Dependence of Light Intensity on Distance
SPN LESSON #25
TEACHER INFORMATION
LEARNING OUTCOME:
After determining through measurement the relationship between the
intensity of a point light source and the distance from the light source, students are able to make
predictions on the basis of the inverse square law and relate that law to other phenomena such as
gravitational and electrical forces.
LESSON OVERVIEW:
The purpose of this lesson is for students to determine the relationship
between the intensity of a point light source and the distance from the light source. Measuring
this relationship gives students experience with the “inverse square” relationship that also
characterizes the gravitational force between two point masses and the electric force between
two point charges. Data from SPN lesson #24,
Efficiency of Energy Conversion
, can be used in
this lesson. Two versions of this lesson are provided in the student section; which one you use
depends on the equipment you have available. Version One uses TI83+/LabPro and Version
Two uses TI83/CBL.
GRADELEVEL APPROPRIATENESS:
This Level III Physical Setting lesson is designed for
use with students enrolled in a high school physics or physical science class.
MATERIALS
Version One:
Student handout (Version One), meterstick, 40 W incandescent light bulb and
fixture, TI83+ graphing calculator, Vernier LabPro unit
Version Two:
Student handout (Version Two), meterstick, 40 W incandescent light bulb and
fixture, TI83 graphing calculator, TI CBL unit, TI light probe
SAFETY:
Students need to take the usual precautions for handling electric appliances in setting
up the bulb.
TEACHING THE LESSON:
Display a lighted light bulb in a fixture on a lab table. Given the
supposition that the intensity of the light is “
x
” at a distance of 1 meter, ask students to predict
what the intensity of the light will be at a distance of 2 meters (as a fraction of
x
). Explain to
students how they will be carrying out an investigation to test their prediction. If it is appropriate,
in conjunction with previous learning about the dependence of gravitational and electric force on
distance between point masses and/or charges, point out that the same relationship applies
between those forces and the distance between objects.
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View Full DocumentACCEPTABLE RESPONSES FOR DEVELOP YOUR UNDERSTANDING SECTION:
The following data and calculations were obtained with a TI83+/LabPro system:
Version One:
Distance from center of
bulb (m)
Intensity (W/m^2)
0.4
4.5265
0.5
4.4366
0.6
2.7715
0.7
2.2618
0.8
1.7302
0.9
1.3297
1.0
1.0554
These data fit a power regression on the TI83+ of the following form:
Intensity = 1.15*distance^1.686.
If the exponent equaled –2, the relationship would have been exactly that of intensity varying as
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 Spring '08
 NA
 Light, Distance, Incandescent light bulb, Fluorescent lamp, Compact fluorescent lamp, light probe

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