Absorbing light and heat (Erica Brauer)
Author
Erica Brauer, Granada Hills Charter High School, California State University, Northridge
Principles
Dark surfaces absorb more visible light energy than bright surfaces • Dark surfaces reflect less visible light energy than bright surfaces • Energy can change forms, in this case from radiant light energy to heat • Clouds, being bright, reflect significant amounts of sunlight and help to regulate Earth’s temperature
Standards
4-PS3-2.
Make observations to provide evidence that energy can be transferred from place to place by sound, light, heat, and electric currents.
Materials needed
2 thermometers
Flood lamp, desk lamp, or area in direct sunlight
Ruler
Construction paper, 1 piece white, 1 piece black, or 2 sheets photocopy paper
Scissors
Cellophane tape or rubber bands
2 empty metal food cans, same size (be sure rims are not jagged)
Procedure
1. Cut a piece of white construction paper to fit around the outside of one of the cans in the same way that a can’s label would. Do the same with the black construction paper and the other can. Alternatively, you can use white photocopy paper and create the black piece by heavily overexposing one of the pieces.
2. Secure one piece of paper to each can with tape or rubber bands.
3. Place one thermometer inside each can.
4. Read and record the temperature on both thermometers in Table 1.
5. Position both cans about 12 in. (30 cm) from the lamp.
6. Turn lamp on.
7. Read and record temperature on both thermometers after 10 minutes.
8. Compare the temperatures just recorded for the two cans.
Explanation
Light from the lamp is absorbed by the cans just as light from the Sun is absorbed by objects. When the light is absorbed it is transformed into heat energy. The more light that is absorbed, the more heat is produced. Dark objects absorb more light. Another way to think of this is bright objects reflect more light. So, dark objects under the same conditions get hotter than bright objects, just as dark surfaces, or dark portions of surfaces get hotter than bright areas under the same conditions. A typical black object may absorb 90% of visible light. A mirror reflects over 90% of incoming visible light. Clouds, ice, and snow are very bright and therefore reflect most of the sunlight that hits them. All have the effect of keeping average temperatures on Earth cooler than they would be otherwise.
EXTENSION The albedo of an object is a quantitative way to measure its brightness and is often used to describe the brightness of a planetary surface. Higher albedo surfaces (bright) reflect more light and absorb less than low albedo (dark) surfaces. Earth, on average, has an albedo of approximately 0.39, partly due to bright clouds. In other words, on average the Earth reflects 39% of the incoming sunlight, thus keeping temperatures cooler. The Moon has an average albedo that is much lower, about 0.08, meaning that it reflects only about 8% of incoming sunlight. The Moon is thus very dark and appears bright at night primarily due to the contrast with the blackness of space
Questions
Which absorbs more heat, the ocean, or polar ice? Why? (The ocean, since it appears much darker than ice)
What might be a better reflector than even white? Silver/metallic
What is a real life example of where you have seen or felt this phenomenon? Answers will vary
How could engineers use this to design things to help the environment? Answers will vary
Everyday examples of the principles illustrated
1. Climate change-increased CO2 causes temperatures to warm, which melts polar ice, which means less reflection. Less reflection means more absorption, which increases temperature, which melts more ice, etc (positive feedback loop).
2. Movement to paint inner city streets light gray instead of black.
Photos
Movies
Include movies that you have taken. Your movies should be placed in your youtube account.
References
Modified from Astronomy for Every Kid by Janice VanCleave, New York: Wiley and Sons, pg. 6-7, 1991.