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Solar Energy on Earth

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What's wrong with this dog?

Nothing! The sensor is detecting infrared radiation from the dog — in other words, heat. The Sun emits infrared radiation among other wavelengths too.

Energy From the Sun

Most of the energy that reaches the Earth’s surface comes from the Sun ( Figure below ). About 44% of solar radiation is in the visible light wavelengths, but the Sun also emits infrared, ultraviolet, and other wavelengths.

An image of the sun taken by the SOHO spacecraft. The sensor is picking up only the 17.1 nm wavelength, in the ultraviolet wavelengths.

Ultraviolet

Of the solar energy that reaches the outer atmosphere, ultraviolet (UV) wavelengths have the greatest energy. Only about 7% of solar radiation is in the UV wavelengths. The three types are:

  • UVC: the highest energy ultraviolet, does not reach the planet’s surface at all.
  • UVB: the second highest energy, is also mostly stopped in the atmosphere.
  • UVA: the lowest energy, travels through the atmosphere to the ground.

Infrared

The remaining solar radiation is the longest wavelength, infrared . Most objects radiate infrared energy, which we feel as heat.

Some of the wavelengths of solar radiation traveling through the atmosphere may be lost because they are absorbed by various gases ( Figure below ). Ozone completely removes UVC, most UVB, and some UVA from incoming sunlight. O 2 , CO 2 , and H 2 O also filter out some wavelengths.

Atmospheric gases filter some wavelengths of incoming solar energy. Yellow shows the energy that reaches the top of the atmosphere. Red shows the wavelengths that reach sea level. Ozone filters out the shortest wavelength UV and oxygen filters out most infrared.

Summary

  • Ultraviolet radiation has the highest energy; infrared the lowest.
  • Ultraviolet is divided into three categories based on wavelength: UVC, with the highest energy, UVB, and UVA, with the lowest energy.
  • Infrared has longer wavelengths than red light and is felt as heat.

Practice

Use these resources to answer the questions that follow.

http://www.youtube.com/watch?v=i8caGm9Fmh0

1. What is infrared light?

2. How can we sense infrared light?

3. What can be used to see infrared light?

4. What happens to infrared radiation when it get to the Earth?

5. What heats the lower atmosphere?

6. What is the Earth's radiation budget?

7. What does near infrared measure?

8. What can studying infrared tell us abut the Earth?

Ultraviolet Waves

http://www.youtube.com/watch?v=QW5zeVy8aE0

9. What are ultraviolet waves?

10. What are three regions of ultraviolet waves?

11. Describe UVA.

12. What problem can UVB cause?

13. Why don't UVC rays reach the Earth?

14. What have scientists discovered with ultraviolet waves?

Review

1. Why does more solar radiation of all wavelengths come into the exosphere than reaches Earth's surface?

2. Why does ultraviolet radiation, especially UVC, damage life?

3. Look at the graph above. What happens to the highest wavelengths of energy at Earth's surface?

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