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Name: Jen
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Question:
Why is it colder in shaded areas? Does it have to do with how infrared radiation behaves?



Replies:
Jen,

Light contains energy. This includes infrared light, visible light, ultraviolet light, and even microwaves and radio waves. Sunlight is a very strong source of light and of energy. When light strikes a surface, some of the energy is absorbed by the surface and some reflects away. The absorbed energy causes the material to warm up. In the shade, the energy in the sunlight is reaching the tree rather than your body.

There are in fact three ways to warm something up, to transfer heat from one object to another. The first is contact: the hot object touches the cold object. An example is putting something in a hot skillet. Contact requires that both objects be in the same place.

Another is convection: the hot object makes the air around it warm, and then the warm air carries the energy over to the cold object. An example of this is putting something on the rack of a conventional oven. The air is heated by the coils, and then the hot air warms what you put in the oven. Convection requires that there be some material, usually a gas, which is free to move from one object to the other.

Sunlight falls into the third category. This is radiation: the hot material emits particles or waves of energy that then travel to the cold object and are absorbed. An example is putting something in a microwave oven. The circuits of the oven emit radiation that is easily absorbed by the water molecules in food. Radiation is the only way to transfer heat through empty space.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College



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