Principles of Radiation. 1. All object possesses (sensible) heat and emit 1. All object possesses...

Principles of Principles of RadiationRadiation

Principles of RadiationPrinciples of Radiation

1. All object possesses (sensible) 1. All object possesses (sensible) heat and emit heat and emit

radiation energy as long as their radiation energy as long as their temperatures temperatures

is above is above absolute zeroabsolute zero ( ( 00 oo K). K).

(Kelvin Scale: 0(Kelvin Scale: 0oo K = - 273o C ) K = - 273o C )

Principles of RadiationPrinciples of Radiation 2. The character of radiant energy sent out 2. The character of radiant energy sent out depends on the temperature of the emitting depends on the temperature of the emitting object.object. a. The radiant energy emitted is directly a. The radiant energy emitted is directly proportional to the absolute proportional to the absolute temperaturetemperature. . b. The b. The lower the temperaturelower the temperature of the radiating of the radiating

object, object, the longer is the wavelengththe longer is the wavelength of the of the radiant energy emitted.radiant energy emitted.


Wavelength: Wavelength: the actual distance betthe actual distance between two successive wave crests / tween two successive wave crests / troughs.roughs.

FrequencyFrequency : the number of waves p : the number of waves passing a fixed point in a period of tiassing a fixed point in a period of time. me.

(1 hertz = one cycle per second)(1 hertz = one cycle per second)


The Wien’s Law: The Wien’s Law: is useful to find ois useful to find out the ut the maximum wavelengthmaximum wavelength of ene of energy emitted by the radiating objectrgy emitted by the radiating object

Wavelength (max) = Wavelength (max) = 28972897 / (temp. o / (temp. of the object in f the object in ooK) * 10-6 m in K) * 10-6 m in m (mm (micron)icron)


旋轉星圖

http://www.lcsd.gov.hk/CE/Museum/Space/WhatsNew/c_index.htm


4. When an object 4. When an object absorbsabsorbs radiant e radiant energy, nergy,

its surface temperature is its surface temperature is raisedraised. T. This his

represents a process of represents a process of transformatransformation of tion of

radiant energyradiant energy to to sensible heatsensible heat..

Solar Radiation / InsolationSolar Radiation / Insolation

The difference between the two terThe difference between the two terms?ms?


1. The surface temperature of 1. The surface temperature of the sun is the sun is

about about 60006000 ooK. Its surface K. Its surface emits a emits a

spectrum of spectrum of electro-magnetic electro-magnetic radiation radiation

of various wavelengths.of various wavelengths.

Solar Radiation / InsolationSolar Radiation / Insolation X-rays & gamma rays; X-rays & gamma rays;

Ultraviolet raysUltraviolet rays 1/2000 – 1/100 1/2000 – 1/100 m; m; 0.2-0.4 0.2-0.4 m m Visible light 0.4-0.7 Visible light 0.4-0.7 m m Infrared raysInfrared rays 0.7 – 3000 0.7 – 3000 m m

Electromagnetic radiation spectrum


2. The radiation ranging from 2. The radiation ranging from X-ray , visible light to infrared X-ray , visible light to infrared rays.rays.


3. It takes about _ minutes for 3. It takes about _ minutes for the solar the solar

radiation to travel from the radiation to travel from the sun to the sun to the

earth.earth. radiation of various radiation of various wavelengths.wavelengths.


3. It takes about 3. It takes about 88 minutes for minutes for the solar the solar

radiation to travel from the radiation to travel from the sun to the sun to the

earth.earth. radiation of various radiation of various wavelengths.wavelengths.


4. Since the intensity of 4. Since the intensity of radiation decreases radiation decreases

with distance, the earth with distance, the earth intercepts only intercepts only

about about 1 / 2 000M1 / 2 000M of the sun's of the sun's total energy total energy

output. output.


5. On average, the earth intercepts 5. On average, the earth intercepts solar solar

energy of about 2 gm-cal/ cm2/ energy of about 2 gm-cal/ cm2/ min, min,

i.e. about 2 langleys/min above the i.e. about 2 langleys/min above the

atmosphere. This figure is known atmosphere. This figure is known as the as the

solar constantsolar constant..


6. The solar constant undergoes 6. The solar constant undergoes smallsmall

periodic variationperiodic variation of 1 - 2 % due to of 1 - 2 % due to sunspotsunspot

cyclecycle. In the long run, a difference of 2 % . In the long run, a difference of 2 %

in solar constant can change the in solar constant can change the effective effective

mean temperature of the earth surface mean temperature of the earth surface by by

as much as as much as 1.21.2ooCC. (significant?). (significant?)


7. What is the maximum 7. What is the maximum wavelength of the wavelength of the

radiation emitted from the Sun radiation emitted from the Sun ??

- Calculate the value with the - Calculate the value with the help of the help of the

Wien's LawWien's Law..

- Wavelength (max) = 2879 / - Wavelength (max) = 2879 / ________ ________ m = _______ m = _______ mm

Solar Radiation / InsolationSolar Radiation / Insolation Therefore, most of the wavelength Therefore, most of the wavelength

of energy emitted from the Sun beloof energy emitted from the Sun belongs to _____________.ngs to _____________.

** Solar Radiation is a ** Solar Radiation is a shortwave shortwave // longwave longwave energy. energy.

Solar Radiation / InsolationSolar Radiation / Insolation ActivityActivity: : Calculate the Maximum wavelength of Calculate the Maximum wavelength of

the terrestrial radiation (with the eartthe terrestrial radiation (with the earth surface temperature of about 300h surface temperature of about 300oo K). K).

Therefore, most of the wavelength of Therefore, most of the wavelength of energy emitted from the Sun belongs tenergy emitted from the Sun belongs to _____________.o _____________.

** Terrestrial Radiation is a ** Terrestrial Radiation is a shortwave / shortwave / longwave longwave energy. energy.


In what ways does terrestrial In what ways does terrestrial radiation different from solar radiation different from solar radiation ?radiation ?

Solar Radiation vs Terrestrial RadiSolar Radiation vs Terrestrial Radiationation

Spectra of solar and terrestrial radiation

The Incoming & The Incoming & Outgoing of Energy Outgoing of Energy

of the Earthof the Earth

Principles of Radiation. 1. All object possesses (sensible) heat and emit 1. All object possesses...

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