[waves] black body radiation

[ BBC Bitesize 'Waves ⇢ Black body radiation' page 1-3 ]

All bodies (objects) emit and absorb…

… infrared radiation regardless of their temperature

The hotter a body is:

• the more infrared radiation it gives out in a given time → the intensity of the emission will increase. This is a directly proportional relationship.

• the greater the proportion of emitted radiation is visible light → the wavelength of the radiation will decrease. This is an indirectly proportional relationship.

Black Bodies

Bodies which absorb all radiation which falls on them; they do not reflect or transmit any radiation, but they emit radiation. They are the best emitters of radiation. There are no known black bodies, however some of bodies are close to a perfect black body.

A perfect black body is a…

… theoretical object.

An object that is good at absorbing radiation…

… is also a good emitter, so a perfect black body would be the best possible emitter of radiation.

Examples of Black Bodies:

• stars

Stars are very good emitters of most wavelengths in the electromagnetic spectrum. This suggests that stars also absorb most wavelengths. Whilst there are a few wavelengths that stars do not absorb or emit, this figure is very low, so they can be treated as black bodies.

• planets

• black holes

Poor Absorbers and Emitters:

• white

• shiny

• silvery

Radiators in homes are usually painted white so that the infrared radiation is emitted gradually.

Apparatus for Required Practical 10:

• kettle

• infrared detector

• heat-proof mat

• Leslie cube

• 30cm ruler

• stopwatch

Step 1 of Required Practical 10:

Place a Leslie cube on a heat-proof mat and fill it, almost to the top, with boiling water. Leave for two minutes. This is to enable the surfaces to heat up to the temperature of the water.

Step 2 of Required Practical 10:

Then pour out the water carefully. Use the infrared detector to measure the intensity of infrared radiation emitted from each surface, or the temperature of the surface. Make sure that the detector is the same distance from each surface for each reading (30cm).

Step 3 of Required Practical 10:

Repeat with other sides of Leslie cube. Calculate average radiation emitted or temperature.

Results of Required Practical 10:

Surface type	Infrared intensity (W/m2)
matt black	19.5 (best emitter)
matt white	5.1
shiny black	14.2
shiny silver	3.8 (worst emitter)

Hazards and Control Measures in Required Practical 10:

• boiling water → do not touch cube; complete investigation standing up; be cautious

Rate of Absorption: greater than the rate of emission

Temperature of the Body: increasing

Rate of Absorption: equal to the rate of emission (balanced)

Temperature of the Body: constant

Rate of Absorption: less than the rate of emission

Temperature of the Body: decreasing

Factors affecting Earth’s Temperature:

• rates at which light and infrared radiation are absorbed by the Earth’s surface and atmosphere

• rates at which light and infrared radiation are emitted by the Earth’s surface and atmosphere

Through these systems, radiation is kept in balance and therefore the temperature of the Earth can also be kept fairly constant.

Rates at which light and infrared radiation are absorbed by the Earth’s surface and atmosphere

During the day, radiation is emitted by the Sun and absorbed by the Earth. This leads to an increase in temperature. Some of this radiation will be absorbed by the clouds and the atmosphere. This will lead to a decrease in the temperature again.

Rates at which light and infrared radiation are emitted by the Earth’s surface and atmosphere

The atmosphere will reflect some of the radiation from the Sun, leading to a decrease in temperature. At night, the Earth can also emit some radiation back into space. This leads to a decrease in temperature.