Heat Radiation question (skin's reaction to UV versus IR radiation)

[vcsharp2003]

Homework Statement

Why is the sun's infra-red light felt by us instantly compared to ultra-violet radiation that takes a day or a few hours to show sun burn marks?

Relevant Equations

None

I am not sure of the answer. But I am guessing that infra-red rays carry more heat radiation than ultra-violet rays, and the heat is instantly felt on our skin when exposed to both these rays. So what we feel is the heat sensation from infra-red rays and that sensation is missing when it's ultra-violet rays.

 

[haruspex]

In order to sense any radiation, what has to happen to the radiation?

 

[malawi_glenn]

Do you think this is specific to the suns radiation or is it a general feature how different radiation types are interacting with the human body?

If this indeed is a HW question, what are your own thoughts? What kind of work (litterature reading) have you done?

 

[vcsharp2003]

In order to sense any radiation, what has to happen to the radiation?

It must be felt by the skin.

 

[malawi_glenn]

It must be felt by the skin.

Is the skin feeling anything? Feelings is an interplay between the cells in the tissue and the brain.

You must separate the brain signals that makes you "feel" things and registred damage in the tissue.

 

[vcsharp2003]

Do you think this is specific to the suns radiation or is it a general feature how different radiation types are interacting with the human body?

If this indeed is a HW question, what are your own thoughts? What kind of work (litterature reading) have you done?

Ultraviolet and infrared rays should be the same whether it comes from the sun or any other source, since these rays have their unique properties independent of the source of these rays.

I did read in a textbook by Sears and Zemansky that most of the thermal radiation is within the infrared wavelengths at all temperatures of the source body and so I came up with the attempted answer in my OP.

 

[vcsharp2003]

Is the skin feeling anything?

The skin sensation is how a person senses heat radiation. Of course, internally the process of feeling heat radiation by the human body is probably very complex.

 

[malawi_glenn]

I did read in a textbook by Sears and Zemansky that most of the thermal radiation is within the infrared wavelengths at all temperatures of the source body and so I came up with the attempted answer in my OP

A body that is 50 000 K does not have most of its thermal radiation withing the IR wavelengths.

Is this really a HW question? This is more biology than physics anyway.

 

[vcsharp2003]

In order to sense any radiation, what has to happen to the radiation?

Also, heat radiation must be absorbed by human skin, and probably that's what causes an immediate sensation.

 

[vcsharp2003]

Is this really a HW question?

What do you mean by HW? Heat Wave?

 

[malawi_glenn]

Also, heat radiation must be absorbed by human skin, and probably that's what causes an immediate sensation.

UV light is also abosrbed by human skin cells, but in a different way than IR does.
IR causes the water molecules in our cells to vibrate.
UV light is ionizing and breaks down DNA.

Why does not our skin register visible light or radio waves you think?

What do you mean by HW? Heat Wave?

Homework, you have +600 posts on this forum and been member for 5 years. I assumed you knew the common acronyms

 

[vcsharp2003]

I came across an excellent explanation that appears to be closely related to my question at https://www.quora.com/If-UV-rays-ar...4&share=276a3ba9&srid=2W7O&target_type=answer

 

[vcsharp2003]

Homework, you have +600 posts on this forum and been member for 5 years. I assumed you knew the common acronyms

I am not good at remembering acronyms unless I use them in my everyday life. But, knowledge is my final goal.

 

[vcsharp2003]

Why does not our skin register visible light or radio waves you think?

My guess is that they contain less thermal radiation i.e. less heat radiation per unit area per unit time as compared to a radiation that is instantly felt.

 

[vcsharp2003]

Homework, you have +600 posts on this forum and been member for 5 years. I assumed you knew the common acronyms

If this platform had called this forum category as Physics HW Help, then perhaps I would surely remember what HW stood for. But, for some reason, this platform has chosen the full form for the tittle of this forum without acronyms, which makes sense to me because it expresses the whole truth rather than hiding some subtle meaning behind an acronym.

 

[malawi_glenn]

My guess is that they contain less thermal radiation i.e. less heat radiation per unit area per unit time as compared to a radiation that is instantly felt.

What is heat radiation and why does it differ from thermal radiation?

Study https://en.wikipedia.org/wiki/Thermal_radiation

You might wanna read about the photoelectric effect absorption spectra too.

It has to do with the energy carried by each light quanta (photon). Different wavelenghts of light interact differently with atoms and molecules due to quantum physics. This is somewhat analogous to the atmosphere and the green-house effect, the atmosphere is good at letting visible light pass through and "good" at blocking IR radiation. Why is that? It has to do with the chemical composition of the atmosphere, what wavelenghts of light the molecules can absorb.

 

[haruspex]

What is heat radiation and why does it differ from thermal radiation?

@vcsharp2003 did not imply a difference between those. Did you mean to ask how thermal radiation (or heat radiation if you want to call it that) differs from electromagnetic radiation?

 

[malawi_glenn]

@vcsharp2003 did not imply a difference between those.

My guess is that they contain less thermal radiation i.e. less heat radiation per unit area per unit time

That was my interpretation (english is not my native language so I might have misunderstood what OP meant)

@vcsharp2003 many remote controls emit IR radiation, but it is not due to thermal radiation.

