Lab
Illuminance by a Light Source
Printer Friendly Version
The
purpose
of this lab is to measure the properties of illuminance provided by a point source of light.
Illuminance
is measured in lux (lx) and is a metric system unit that represents the ratio of lumens/m
^{2}
. This unit measures how the rays from a light source illuminate a surface placed at different distances form a point source. The unit of lumens measures luminous flux; that is, the "number" of light rays that pass through a surface which is at right angles to their direction of motion. This term has the same conations as magnetic flux and electric flux.
Suppose a light source emits 800 lumens. As the light rays move away from the source, they form light spheres. If one such light sphere had a total surface area of 800 m
^{2}
, then the illumination on one square meter would be 1 lux. If a different, smaller light sphere only has a surface area of 4.19 m
^{2}
, then the illuminance on one square meter would be 193.6 lux.
Creative Commons License
As shown in the diagram above, illuminance is an
inverse square
property of light.
Equipment
optics bench (or a meter stick to measure distances)
point source of light
rigid, moveable screen
lens holder to secure light probe
computer with LoggerPro and appropriate interface for a light probe (measuring in lux)
shown: Pasco's
Optics System OS-8515C
Vernier's
Light Sensor
Procedure - Part I
Take a sheet of unlined paper and trace out the size of the moveable screen. Cut out the outline. Next draw two circles that are centered on the paper. Make one circle approximately 3-cm in diameter and the other approximately 15-cm in diameter. The actual size does not matter as long as the circles are drastically difference in diameter, are concentric, and fit completely on the paper.
Secure the paper to the front of the screen and mount the screen on the optics bench.
Turn on the light source (which has been placed at the "0" mark on the optics bench) and COMPLETELY DARKEN the room. Move the screen to a position where the beam from the light source completely fits the smaller circle. Record that position in the blank provided below. Then move the screen until the light beam completely fills the larger circle and record the second position in the blank provided below.
diameter of small circle (cm)
position on bench where the beam completely filled the small circle (cm)
diameter of larger circle (cm)
position on bench where the beam completely filled the larger circle (cm)
Procedure Part II
Leaving the light source in place remove the screen from the optics bench. Secure a light probe into the lens holder so that it does not move and its front surface is lined up with the lens holder's pointer that moves along the optic's bench.
Again, the room must be COMPLETELY DARKENED. Begin with the light probe (set to 0-6000 lux) 5 cm from the light source and read the lux value from the LoggerPro program. You do not need to graph the data, just observe the numerical data that is displayed on the lower left side of the screen. Once the reading stabilizes, record the value for 5 cm. Then move the lens holder back in 5-cm increments, recording the illuminance each time. As long as you can discriminate between readings, keep recording until you reach 100 cm.
Record your values in the chart below.
probe position
illuminance
trial
(cm)
(lux)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Analysis
After collecting all of your data, you will now create two graphs in EXCEL. The first will be called
Illuminance vs Distance
and the second will be called
Illuminance vs (1/D
^{2}
)
. Make each graph a scatter plot and fit a trend curve. The graphs will NOT BOTH be linear. Record the equation of each trend curve and its correlation coefficients (R
^{2}
) in the spaces below.
When stating your equations, use these variables (NOT y and y):
I for illuminance
D for distance
1/D
^{2}
for the reciprocal of the distance squared
Refer to the following information for the next two questions.
Graph of
Illuminance vs Distance
equation
correlation coefficient (R
^{2}
)
Refer to the following information for the next two questions.
Graph of
Illuminance vs 1/D
^{2}
equation
correlation coefficient (R
^{2}
)
Which graph was linear?
Illuminance vs Distance
Illuminance vs 1/D
^{2}
Which graph had the better correlation coefficient (R
^{2}
)?
Illuminance vs Distance
Illuminance vs 1/D
^{2}
both graphs had the same R
^{2}
Conclusions
We now return to your data from the paper screen in Part I. Since the same number of lumens illuminated the areas of both the small and large circles we can see how well all of our data conforms to an inverse-square relationship.
Start by substituting the distance of each circle from the light source into either equation generated by EXCEL to calculate the expected illuminance.
small circle's illuminance (lux)
large circle's illuminance (lux)
Calculate the ratio of I
_{small}
:I
_{large }
. Be careful to state your answer as a decimal with three significant figures.
Next calculate the area of each circle. A =
π
r
^{2}
area of small circle (cm
^{2}
)
area of large circle (cm
^{2}
)
Calculate the ratio of A
_{large}
:A
_{small}
. Be careful to state your answer as a decimal with three significant figures.
Error Analysis
If all of your measurements supported each other then the two ratioes you just calculated in the previous section should be the same. But we do not know THE CORRECT ANSWER; we just know that the two ratios should be identical. Therefore your error should be calculated as the percent difference between your two values.
What was your group's percent difference?
The light probe's manufacturer states that the probe's readings are accurate to ±20%. Using this information, discuss how well your group conducted this experiment.
