[Duffy]

Hi Brian

Roger and I are engaged in terminator analysis, but it has just dawned on me (no pun intended) about what we see after the sun sets.   The visible spectrum is somewhere around 400 nm to 700 nm, the longest wavelength being in the red portion.  That why we get a red sky at night because the suns light is being stretched as it sets ... but what do we see after that ?  The spectrum continues into the Infrared band, we can't see this but is it present ? ... sorry if its a strange question.


Duffy

[Skywise]

(02-11-2017, 08:07 PM)Duffy Wrote: Hi Brian

Roger and I are engaged in terminator analysis, but it has just dawned on me (no pun intended) about what we see after the sun sets.   The visible spectrum is somewhere around 400 nm to 700 nm, the longest wavelength being in the red portion.  That why we get a red sky at night because the suns light is being stretched as it sets ... but what do we see after that ?  The spectrum continues into the Infrared band, we can't see this but is it present ? ... sorry if its a strange question.


Duffy

Not strange at all. Nor dumb. But nothing is stretched.

The answer is very related to why the sky is blue. As sunlight passes through the atmosphere, light is scattered off the molecules and other small particles in a process call Rayleigh scattering. Different wavelengths of light scatter more than others. Blues scatter more than reds.

The sky is blue because more of the blue light from the sun is scattered. The red keeps going.

The sun appears red at sunrise/sunset because you are looking at it through more atmosphere than you would when it's straight up at noon. Even more blue light is scattered away and now some of the greens as well, leaving behind only the reds.

Infrared is scattered less than red, and although it's still present at sunset, the sky isn't turning infrared. Instead, the sky is getting darker simply because we are in shadow.

BTW, the sun is absolutely stark white. It only appears yellowish due to the scattering. Take away a bit of blue light from white and you are left yellow.

Taking this further, opalescence is also caused by Rayleigh scattering.

Brian

[Duffy]

Brian 

Thank you for your politeness, I actively participated in astronomy for 10 years and should have known this. I also had a de'ja' vu moment because I think we covered this in our "radon" discussions. 

When we are in the shadow of the sun i.e. night, we can see infrared radiation with thermal imaging as heat, but the source of heat we can see depends on the wavelength it is being emitted at eg a hot rock in the desert after sunset (black body) or a human being (continuous). Is this a fair assumption of the infrared spectrum, or do you need to be polite again ?


Duffy

[Skywise]

(02-12-2017, 09:41 PM)Duffy Wrote: When we are in the shadow of the sun i.e. night, we can see infrared radiation with thermal imaging as heat, but the source of heat we can see depends on the wavelength it is being emitted at eg a hot rock in the desert after sunset (black body) or a human being (continuous). Is this a fair assumption of the infrared spectrum, or do you need to be polite again ?

Actually, both the rock and the human body would radiate infrared 'heat' in the same way. That the human body continuously generates heat is irrelevant. All that means is it doesn't cool off as a rock eventually would.

A black body is an ideal object that absorbs all radiation. Not real.

A black body radiator is an object that is in thermal equilibrium with it's surroundings. It radiates as much energy as it absorbs.

The temperature of an object dictates the frequencies of electromagnetic radiation the object will emit. It's not just infrared.

The Sun (and other stars) are black body radiators. Even the visible light they give off is black body radiation. Stars range in color from red to orange, yellow, white, and to blue as their temperature goes up. Very hot objects emit visible light.

On the other extreme, very cool objects might not even emit infrared. The universe itself has cooled to only 3 degrees kelvin and emits radiation in the microwave region. This is the cosmic microwave background.

Brian