Radiation and Albedo

Purpose:

To examine the effect of albedo on infrared and visible radiation
 
 

Equipment:

Two radiation sources: a hot plate and a 150W light bulb; board with sheet of white paper and sheet of black paper mounted on it; infrared thermometer
 
 

Procedure:

1)    Hot plate case

Set up the board with the paper on it and the hot plate so that the hot plate is facing the board and pointing evenly at each sheet from about two feet away.
 
Before turning on the hot plate, measure the temperature of each sheet of paper.  They are the same, at room temperature.

 
Turn on the hot plate.  Begin measuring the temperature of the sheets.  Be sure to measure in multiple locations on the sheets, because just as different geographical locations within a given area will have different temperatures, so too will the paper.  After about five minutes both sheets of paper will have warmed to approximately the same temperature, about 110 degrees Fahrenheit. 

Turn off the hot plate and cool  the sheets of paper back to room temperature.
 
 
 

2)    Light bulb case

Now set up the board with the paper on it and the light bulb so that the light bulb is facing the board and pointing evenly at each sheet from about two feet away.
 
Before turning on the light bulb, measure the temperature of each sheet of paper.  They are again the same, at room temperature.

 
Turn on the light bulb.  Begin measuring the temperature of the sheets, again in multiple locations to account for slightly uneven heating and sampling variations.  Very quickly you should notice a difference compared to the case using the hot plate.  The black sheet of paper warms up rapidly while the white sheet hardly warms at all.  After about five minutes the black sheet will be at about 200 degrees Fahrenheit while the white sheet will be at about 80 degrees Fahrenheit.

Explanation:

In the case with the hot plate, it was emitting only infrared radiation.  The sheets of paper absorbed the same amount of infrared radiation and warmed the same amount, regardless of their color.  However, the light bulb emits mostly visible light.  In that case, the albedo is very important.  The white sheet has a high albedo and thus reflects most of the visible light incident upon it, while the black sheet has a low albedo and absorbs most of the visible light incident upon it.  The black sheet then warms significantly because it absorbs so much energy.  The white sheet absorbed only some energy from the visible light (as well as some infrared energy which the light bulb also emits) and warmed only slightly.
 
 

Conclusions:

1) Energy and heat are transferred by electromagnetic radiation, both at the wavelengths of visible light and in the infrared.

2) Black and white paper (and indeed most terrestrial surfaces) have very similar albinos in the infrared.

3) The albedo of terrestrial surfaces is, however, highly non uniform at the wavelengths of visible radiation.  Surfaces with high albedo like ice, snow, and white sand reflect most of the visible radiation that falls upon them, while surfaces with low albedo like asphalt, dark soil, and vegetation absorb much of the incident visible radiation.