Understanding the Infrared Glow of Objects: Hotter is Not Always Brighter

[shmijda]

Homework Statement

Which statement is true?
The color an object glows has nothing to do with how hot it is.

As you heat an object, it glows more in the visible and less in the infrared.

The day side of the Earth reflects more light, but the night side glows brighter in the infrared.

You glow brighter in every wave length than a frozen body would.

Homework Equations

none

The Attempt at a Solution

Choice 1 is wrong, choice 2 appears to be right, choice 4 also seems right. Choice 3 is wrong

 

[phinds]

Science is not a democracy and "voting" for the right answers is ridiculous so a poll is not a good idea.

#1 I agree it's wrong
#2 You might want to this think through again
#3 Well since the day side GETS more light, I'd say, yeah, it reflects more light, but is that a property of the day side or a property of the incident light? I think the question is a poor one. In any case, do you think the day side glows brighter in the infrared? Is that why you consider this one wrong? (I would agree, by the way)
#4 if we're only talking about radiation as opposed to reflection I agree w/ you but it seems like another ambiguously worded question.

 

[shmijda]

Science is not a democracy and "voting" for the right answers is ridiculous so a poll is not a good idea.

#1 I agree it's wrong
#2 You might want to this think through again
#3 Well since the day side GETS more light, I'd say, yeah, it reflects more light, but is that a property of the day side or a property of the incident light? I think the question is a poor one. In any case, do you think the day side glows brighter in the infrared? Is that why you consider this one wrong? (I would agree, by the way)
#4 if we're only talking about radiation as opposed to reflection I agree w/ you but it seems like another ambiguously worded question.

For choice 2, as you heat an object, its wavelength becomes shorter i.e. less in infrared more in visible no?

 

[phinds]

Perhaps you think "in the infrared / not in the infrared" is pure binary? Actually there is quite a range of infrared and as you heat up an object it moves TOWARDS the visible but until it starts to glow (faint orange) it isn't there yet and will be radiating more strongly but still in infrared, just in shorter wavelengths

 

[shmijda]

Perhaps you think "in the infrared / not in the infrared" is pure binary? Actually there is quite a range of infrared and as you heat up an object it moves TOWARDS the visible but until it starts to glow (faint orange) it isn't there yet and will be radiating more strongly but still in infrared, just in shorter wavelengths

Yes, but if it is at the range where it is about to be visible light, then wouldn't the statement be true?

 

[vela]

You might want to compare the spectra of blackbodies at different temperatures.

http://en.wikipedia.org/wiki/Black_body

 

[phinds]

Yes, but if it is at the range where it is about to be visible light, then wouldn't the statement be true?

So your point of view is "eh ... it COULD be true. If I set up things just right, there might be a special case where it's true. SO ... by me, it's true. End of story"

Sounds more like a negotiation than a search for reality.

 

[shmijda]

So your point of view is "eh ... it COULD be true. If I set up things just right, there might be a special case where it's true. SO ... by me, it's true. End of story"

Sounds more like a negotiation than a search for reality.

No that's not my point of view I was just examining all possibilities because I don't know what the answer is but ok

 

[vela]

Yes, but if it is at the range where it is about to be visible light, then wouldn't the statement be true?

You seem to be assuming the total amount of light radiated is constant with temperature. It's not.

 

[shmijda]

You seem to be assuming the total amount of light radiated is constant with temperature. It's not.

Doesn't it increase with temperature?

 

[vela]

Yes. Did you look at the plots of the blackbody spectrum at different temperatures? Does the amount of IR radiation decrease with rising temperature?

 

[shmijda]

Yes. Did you look at the plots of the blackbody spectrum at different temperatures? Does the amount of IR radiation decrease with rising temperature?

nope it rises

 

[phinds]

nope it rises

And what do you conclude from this, as regards your problem?

 

[shmijda]

And what do you conclude from this, as regards your problem?

I guess answer 4 is correct despite its ambiguity

 

[phinds]

I guess answer 4 is correct despite its ambiguity

Uh ... we were talking about question #2

 

[shmijda]

Uh ... we were talking about question #2

yep answer 2 must be false then from the black body spectrum

 

[phinds]

yep answer 2 must be false then from the black body spectrum

Good. Now you see why I suggested you might want to rethink your original answer.