Abstract

Objective

The goal of this project is to see how the color of an object affects how much heat it absorbs when exposed to incandescent light.

Introduction

Light is an example of an electromagnetic wave. Electromagnetic waves can travel through the vacuum of interstellar space. They do not depend on an external medium—unlike a mechanical wave such as a sound wave which must travel through air, water, or some solid medium. Electromagnetic waves cover a huge range of frequencies, from high-frequency gamma rays and x-rays, to ultraviolet light, visible light, and infrared light, and on into microwaves and radio waves. As the frequency decreases, so does the energy. The wavelength of an electromagnetic wave is inversely proportional to its frequency. So waves with high frequency have short wavelengths, and waves with low frequency have long wavelengths.

Electromagnetic waves interact with materials in different ways, depending on the nature of the material and the frequency of the electromagnetic wave. Light is the range of electromagnetic waves that are visible (Figure 1). For humans, the range of visible wavelengths is from 400 to 700 nm (1 nm = 1 ×10⁻⁹ m).

Figure 1. The visible spectrum. X-rays, light, and radio waves are examples of electromagnetic waves. Light is the part of the electromagnetic spectrum that we can detect with our eyes. At the blue end of the visible spectrum, the wavelength of light is shorter (about 400 nm). At the red end of the spectrum, the wavelength of light is longer (about 700 nm) (Illustration from Abrisa Glass & Coatings, 2005).

This range of wavelengths is called the visible spectrum of light. When you see a rainbow in the sky, or white light that has been refracted through a prism, or diffracted by the regular surface of a CD, you are seeing a spectrum of colors. The different colors are related to the different wavelengths of light. Violet light is at the short-wavelength end of the visible spectrum (400 nm), and red light is at the long-wavelength end of the visible spectrum (700 nm), with the rainbow of colors in between.

We perceive different colors because our visual system has evolved to make use of the spectral information in reflected light. When light interacts with an object, the light can be absorbed by the object, reflected by the object, or transmitted by the object.

For example, when you look at yourself in the mirror, the light that you are seeing has been relected by the mirror, transmitted through the air, through your cornea, through the lens of your eye, and through two layers of cells in your retina before it is absorbed by light-sensitive pigments in your photoreceptor cells. The energy from the absorbed light starts a cascade of chemical reactions in your photoreceptors that ultimately leads to your percpeption: seeing yourself in the mirror.

Objects in the world have different colors depending on which parts of the visible spectrum they absorb, and which parts of the visible spectrum they reflect. Red objects reflect long wavelengths of light (and absorb shorter wavelengths), while blue objects reflect short wavelengths of light (and absorb longer wavelengths). Black objects absorb all visible wavelengths about equally, and white objects reflect all visible wavelengths about equally.

Light that is absorbed by an object is usually converted into heat energy. The goal of this project is to measure how much heat is produced by the absorption of light by different colors. You'll use an incandescent light (a heat lamp), and water-filled jars wrapped with different colors of construction paper. By measuring how much the temperature of the water increases, you'll have a measure of how much light was absorbed by each color.

Before you get started, study Figure 2 below and then try to predict what your results will be. The graph compares the spectrum of sunlight with the spectrum of an incandescent bulb. You can see that sunlight has much more energy (brightness) in the range of visible wavelengths (gray shaded region), while the incandescent bulb has more energy in the red and infrared (invisible, longer-wavelength electromagnetic radiation) region of the spectrum.

Figure 2. A comparison of the spectrum of sunlight vs. the spectrum of an incandescent bulb (Schroeder, 2003). The x-axis shows the wavelength (in microns), and the y-axis shows the relative energy (brightness) at each wavelength. The gray region corresponds to the visible region of the spectrum (0.4–0.7 μm = 400–700 nm).

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:

• electromagnetic spectrum,
• visible light,
• ultraviolet light,
• infrared light,
• absorption of light,
• reflection of light.

Questions

• How does an incandescent light bulb work?
• What wavelength is the peak output for an incandescent bulb?

