Why aren't we getting burned up by light?

[bbbl67]

Considering that infrared is what we call heat, and we can feel it. Also considering that UV is what causes sunburns, why aren't we also being burned by normal visible light? It's not like as if we're transparent and all of that light passes right through us. Visible light has a higher radiation temperature than IR too.

 

[jbriggs444]

Considering that infrared is what we call heat, and we can feel it. Also considering that UV is what causes sunburns, why aren't we also being burned by normal visible light? It's not like as if we're transparent and all of that light passes right through us. Visible light has a higher radiation temperature than IR too.

How many watts per square meter of visible sunlight do we absorb? How many watts per square meter do we emit?

Assume that a human is a decent approximation to a black body and use the Stephan Boltzman law for the latter. Google up some representative values for the former.

You may assume that humans approximate spherical cows if that is helpful.

 

[anorlunda]

You may assume that humans approximate spherical cows if that is helpful.

Wow! I learn something new every day on PF. I knew about ANSI standard rubber chicken equivalents before. Now I can add sperical cows. We engineers are so clever.

 

[russ_watters]

Considering that infrared is what we call heat, and we can feel it.

This isn't true (or is too limited): all blackbody radiation is thermal. In other words, all of the visible, infrared and UV radiation from the sun is "heat" that you feel.

Also considering that UV is what causes sunburns, why aren't we also being burned by normal visible light?

My understanding of sunburns is that they aren't burns in the normal sense of the word - damage due to heat. They are, rather, a defense mechanism/reaction to your body's "known" threat from UV radiation.

 

[ZapperZ]

Wow! I learn something new every day on PF. I knew about ANSI standard rubber chicken equivalents before. Now I can add sperical cows. We engineers are so clever.

Don't tell me you also missed the banana equivalent of radiation dose! :)

Coming back to the original question:

Considering that infrared is what we call heat, and we can feel it. Also considering that UV is what causes sunburns, why aren't we also being burned by normal visible light? It's not like as if we're transparent and all of that light passes right through us. Visible light has a higher radiation temperature than IR too.

Think. Why is it that some kids had to use a magnifying glass to heat or burn up something using sunlight?

Zz.

 

[davenn]

A very indepth article on UV damage to the skin ...

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3709783/

 

[davenn]

My understanding of sunburns is that they aren't burns in the normal sense of the word - damage due to heat. They are, rather, a defense mechanism/reaction to your body's "known" threat from UV radiation.

I don't entirely agree with that ... serious sunburn is burning/searing of the skin = a burn

https://www.dnrme.qld.gov.au/busine...d-other-ultraviolet-radiation-risk-management

UV light causes both acute (short-term) and chronic (long-term) health problems. These problems can be painful, disfiguring and sometimes fatal. Sunburn is painful evidence of overexposure, with sunburn that blisters considered a second-degree burn

https://en.wikipedia.org/wiki/Sunburn

Signs and symptoms[edit]
Blisters on a shoulder caused by sunburn.
Typically, there is initial redness (erythema), followed by varying degrees of pain, proportional in severity to both the duration and intensity of exposure.

Other symptoms can include blistering, swelling (edema), pruritus (itching), peeling skin, rash, nausea, fever, chills, and fainting (syncope). Also, a small amount of heat is given off from the burn, caused by the concentration of blood in the healing process, giving a warm feeling to the affected area. Sunburns may be classified as superficial, or partial thickness burns. Blistering is a sign of second degree sunburn.[4]

https://en.wikipedia.org/wiki/Burn

A burn is a type of injury to skin, or other tissues, caused by heat, cold, electricity, chemicals, friction, or radiation.[3] Most burns are due to heat from hot liquids, solids, or fire.[7] While rates are similar for males and females the underlying causes often differ.[4] Among women in some areas, risk is related to use of open cooking fires or unsafe cook stoves.[4] Among men, risk is related to the work environments.[4] Alcoholism and smoking are other risk factors.[4] Burns can also occur as a result of self harm or violence between people.[4]

Burns that affect only the superficial skin layers are known as superficial or first-degree burns.[1][8] They appear red without blisters and pain typically lasts around three days.[1][8] When the injury extends into some of the underlying skin layer, it is a partial-thickness or second-degree burn.[1] Blisters are frequently present and they are often very painful.[1] Healing can require up to eight weeks and scarring may occur.[1] In a full-thickness or third-degree burn, the injury extends to all layers of the skin.[1] Often there is no pain and the burnt area is stiff.[1] Healing typically does not occur on its own.[1] A fourth-degree burn additionally involves injury to deeper tissues, such as muscle, tendons, or bone.[1] The burn is often black and frequently leads to loss of the burned part.[1][9]

so as far as a definition of a burn goes ... UV sunburn is a physical burn, no different to touching an electric heating element, a fire burn etcDave

 

[haruspex]

It's not like as if we're transparent and all of that light passes right through us.

No, but the energy might be absorbed at different depths, so the depth of the heat sensors is significant. Much of the infrared spectrum is well absorbed by water (a powerful greenhouse gas), so will be absorbed in the first mm at most.

 

[gleem]

so as far as a definition of a burn goes ... UV sunburn is a physical burn, no different to touching an electric heating element, a fire burn etc

Not the same. A thermal burn causes the denaturation of proteins in the cells while a UV "burn" is due to the inflammatory reaction of the cell (thus the redness) due to the UV's interaction with DNA. See for example https://www.scientificamerican.com/article/what-happens-when-you-get/.

