# need help on "Black body radiation" (1 Viewer)

#### slyvester63

##### New Member
Hey i got so confused with this definition and also other stuff. i'm just an ESL student so can somebody give me a good definition for this? Thank's

1. what is this black body radiation?
2. whats with the curve of the lines? isn't it as the temperature increases, the peak moves toward the shorter wavelength and we see it blue...
3. is intensity and brightness are the same?
4. why is 'intensity' against 'wavelength'?

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#### alcalder

slyvester63 said:
Hey i got so confused with this definition and also other stuff. i'm just an ESL student so can somebody give me a good definition for this? Thank's

1. what is this black body radiation?
2. whats with the curve of the lines? isn't it as the temperature increases, the peak moves toward the shorter wavelength and we see it blue...
3. is intensity and brightness are the same?
4. why is 'intensity' against 'wavelength'?
Using the Jacaranda HSC text book - Black Body radiation is the electromagnetic radiation emitted by a "black body" at a particular temperature. It is distributed across the entire E/M spectrum but it is not distributed evenly across the E.M spectrum. There is a peak for a particular wavelength which depend on temperature. This is shown using Wien's Law:

Max intensity wavelength x Temperature (in Kelvin) = W (a constant=2.9 x 10-3 mK

Essentially, a blackbody is an ideal absorber and radiator of energy - it is a theoretical thing. However, Stars tend to fit this ideal model for particular temperatures (but not all).

Therefore, as the temperature increases of this blackbody, we tend to see more of the bluer light and the object appears more blue.

INTENSITY AND BRIGHTNESS
Page 271 of Jacaranda Prelim

"The brightness of a radiant object (seen as a lamp or star) is the intensity of light seen some distance away from it. It is the energy received per sqaure metre per second."

I guess intensity and brightness are the same thing.

The Black body graph, then, is wavelength versus intensity because it is showing that there is continuous spectrum of wavelengths emitted from a black body BUT the relative intensities of the different radiations is different. To show the change for different temperatures we must use multiple lines on the graph - a new line for each new temperature.

Hope that helps.

#### slyvester63

##### New Member
hey, that helps a lot! but i have some more question to ask:

1. Where is this black body (is it from the core of the star? or surrounding of it?) and how do they emit?
2. what makes them ideal absorber and emitter?
3. and lastly; 'some distance away from it' means from earth or limited parsec?

Man i see some of you guys are good in explaining this thing... just really sound like you should write a book... very natural is it?

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##### Member
alcalder said:
Using the Jacaranda HSC text book - Black Body radiation is the electromagnetic radiation emitted by a "black body" at a particular temperature. It is distributed across the entire E/M spectrum but it is not distributed evenly across the E.M spectrum. There is a peak for a particular wavelength which depend on temperature. This is shown using Wien's Law:

Max intensity wavelength x Temperature (in Kelvin) = W (a constant=2.9 x 10-3 mK

Essentially, a blackbody is an ideal absorber and radiator of energy - it is a theoretical thing. However, Stars tend to fit this ideal model for particular temperatures (but not all).

Therefore, as the temperature increases of this blackbody, we tend to see more of the bluer light and the object appears more blue.

INTENSITY AND BRIGHTNESS
Page 271 of Jacaranda Prelim

"The brightness of a radiant object (seen as a lamp or star) is the intensity of light seen some distance away from it. It is the energy received per sqaure metre per second."

I guess intensity and brightness are the same thing.

The Black body graph, then, is wavelength versus intensity because it is showing that there is continuous spectrum of wavelengths emitted from a black body BUT the relative intensities of the different radiations is different. To show the change for different temperatures we must use multiple lines on the graph - a new line for each new temperature.

Hope that helps.

I can't believe someone actually just copied this from the Jacaranda book.

#### Forbidden.

##### Banned
Wein's Law is out of the syllabus.