# I Why aren't we getting burned up by light?

1. Nov 27, 2018

### 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.

2. Nov 27, 2018

### jbriggs444

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.

3. Nov 27, 2018

### Staff: Mentor

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.

4. Nov 27, 2018

### Staff: Mentor

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.
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.

5. Nov 27, 2018

### ZapperZ

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

Coming back to the original question:

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

Zz.

6. Nov 27, 2018

### davenn

7. Nov 27, 2018

### davenn

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

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

Dave

8. Nov 27, 2018

### haruspex

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.

9. Nov 27, 2018

### gleem

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.

10. Nov 27, 2018

### Staff: Mentor

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.

11. Nov 27, 2018

### davenn

yes exactly

maybe, maybe not .... the OP was asking about sunburn .... I was just posting some definitions

This from the OP @bbl67 is not really correct .....

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, #8

Dave

12. Dec 5, 2018

### 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.

13. Dec 5, 2018

### davenn

hi there
welcome to PF

but rather the UV light, which is not visible ( to humans)

Dave

14. Dec 5, 2018

### 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.

15. Dec 5, 2018

### davenn

then you should be able to provide some good references

16. Dec 5, 2018

### 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.

17. Dec 5, 2018

### Anthony Castaneda

18. Dec 5, 2018

### 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?

19. Dec 5, 2018

### Anthony Castaneda

Last edited: Dec 5, 2018