Does skin perpendicular to sun’s rays burn faster than if not perpendicular?

In summary, a friend of mine believes that the skin that is perpendicular to the sun's rays burns faster than skin that is closer to parallel to the sun's rays.
  • #1
DaveN123
5
0
Does skin that is perpendicular to the sun’s rays burns faster than skin that is closer to parallel to the sun’s rays? I was certain that it does but a very smart friend/engineer is certain that it doesn’t and was incredulous that I thought it did.

I really would like to know the physics behind it if he is right. I haven’t been able to find an answer to this question online, because most of the information online is about the intensity of the sun relative to the angle of the sun through the atmosphere, versus the object on the ground changing orientation. If he is right, would it mean that if you want to collect the sun’s energy in a small area, it would be best to place collectors at close to parallel to the sun’s rays as possible without overlapping, sort of like louvers? I haven’t seen any solar collectors like that. Thanks!
 
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  • #2
The power density (or energy flux) is maximum when propagation is perpendicular. When exactly parallel, the sun's rays travel past and are never absorbed (at least if your arm is a perfectly planar surface). In between, the flux varies as sine of the angle.

The center picture here
http://en.wikipedia.org/wiki/Flux"
shows how the flux decreases with angle.

Burns occur more rapidly with normal (aka perpendicular) incidence. That's why your nose, shoulders and sometimes feet burn at midday when the sun is overhead, while your chest, legs and arms (if they stay by your side) do not.
 
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  • #3
marcusl said:
That's why your nose, shoulders and sometimes feet burn at midday when the sun is overhead, while your chest, legs and arms (if they stay by your side) do not.
Yep.

Another way is to look at the tan/burn of someone who did not move. If tan/burn was not dependent on angle of incidence, the line between red leg front and white leg back would be knife edge sharp, despite being on a curved form.

It's not. Red-to-white is well-graded around the curve of the arm or leg.
 
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  • #4
Thanks so much for the replies. His view is that there is no definite line on a sun burn because the skin that was closer to parallel was actually moving in and out of the sun as the sun moved or the person moved, so the amount of sun burn is based totally on the time of the exposure, versus the angle of the exposure.

Thanks again for your replies!
 
  • #5
DaveN123 said:
Thanks so much for the replies. His view is that there is no definite line on a sun burn because the skin that was closer to parallel was actually moving in and out of the sun as the sun moved or the person moved, so the amount of sun burn is based totally on the time of the exposure, versus the angle of the exposure.
Certainly. No question that is a confounding factor (I was hoping you wouldn't think of it :tongue: ). Simply seeing a continuous gradient from red to white on the side of a leg does not conclusively point to incidence of light.

Motion will produce a similar result, so will movement of the sun over time (at least in principle).
 
  • #6
And the same effect is why solar panels make the most power when perpendicular to the suns rays, and why windows absorb the most solar heat when the sun is roughly perpendicular to the glass surface...
 
  • #7
...and why the Earth has ice caps. And seasons. And is hot at the equator.

What is the smart friend's explanation for these phenomena?
 
  • #8
I missed the part that your "smart" engineer friend insisted this is so.

I would like to know the following:
Please ask your fiend what kind of engineer he is. Civil?
Can you please ask him for a list any bridges or structures he has built, I'd like to know which ones to avoid.
 
  • #9
DaveN123 said:
If he is right, would it mean that if you want to collect the sun’s energy in a small area, it would be best to place collectors at close to parallel to the sun’s rays as possible without overlapping, sort of like louvers? I haven’t seen any solar collectors like that. Thanks!
Waitaminnit! Your friend is really on to something! He could make millions!

See diagram.
PF20100304solarpanels.gif
 
  • #10
DaveC426913 said:
Waitaminnit! Your friend is really on to something! He could make millions!

See diagram.
PF20100304solarpanels.gif

HAHA!YES!Thats very interesting idea! in your second graph, indeed won't much light will reflect, the efficiency can be maximised, however I m just wonder how much cost for one sun panel.

Great idea:)
 
  • #11
Hello again. Thanks again for all of the information.

What started my conversation with my friend was that I thought that the sun was strongest on my front/back (when standing) at about 6pm versus 1pm. He thought that due to the increased thickness of the atmosphere as the sun gets lower in the sky, that the sun is not nearly as strong, even on vertical surfaces, at 6pm.

He still thinks that is the case, so thanks to some of your information I looked up some equations and tried to calculate it. Attached is an Excel spreadsheet and picture of what I came up with, including the sources. It does show the sun stronger at 6pm on vertical surfaces, but I'm not 100% sure that I did it correctly. Please feel free to correct me if you find an error on my calculations. I don't need super accurate equations. I just need a relative value between close to high noon and closer to sunset. Thanks!
 

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  • #12
Well, now you've thrown a monkey wrench into the works.

Your question has changed to become:

Which is a bigger influence on exposure: incident angle of sun to exposed surface? or thickness of atmosphere due to height of sun above horizon?

That is a completely different animal!
 
  • #13
Yes DaveC426913, you are correct. As we were discussing about sunburn on the back/front of a standing person, we got into the disagreement about what I initially posted the question about, which was so black-and-white that I had to try to confirm one way or another. He now accepts that correction and says that was just a minor point of his view (it seemed major to me at the time), and says that his major point was/is the thickness of the atmosphere. Thanks!
 
  • #14
I think I will start a new topic to discuss sun exposure on the back/front at near sunset versus exposure on top of head at noon, since it definitely is a different topic. Thanks.
 

1. How does the angle of the sun's rays affect skin burning?

The angle of the sun's rays does not directly affect skin burning. The intensity of UV radiation from the sun is what causes skin burning.

2. Does standing perpendicular to the sun increase the risk of skin burns?

Standing perpendicular to the sun does not necessarily increase the risk of skin burns. The intensity of UV radiation is affected by many factors, including time of day, weather conditions, and altitude.

3. Do people with darker skin tones burn faster when standing perpendicular to the sun?

People with darker skin tones have more melanin, which provides natural protection from the sun's UV radiation. However, they are still at risk for skin burns if exposed to intense UV radiation, regardless of the angle of the sun's rays.

4. Is it safer to stand with your back to the sun to avoid burning?

Standing with your back to the sun can provide some protection from UV radiation, but it is not a foolproof method to avoid burning. It is important to wear sunscreen and protective clothing regardless of your position in relation to the sun.

5. Are there any benefits to standing perpendicular to the sun's rays?

Standing perpendicular to the sun's rays can provide some benefits, such as increased vitamin D production. However, it is crucial to balance exposure to the sun with proper protection to avoid skin burning and other harmful effects.

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