explain your reasoning. Incorrect responses to question 1 “Blackbody distributions are dependent on high temperatures.” “The others would not have a high enough temperature to show up in blackbody radiation.” After this question, the students were directed to the PhET simulation on blackbody radiation. 1-2 ResultsSample Student Responses Question 1 Fire60% Incandescent65% Star80% Fluorescent35% LED25% Tree30% Question 2ab Correct95%85% w/ reasoning35%65% Question 3 Correct60% w/ reasoning40% Correct response to question 2 “A light bulb is … inefficient … because the majority of the radiation is in the infrared spectrum which we cannot see.” Incorrect response to question 2 “… more than 80% of the energy emitted is in the form of heat and not light that we can see.” These responses to question 3 have incomplete student reasoning because they do not include temperature. “The different objects must emit light at the different frequencies (or energies) so that they exhibit different colors and are therefore recognized distinctly from each other.” “Night vision goggles rely on the fact that different objects emit different magnitudes of infrared radiation.” Description The following question was given as part of a hand-written pretest on the first day of class before any lecture material. Question: Sketch a possible graph for the electromagnetic spectrum of an incandescent light bulb. Only 30% of students drew something that might be interpreted as a blackbody curve, and very few labeled the spectrum or explained how they arrived at their answer. Pretest We asked a free-response exam question approximately one week following the completion of the online homework. The question asked students to draw a graph similar to the one in the pretest. It also asked students to draw the graph for a light bulb emitting less total power, and to describe any differences between the graphs. The table below summarizes the exam results, as well as the pretest results, where applicable. Exam Question Correct …Pretest (N=94) Exam (N=107) Blackbody curve*30%80% with explanation<5%20% Change in intensityN/A85% Shift in frequencyN/A40% Shift in temperatureN/A30% *includes all students who drew a curve consistent with blackbody radiation Current Research This research focuses on students in a sophomore modern physics course. This course also serves as an introduction to quantum mechanics. It consists of three 50-minute lectures per week for ten weeks. All physics majors at the University of Washington are required to take this course. There is a clear improvement from pre- to post- homework. However, a majority of students still struggle with some key features of blackbody radiation as it relates to real-world objects, particularly the key role that temperature plays. These results are consistent with additional student interviews conducted after the conclusion of the modern physics course.