Bibliography

• For more information on how light interacts with matter, see:
  Hoff, K. Mellendorf and V. Calder, date unknown. "Reflection and Absorption," NEWTON Ask a Scientist, Physics Archive, Argonne National Laboratory, Division of Educational Programs [accessed February 23, 2007] http://www.newton.dep.anl.gov/askasci/phy00/phy00232.htm.
• For high-school level references on color and vision, see:
  • Henderson, T., 2004. "Color and Vision," The Physics Classroom, Glennbrook South High School, Glennbrook, IL [accessed February 23, 2007] http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/light/u12l2a.html.
  • Wikipedia contributors, 2007. "Color," Wikipedia, the Free Encyclopedia [accessed February 23, 2007] http://en.wikipedia.org/w/index.php?title=Color&oldid=110725908.
• The visible spectrum illustration in the Introduction is from:
  Abrisa Glass & Coatings, 2005. "Understanding Light and Colort," Abrisa Glass & Coatings [accessed March 5, 2007] http://www.abrisa.com/guide/understandinglight/light.spectrum.asp.
• Advanced. For more information on the distribution of energy in light sources, see:
  • ACEPT W3 Group, 2005. "Sources of Light: The Sun and Lamps," Department of Physics and Astronomy, Arizona State University [accessed February 23, 2007] http://acept.asu.edu/PiN/rdg/color/source.shtml.
  • Schroeder, D.V., 2003. "Radiant Energy," online chapter for the course, 'Energy, Entropy, and Everything,' Physics Department, Weber State University [accessed February 23, 2007] http://physics.weber.edu/schroeder/eee/chapter6.pdf.

Materials and Equipment

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

• 6–8 identical glass jars, 1 quart size, with lids,
• 6–8 sheets of colored construction paper (different colors),
• scissors,
• tape,
• water,
• thermometer,
• modeling clay,
• heat lamp,
• timer or clock,
• drill and bit for making holes in jar lids.

Experimental Procedure

1. Drill a hole slightly larger than the diameter of your thermometer in the lid of one jar. (In this project you'll be testing the jars one at a time, so you can use the same lid with each jar.)
2. Tightly wrap each jar with a different color of construction paper.
3. Carefully fill each jar with water.
   1. Keep the construction paper dry.
   2. You need to have the same starting temperature for each jar. The easiest way to do this is to have all of the jars at room temperature. Fill them with water that is about the same temperature the day before you want to start your experiment. Cover the jars and leave them to equilibrate to room temperature overnight.
4. Cover the jar to be tested with the lid with the drilled hole.
5. Put the thermometer in through the hole so that its bulb is completely immersed in the water. Use the clay to seal the hole and hold the thermometer in place. The rest of the thermometer will be out of the jar, and you should still be able to take readings with it. Keep the thermometer at the same height with respect to the jar lid for all of the tests.
6. Note the starting temperature for each jar.
7. Next, set up your heat lamp in a convenient location, so that it can shine directly at the side of a jar placed between 15–30 cm away. (Decide on an exact distance, and use it for all of the tests.)
8. Set the jar to be tested at the correct distance, centered in front of the heat lamp.
9. Leave the jar in front of the lamp for a set amount of time (e.g., 30 min), and check the temperature of the jar when that time has elapsed.
   1. Be sure to make the time interval long enough so that there is a measurable increase in temperature.
   2. Be sure to use the same time interval for each jar.
10. Repeat until all of the jars have been tested.
11. You should do at least three separate trials for each color, with each trial starting with water at room temperature. (It may take more than one day to do your measurements, so plan ahead!)
12. Analyze your results.
    1. What was the average increase in temperature for each color?
    2. Make a bar graph to show your results, ordering the colors from lowest to highest temperature increase.
    3. How does the arrangement of the colors in your bar graph compare to the spectrum of incandescent light?

Variations

• Compare temperatures of the jars when exposed to sunlight instead of incandescent light. You'll need a separate thermometer for each jar, and a location where all of the jars receive the same amount of sunlight for a defined time period. As in the experiment described above, you should conduct at least three separate trials. How does the spectrum of sunlight compare to the spectrum of incandescent light? Are the results of your experiment the same or different with sunlight compared to incandescent light?
• For a more basic experiment, see the Science Buddies project Can the Color of Your House Reduce Your Energy Bill?.

• For more science project ideas in this area of science, see Physics Project Ideas.

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

• This idea is from an entry to the 2007 San Mateo County science fair (author's name not provided).

Last edit date: 2007-04-09 13:00:00