Transcript the Contributed Poster

Paul J. Emigh
Gina Passante
Peter S. Shaffer
Student Understanding of Blackbody Radiation and Its
Application to Everyday Objects
Introduction
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.
Previous Research
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
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.
Pretest
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.
Contact information: [email protected]
This work has been supported in part by
the National Science Foundation Grant
No. DUE-1022449
Physics
Education
Group
Online Instructional Homework
Description
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.
Results
Question 1
Fire
Incandescent
Star
Fluorescent
LED
Tree
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.
After this question, the students were directed to the PhET
simulation on blackbody radiation.1-2
Sample Student Responses
Question 2
Correct
w/ reasoning
a
95%
35%
60%
65%
80%
35%
25%
30%
b
85%
65%
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 3
Correct
w/ reasoning
60%
40%
Blackbody curve*
with explanation
Change in intensity
Shift in frequency
Shift in temperature
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.”
Future Research
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.
Correct …
“The others would not have a high enough temperature to
show up in blackbody radiation.”
“Night vision goggles rely on the fact that different objects
emit different magnitudes of infrared radiation.”
Exam Question
There is a clear improvement from pre- to posthomework. 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.
Incorrect responses to question 1
“Blackbody distributions are dependent on high
temperatures.”
Pretest
(N=94)
30%
<5%
N/A
N/A
N/A
Exam
(N=107)
80%
20%
85%
40%
30%
*includes all students who drew a curve
consistent with blackbody radiation
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