 

[haruspex]

I came across an excellent explanation that appears to be closely related to my question at https://www.quora.com/If-UV-rays-ar...4&share=276a3ba9&srid=2W7O&target_type=answer

Certainly the distribution of energy across the wavelengths is important to the question. The diagram at that link makes it look as though most of it is infrared or below. According to https://sos.noaa.gov/catalog/datasets/climatebits-solar-radiation/, it's about equal.
But both consider what arrives at the top of the atmosphere. By the time it gets to us, a significant slice of the IR (and UV) has been absorbed in the atmosphere. Also, a chunk of the radiation below visible is radio waves, which simply pass through us.

So as regards answering the question in post #1, the only other piece to the puzzle you need is why the UV can make our skin feel hot later.

But to understand why the visible light does not immediately feel so warm on the skin, it is necessary to go into how those wavelengths interact with our bodies and how we sense warmth.
Surprisingly, perhaps, we do not directly sense warmth: we detect temperature gradient.
That's why I can feel hot at night in an air temperature I would find comfortable in daytime. In sleep, core temperature drops. It also explains the paradox that people who get hypothermia can feel so hot they start taking clothes off.
So maybe the IR is absorbed so readily (by water molecules in particular) that it sets up a sharp temperature gradient across the sensing cells, while the energy from visible light gets absorbed across a deeper layer.

 

[vcsharp2003]

The diagram at that link makes it look as though most of it is infrared or below.

It does say the following, which appears relevant.

"Even at 5000 K when the peak has moved into the visible, there's a long tail on the IR that accounts for most of the heat."

The area under the curve gives the total radiation per unit area, and the area under the curves shown at that link appear to be substantial in infrared range of wavelengths.

 

[haruspex]

It does say the following, which appears relevant.

"Even at 5000 K when the peak has moved into the visible, there's a long tail on the IR that accounts for most of the heat."

Yes, but it does not discriminate IR from radio waves, does not quote actual percentages like the NASA link does, and does not consider what is lost in coming through the atmosphere.

 

[Tom.G]

The answer in short form was answered back in post #11:

UV light is also abosrbed by human skin cells, but in a different way than IR does.
IR causes the water molecules in our cells to vibrate.
UV light is ionizing and breaks down DNA.

You would do well to follow thru with questions related to what @malawi_glenn posted.

Cheers,
Tom

 

[vcsharp2003]

But both consider what arrives at the top of the atmosphere. By the time it gets to us, a significant slice of the IR (and UV) has been absorbed in the atmosphere.

I get it now.

I came across an interesting plot which confirms what you mentioned about filtering of sun's radiation by the atmosphere. Below is a screenshot that shows in orange color the actual wavelengths from sun reaching us and it's taken from https://www.quora.com/What-are-the-...9&share=a89917a3&srid=2W7O&target_type=answer

As can be seen, the area under the curve is much less for the infrared wavelengths as compared to the area under other wavelengths of the sun's radiation spectrum. The area tells us if the heat radiation is high or low. Clearly the infrared region area is not that much.

 

[vcsharp2003]

But to understand why the visible light does not immediately feel so warm on the skin

But when I go out under the sun in winter time when daytime temperatures are like 10 degrees Celsius, then I feel warmth when sunlight falls on my face. So, I do feel the heat radiation from the sun which is predominantly in the visible light wavelengths.

Or am I feeling the infrared part of the sun's rays even though heat radiation is not that much in the infrared part when I step out in the daytime in winters? Assuming that visible light heat radiation is not easily and quickly felt by the human body.

 

[haruspex]

Or am I feeling the infrared part of the sun's rays

Largely, I would guess.

heat radiation is not that much in the infrared part when I step out in the daytime in winters?

Why do you say that? Merely being a cold day does not change the power spectrum of the light from the sun. What does weaken it in winter is the extra thickness of atmosphere it has to pass through, but as you saw in the chart you posted, that affects both visible and IR.

 

[malawi_glenn]

And ozone (O₃) is absorbing a lot of the suns UV light. Without it, we would have to use sun protector lotion with factor 500 or something all year around

 

[Tom.G]

And ozone (O₃) is absorbing a lot of the suns UV light. Without it, we would have to use sun protector lotion with factor 500 or something all year around

Or we would have grown fur!

(or at least not evolved to NOT grow it.)

 

[Frabjous]

Here‘s a brief from Scientific American “What happens when you get a sunburn”
https://www.scientificamerican.com/article/what-happens-when-you-get/

 

[vcsharp2003]

Yes, but it does not discriminate IR from radio waves, does not quote actual percentages like the NASA link does, and does not consider what is lost in coming through the atmosphere.

Regarding what fails to reach us as sun's radiation, can we say the following?

• Visible part: The atmosphere above the earth allows most visible light to pass through to earth's surface with some of it being reflected back into space from molecules in the atmosphere.
• UV part: The ozone gas molecules in the atmosphere absorbs UV and some UV rays that fail to get absorbed reach the earth's surface.
• Infrared part: The greenhouse gas molecules in the atmosphere of carbon dioxide, methane and nitrous oxide absorb infrared radiation and only some of it manages to reach the earth's surface.

So, the peak of heat radiation from the sun comes in the form of visible light as it's not absorbed by any molecules in the earth's atmosphere.

 

[malawi_glenn]

Depends on what you mean by "peak"...

What is interesting to know is perhaps "most energy" i.e the area in the diagram in post #23