Black Bodies and Infrared Radiation

• All bodies (objects) emit and absorb infrared radiation
• What 2 factors affect the rate at which an object emits radiation?

  The nature of the surface and the temperature of the object - A hotter object emits infrared radiation at a faster rate
• What does the wavelength and intensity of radiation depend on?

  The temperature of the object
• Very hot objects emit shorter wavelength radiation than cooler objects - This is why very hot objects produce visible light (e.g. a very hot piece of metal)
• What is a perfect black body?

  An object that absorbs all of the infrared radiation incident on it. It does not reflect or transmit any infrared radiation.
• Why is a perfect black body the best possible emitter?

  Because a good absorber is also a good emitter
• What will happen if an object is warmer than its surroundings?

  It will emit more radiation than it absorbs and its temperature will decrease (e.g a cup of hot coffee)
• What will happen if an object is cooler than its surroundings?

  It will absorb more radiation than it emits and its temperature will increase (e.g. burgers on a barbecue)
• What happens if an object is at a constant temperature?

  It is absorbing radiation at the same rate it is emitting radiation
• What are the only ways the Earth can gain or lose energy?

  Absorbing or emitting radiation
• Explain how the Earth absorbs radiation from the Sun

  The sun emits short wavelength radiation (e.g. visible light or UV) and this radiation travels to earth, where some of it is reflected (e.g. by clouds) but the remaining radiation is absorbed by the surface of the Earth, increasing the Earth's temperature
• Describe what happens when the Earth tries to emit infrared radiation back into space?

  When the earth absorbs radiation and its temperature increases, It emits infrared radiation back into space. But some of the energy is trapped by greenhouse gases in the atmosphere (e.g. carbon dioxide). Human activity increases greenhouse gas levels so more heat energy is trapped in the atmosphere and less is radiated to space
• How can cloud cover affect how much energy is radiated from the Earth?

  Clouds can reflect infrared back to the earth and prevent it from being radiated into space - this is why cloudy nights tend to be warmer than clear nights