UV while strictly not ionizing can break molecular bonds like x-rays and causes far more biological damage than its thermal energy content can cause.

 

[russ_watters]

I don't entirely agree with that ... serious sunburn is burning/searing of the skin = a burn

https://www.dnrme.qld.gov.au/busine...d-other-ultraviolet-radiation-risk-management
https://en.wikipedia.org/wiki/Sunburn

https://en.wikipedia.org/wiki/Burn
so as far as a definition of a burn goes ... UV sunburn is a physical burn, no different to touching an electric heating element, a fire burn etc

Yes, I guess I'm not very clear, physiologically, on what a "burn" is. I tend to associate it with damage due to high temperature, but evidently it's about the skin's response to damage? E.G, chemical and radiation burns look similar to high temperature burns even though the cause is different.

Not sure this is critical to the thread though.

 

[davenn]

but evidently it's about the skin's response to damage? E.G, chemical and radiation burns look similar to high temperature burns even though the cause is different.

yes exactly

Not sure this is critical to the thread though.

maybe, maybe not ... the OP was asking about sunburn ... I was just posting some definitions This from the OP @bbl67 is not really correct ...

Visible light has a higher radiation temperature than IR too.

Photons are not particles that "have temperature", and for particles, no single particle has a temperature

Photons have a energy level as per e=hf and yes, visible light photons have more energy and UV or X-ray photons more still

but the way it interacts with our skin/bodies is dependent on wavelength, absorption, intensity (W/m²) etc as hinted at in posts #2, #8Dave

 

[Anthony Castaneda]

We are getting burned up by visible light. We get sunburns all the time, and skin cancer, skin thickening and discoloration, and you can burn your retinas from desert flashes, or even snow and water reflection. We (Humans) have simply adapted to the level of burning that is happening. Wait for 40 years, and see what damage it does to you.

 

[davenn]

We are getting burned up by visible light. We get sunburns all the time, and skin cancer, skin thickening and discoloration,

hi there
welcome to PF

If you read through the thread and follow and read some of the links, you will learn that is not the visible light causing those problems,
but rather the UV light, which is not visible ( to humans)Dave

 

[Anthony Castaneda]

The UV is worse, but it's NOT the only thing causing problems. NORMAL light does the same thing, and the broad spectrum of the SUN's light, actually damages the skin at different levels and depths into the Dermis. Visible light, for instance, can and will blind you as a reflection off a snow bank, while the UV light penetrates the snowbank, and doesn't approach your eyes at all. THEN there is InfraRed, putting your skin through heat cycles that weaken proteins in skin. I don't speak of this in ignorance. I evaluated sunlight damage on plastics, and degredation of bacterial life on exposed surfaces for years.

 

[davenn]

THEN there is InfraRed, putting your skin through heat cycles that weaken proteins in skin. I don't speak of this in ignorance. I evaluated sunlight damage on plastics, and degredation of bacterial life on exposed surfaces for years.

then you should be able to provide some good references

 

[Anthony Castaneda]

Jeez, what a challenge.
Ever look at the dash below your windshield? Your skin gets the same damage to IT's polymers.

Today there are primarily seven commodity polymers in use: polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalate (PET, PETE), polystyrene, polycarbonate, and poly(methyl methacrylate) (Plexiglas). These make up nearly 98% of all polymers and plastics encountered in daily life.[citation needed] Each of these polymers has its own characteristic modes of degradation and resistances to heat, light and chemicals. Polyethylene, polypropylene, and poly(methyl methacrylate) are sensitive to oxidation and UV radiation,[1] while PVC may discolor at high temperatures due to loss of hydrogen chloride gas, and become very brittle. PET is sensitive to hydrolysis and attack by strong acids, while polycarbonate depolymerizes rapidly when exposed to strong alkalis.

UV light is the most energetic (if you count it as a visible light), but it's NOT the only light with energy. Infrared damage is pretty obvious on your skin. Ever see the pigmentation of your skin after years of life in the desert? We used to call the people who had that, Gila Monsters, and I have personally seen 80 yo women who could cut their skin without bleeding.

https://en.wikipedia.org/wiki/Polymer_degradation

 

[Anthony Castaneda]

As far as SUNLIGHT killing bacteria, that's pretty obvious.
http://sunlightinstitute.org/sanitizing-with-sunlight-the-best-disinfectant-know/

 

[Anthony Castaneda]

And of course, how different wavelengths of light penetrates flesh.
https://en.wikipedia.org/wiki/Penetration_depth

Were you aware that INFRARED light, from a hot fire, was the preferred way of "Seeing" broken bones for over 400 years?

 

[Anthony Castaneda]

http://copublications.greenfacts.org/en/artificial-light/figtableboxes/8.htm

Technically, attenuation down to 1% occurs for light wavelengths of 250-280 nm at around 40 μm depth; for 300 nm at 100 μm; for 360 nm at 190 μm; for 400 nm at 250 μm; for 700 nm at 400 μm; for 1.2 μm at 800 μm; for 2 μm at 400 μm; for 2.5 μm at 1μ; and for 400 μm at 30 μm