Related Documents
Lab:
Labs -
Directions: Constructive and Destructive Interference
Labs -
Doppler Effect: Source Moving
Labs -
Frequency of Vibrating Strings
Labs -
Hydrogen Spectrum
Labs -
Hydrogen Spectrum
Labs -
Inertial Mass
Labs -
Interference Shading
Labs -
Pipe Music
Labs -
Reflection Gratings: Wavelength of a Helium-Neon Laser
Labs -
Relationship Between Tension in a String and Wave Speed
Labs -
Relationship Between Tension in a String and Wave Speed Along the String
Labs -
Ripple Tank Checklists
Labs -
Ripple Tank Checklists
Labs -
Ripple Tank Sample Solutions
Labs -
Ripple Tank Student Involvement Sheet
Labs -
Simple Pendulums: Class Data
Labs -
Simple Pendulums: LabPro Data
Labs -
Speed of a Wave Along a Spring
Labs -
Speed of Sound in Air
Labs -
Speed of Sound in Copper
Labs -
Using Young's Equation - Wavelength of a Helium-Neon Laser
Labs -
Video: Law of Reflection
Labs -
Video: Law of Reflection Sample Diagram
Resource Lesson:
RL -
Barrier Waves, Bow Waves, and Shock Waves
RL -
Beats: An Example of Interference
RL -
Incandescent Solids and Radiation
RL -
Interference of Waves
RL -
Interference: In-phase Sound Sources
RL -
Introduction to Sound
RL -
Law of Reflection
RL -
Physical Optics - Interference and Diffraction Patterns
RL -
Physical Optics - Thin Film Interference
RL -
Resonance in Pipes
RL -
Resonance in Strings
RL -
Ripple Tank Video Guides
RL -
SHM Equations
RL -
Simple Harmonic Motion
RL -
Sound Level Intensity
RL -
Speed of Waves Along a String
RL -
The Doppler Effect
RL -
Vibrating Systems - Simple Pendulums
RL -
Vibration Graphs
RL -
Wave Fundamentals
RL -
Waveform vs Vibration Graphs
REV -
Orbitals
Review:
REV -
Chapter 26: Sound
REV -
Honors Review: Waves and Introductory Skills
REV -
Physics I Review: Waves and Introductory Skills
REV -
Sound
REV -
Waves and Sound
REV -
Waves and Sound
Worksheet:
APP -
Echo Chamber
APP -
Santa's Helper
APP -
The Dog-Eared Page
APP -
The Low-Calorie Beer
APP -
The Perfect Pew
CP -
Colors
CP -
Interference
CP -
Light Properties
CP -
Polarization
CP -
Reflection
CP -
Shock Waves
CP -
Sound
CP -
Waves and Vibrations
NT -
Apparent Depth
NT -
Atmospheric Refraction
NT -
Concert
NT -
Electromagnetic Radiation
NT -
Light vs Sound Waves
NT -
Photographing Rainbows
NT -
Polaroid Filters
NT -
Shadows #1
NT -
shadows #2
NT -
Shock Cone
NT -
Soap Film Interference
NT -
Sound Waves
NT -
Standing Waves
NT -
Sunglasses
WS -
Beats
WS -
Beats, Doppler, Resonance Pipes, and Sound Intensity
WS -
Counting Vibrations and Calculating Frequency/Period
WS -
Doppler - A Challenge Problem
WS -
Doppler Effect
WS -
Double Slits
WS -
Fixed and Free-end Reflections
WS -
Fundamental Wave Terms
WS -
Illuminance 1
WS -
Illuminance 2
WS -
Interference: In-phase Sound Sources
WS -
Lab Discussion: Inertial and Gravitational Mass
WS -
More Practice with Resonance in Pipes
WS -
More Practice with the Doppler Practice
WS -
Practice with Resonance in Pipes
WS -
Practice with the Doppler Effect
WS -
Practice: Speed of a Wave Along a String
WS -
Pulse Superposition: Interference
WS -
Ripple Tank Review
WS -
Sound Vocabulary
WS -
Speed of Sound
WS -
Speed of Sound (Honors)
WS -
Standing Wave Patterns #1
WS -
Standing Wave Patterns #2
WS -
Standing Wave Patterns #3
WS -
Standing Wave Patterns #4
WS -
Thin Film Interference
WS -
Vibrating Systems - Period and Frequency
WS -
Wave Phenomena Reading Guide
WS -
Wave Pulses
WS -
Waveform and Vibration Graphs #1
WS -
Waveform and Vibration Graphs #2
TB -
25A: Introduction to Waves and Vibrations
TB -
25B: Vibrations and Waves
TB -
25C: Wave Speed
TB -
25D: Interference
TB -
25E: Doppler
TB -
25F: Doppler Effect (continued)
TB -
26B: Speed of Sound
TB -
26C: Resonance
TB -
26D: Beats
TB -
26E: Decibels
TB -
27A: Light Properties
TB -
27B: Properties of Light and Refraction
TB -
Decibels and Sound Intensity #1
TB -
Decibels and Sound Intensity #2
TB -
Interference Re-examined
TB -
Refraction Phenomena Reading Questions
TB -
Sound: Mixed Practice
TB -
Waves and Vibrations
PhysicsLAB
Copyright © 1997-2024
Catharine H. Colwell
All rights reserved.
Application Programmer
Mark Acton