6.17 - Absorbing & Emitting Radiation

Infrared emission and absorption

• Every object constantly emits and absorbs infrared radiation from its surface.
• The hotter an object is, the more infrared radiation it emits in a given time.

Emission vs absorption sets the temperature

• Emits more than it absorbs - The object cools down (e.g. a mug of hot tea cooling).
• Emits less than it absorbs - The object warms up (e.g. a glass of iced water on a warm day).
• Emits at the same rate as it absorbs - The object stays at the same temperature.

How surfaces affect emission and absorption

• A matt black surface is the best absorber and the best emitter of radiation.
• A light, shiny surface is a poor absorber and a poor emitter, but the best reflector of radiation.

Perfect black bodies

• A perfect black body absorbs all the radiation that hits it - none is reflected and none is transmitted through it.
• Because a perfect absorber is also the best possible emitter, a perfect black body is also the best possible emitter of radiation.
• The radiation emitted covers a continuous range of wavelengths.

Intensity and temperature

• Intensity is the power emitted per unit area (how much energy is transferred to a given area each second).
• The intensity and the spread of wavelengths emitted depend only on the body's temperature.
• Real objects are not perfect black bodies, but they follow the same pattern of emission.

How temperature changes the radiation curve

• An object emits radiation across a continuous range of wavelengths.
• How that radiation is spread across wavelengths depends on the object's temperature.

As the temperature increases

1. The intensity of every wavelength increases.
2. The intensity increases more rapidly for shorter wavelengths than for longer ones.
3. The peak wavelength (the wavelength emitted with the highest intensity) decreases, shifting the peak towards shorter wavelengths.

Visible changes with heating

• At everyday temperatures, the peak wavelength lies in the infrared.
• As an object gets hotter, it begins to glow red and then white as more of its radiation moves into the visible light spectrum.

Radiation balance and the Earth's temperature

The Earth's temperature depends on the balance between the radiation it absorbs and the radiation it emits.

• Absorbs faster than it emits - Temperature rises.
• Emits faster than it absorbs - Temperature falls.
• Absorbs and emits at the same rate - Temperature stays constant.

Day and night

• During the day - Radiation from the Sun is absorbed by the atmosphere and surface, so local temperatures rise.
• At night - Less radiation is absorbed than is emitted, so local temperatures fall.

The greenhouse effect

1. Greenhouse gases in the atmosphere (such as carbon dioxide and methane) absorb infrared radiation emitted by the Earth's surface.
2. They re-emit this radiation in all directions, including back towards the surface.
3. This reduces the rate at which energy is lost to space, causing the Earth's temperature to rise until absorption and emission balance again at a higher temperature.