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Project Summary

Difficulty  6 
Time required Average (about one week)
Prerequisites None
Material Availability Readily available
Cost Low ($20 - $50)
Safety Adult supervision recommended for cutting wax.

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Abstract

You've probably heard that compact fluorescent light bulbs are more efficient than incandescent bulbs. More of the electricity they use goes into producing light, and less into producing heat than with incandescent bulbs. How much more efficient are compact fluorescent bulbs? You can find out for yourself by making a simple photometer to compare the light output from different bulbs. This project shows you how.

Objective

The goal of this project is to measure the relative intensity of different light bulbs, using a simple photometer that you can build yourself.

Introduction

Intro image

As you move away from a light source, the light gets dimmer. No doubt you've noticed this with reading lamps, streetlights, and so on. The diagram at right shows what is happening with a picture. At the center, the yellow star represents a point source of light. Imagine the light from the star spreading out into empty space in all directions. Now imagine the light that falls on a square at some arbitrary distance from the star (d = 1, yellow square). Move away, doubling the distance from the star (d = 2). The light from the original square has now "spread out" over an area of 4 (= 22) squares. Thus, at twice the original distance, the intensity of the light passing through a single square will be 1/4 of the original intensity. Going out still further, tripling the original distance (d = 3), and the light from the original square now covers an area of 9 (= 32) squares. Thus, at three times the original distance, the intensity of the light passing through a single square will be 1/9 of the original intensity. This is what is meant by the "Inverse Square Law." As you move away from a point light source, the intensity of the light is proportional to 1/d2, the inverse square of the distance. Because the same geometry applies to many other physical phenomena (sound, gravity, electrostatic interactions), the inverse square law has significance for many problems in physics.

In this project you'll build a simple photometer, invented by the Irish scientist, John Joly. As you'll see, the design of the photometer is based on the inverse square law. In the Joly photometer, two equal-sized blocks of paraffin wax are separated by a layer of aluminum foil. The wax blocks are mounted in a box with windows cut out on the left, front, and right sides, as shown in Figure 1.

mount the wax blocks in a small cardboard box with windows cut in the left, front, and right sides
Figure 1. Diagram of a Joly photometer. Inside the box are two equal-sized blocks of paraffin wax, separated by a sheet of aluminum foil.

The photometer is positioned between two light sources (see Figure 2). The two light sources and the center of the photometer should all be at the same height. Light from the first source illuminates the left-hand paraffin block. Light from the second source illuminates the right-hand paraffin block. To insure uniform illumination, the distance from each light source to the photometer should be relatively large compared to the size of the wax block. Also, there should be no other light sources in the room. The experimenter views the photometer through the front window and moves it back and forth between the two light sources until both blocks appear equally bright. The photometer should be moved along an imaginary straight line connecting the two light sources.

schematic diagram of Joly photometer experimental setup
Figure 2. schematic diagram of Joly photometer experimental setup. See text for details.

When the two wax blocks are equally illuminated, the relationship between the intensities of the two light sources is determined by the inverse square law. Here is the relationship in the form of an equation:

inverse square law relation between the intensities of the two light sources

You can build your own Joly photometer and use it to measure the relative intensity of different light bulbs. Using the wattage of each bulb, you can also compare how efficient different bulbs are at producing light.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

Questions

Bibliography

Materials and Equipment

To do this experiment you will need the following materials and equipment:

Experimental Procedure

Building the Photometer

  1. You should be able to find one-pound boxes of paraffin wax at your local grocery or hardware store. Each box contains four slabs of paraffin wax.
  2. Cut one slab of the wax in half with a sharp knife. Work carefully so that you don't chip or break the slab.

    cutting the wax slab in half

  3. Cut a piece of aluminum foil to the same size as your two blocks of wax, and place it in between them.

    aluminum foil goes in between the two blocks of wax

  4. Use tape and small pieces of cardboard to mount the wax blocks inside a small cardboard box, with windows cut on three sides, as in the diagram below.

    mount the wax blocks in a small cardboard box with windows cut in the left, front, and right sides

Experimental Setup

  1. The illustration below is a schematic diagram of the experimental setup.

    schematic diagram of Joly photometer experimental setup

  2. Place the photometer in between two light sources.
    1. Each wax block is illuminated by only one of the sources. The aluminum foil prevents light from passing between the blocks.
    2. The light sources and the photometer should be at the same height.
    3. The photometer should be positioned on the straight line between the two sources.
    4. The two light sources should be the only sources of light in the room. No bright sunlight!
    5. To insure uniformity of illumination at the photometer, the distance from the photometer to the nearest light source should be large compared to the size of the wax block.
  3. Move the photometer back and forth between the two light sources until the the two wax blocks are equally bright.

Analyzing Your Results

  1. When the wax blocks are equally illuminated, the inverse square law says that the intensities of the two light sources are related by the following equation:

    inverse square law relation between the intensities of the two light sources

  2. Choose one light bulb as your standard, for example, a 60 W soft white bulb. Call this light I1. The intensity of the second light is then given by:

    calculating the relative intensity of the second light source

  3. Measure the distance from each light source to the aluminum foil layer of the Joly photometer.
  4. Calculate the relative intensity of each bulb compared to your standard bulb. (Your standard bulb will have an intensity of 1.0. You can check this by using two identical bulbs. It's a good way to show that your photometer works as expected.)
  5. To calculate the efficiency of each bulb, divide the relative intensity by the bulb wattage.

Variations

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

This project is based on:


Last edit date: 2007-04-12 14:00:00


Career Focus

If you like this project, you might enjoy exploring careers in Physics.

Physicist
Physicists have a big goal in mind—to understand the nature of the entire universe and everything in it! To reach that goal, they observe and measure natural events seen on Earth and in the universe, and then develop theories, using mathematics, to explain why those phenomena occur. Physicists take on the challenge of explaining events that happen on the grandest scale imaginable to those that happen at the level of the smallest atomic particles. Their theories are then applied to human-scale projects to bring people new technologies, like computers, lasers, and fusion energy.
 



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