Looking Into The Sun

**Nazgûl Killer** · August 06, 2009, 11:14 PM

I know, I know, you looked at the thread title and started laughing (Probably), I would've too, but, this is a serious question.

I did a little test the other day, I looked into a (VERY) bright spotlight for like 3 seconds without blinking, and I managed to stay like that, with only a slight pain in my eye.

Now, after that, I looked at the sun (Which was behind a cloud, thus, it was not very bright at all), but I couldn't look even for just one second, it immediately hurt me as if in the back of my eye and I had to look away.

I did this experiment twice, yesterday and today, and now I must ask: WHY is that? Is it possible that the sun's radiation affects me in such a way that it creates unbearable pain in my eyes, or is it just psychology that tells me "DON'T LOOK INTO THE SUN!" and forces me to look away?

**DekuTrash** · August 06, 2009, 11:35 PM

Because the sun is an enormous fireball as opposed to a wee little light bulb.

**Tiwaz** · August 07, 2009, 02:47 AM

Enormous ball of fusion which radiates on huge spectrum of EM of which visible light is only fraction.

Nazgul, how much UV-A and UV-B you think you get from spotlight? Or infrared?

**Baron von Sky Hat** · August 07, 2009, 04:24 AM

An interesting question, one for which it's possible to do a quick calculation to show whether the two are comparable sources of brightness.

Flux (amount of energy per second striking one unit area) from the sun at the distance that the earth is is
I = L / 4 pi r^2
L is 3.8E26 Watts (the luminosity of the sun), and r is 1 AU or 1.5E11 meters or 93 million miles
This gives I_sun = 1330 W/m^2

A spotlight might be, maybe, 5000 Watts rated (I doubt much more than that, I think stage lights are 2000 ish, so spotlights won't be much more than that)? A halogen lamp has an efficiency of perhaps 30 lm per Watt (according to wiki), or 50mW of radiated light per Watt of power rating, so 250 Watts of radiated light for this spotlight. We can also assume that it radiates into only pi radians of solid angle, rather than the spherical 4 pi radians used for the sun. (If you have better values for the situation that you mention, they can be substituted in instead).

So, if you're, maybe, 5 meters from the thing while looking at it (rather closer than I'd recommend staring into a bright source of light), then the same equation results in:
I_spotlight = 3.2 W/m^2

Now, the solar value doesn't account for any attenuation by the Earths atmosphere, but even so, it's clear that it's a good few orders of magnitude difference. Even if you assume that merely one percent of the sun's light gets through to the ground it's still a good few times greater than the value for the spotlight (I'm confidant that we get rather more than 1% at ground level, though I can't seem to find any number in the time I've got). So it shows that, even comparitively close to a spotlight is still going to be very much less radiative power per unit area of your eye than the sun.

In addition, as Tiwaz mentioned, the sun has a much higher amount of UV than a normal lamp source will have. The Earth's atmosphere shields us from a fair portion of this, but a not unreasonable amount will still get through. Our eyes don't `see' UV, but they can be affected by the high energy that it has, hence sunglasses designed to filter UV specifically to prevent eye damage over long periods of exposure.
As far as I'm aware, IR isn't so much of a problem, as it's lower energy, much mid to far IR is blocked by the atmosphere (which is why we don't have many IR telescopes on the ground, though there are certain bands of IR that can get through) and most lamps will emit a lot in the IR too, making them more similar in that regard. IR is what gives sunlight the heat that it seems to have.

But as you mention, I also wouldn't be surprised if there was additionally a psychological element. Knowing specifically that we shouldn't look straight at the sun. However, given how much brighter the sun is, I would doubt that this would be the dominant mechanism for not being able to look at the sun for long.

Hope that's of some interest and help

**Wheelchair** · August 07, 2009, 06:53 AM

Originally Posted by Baron von Sky Hat

An interesting question, one for which it's possible to do a quick calculation to show whether the two are comparable sources of brightness.

Flux (amount of energy per second striking one unit area) from the sun at the distance that the earth is is
I = L / 4 pi r^2
L is 3.8E26 Watts (the luminosity of the sun), and r is 1 AU or 1.5E11 meters or 93 million miles
This gives I_sun = 1330 W/m^2

A spotlight might be, maybe, 5000 Watts rated (I doubt much more than that, I think stage lights are 2000 ish, so spotlights won't be much more than that)? A halogen lamp has an efficiency of perhaps 30 lm per Watt (according to wiki), or 50mW of radiated light per Watt of power rating, so 250 Watts of radiated light for this spotlight. We can also assume that it radiates into only pi radians of solid angle, rather than the spherical 4 pi radians used for the sun. (If you have better values for the situation that you mention, they can be substituted in instead).

So, what your saying is, you have alot of time.

**Swordfish08** · August 07, 2009, 11:18 PM

Originally Posted by ZainyAntics

So, what your saying is, you have alot of time.

Especially since he could have just looked up what the solar constant was and he wouldn't have had to do that first flux calculation.

Though, props for doing the math, anyway. It does make a better explanation, as it shows how we actually come up with these numbers.

**Baron von Sky Hat** · August 08, 2009, 05:42 AM

Originally Posted by Swordfish08

Especially since he could have just looked up what the solar constant was and he wouldn't have had to do that first flux calculation.

Though, props for doing the math, anyway. It does make a better explanation, as it shows how we actually come up with these numbers.

Heh, if anything, I'd say that, of the free time I have, I spend too much of it on things like that

But as to too much time, it was only a minute or two to write that post, the work I do involves flux and luminosity calculations so they're somewhat second nature.

I could have gone straight to the solar constant value for flux at Earth, but it wasn't really any additional time and thought it'd be good to show the maths behind the idea.

**M D** · August 07, 2009, 08:21 AM

AHAHAHHA, rep for you

**MrMofo** · August 07, 2009, 08:31 AM

Essentially, a spotlight will be uncomfortably bright whereas the sun will make bacon out of your retinas.

**Nazgûl Killer** · August 11, 2009, 02:21 AM

Thank you, what I've gathered from all these replies is that I was right, it was the excess radiation that does not make itself visible to the naked eye as light that did the trick.

Thanks everyone!

**Baron von Sky Hat** · August 11, 2009, 06:00 AM

Well, I'd say it's not really the UV, IR or other non-visibles specifically. Most of both is blocked by the atmosphere, where visible isn't, and the sun's output peaks in about the visible anyway. It's simply that the sun is way brighter than the lamp, even in the visible.

**Ludicus** · August 13, 2009, 09:53 AM

I looked into a (VERY) bright spotlight for like 3 seconds without blinking, and I managed to stay like that, with only a slight pain in my eye

A very bad idea.
Take care with the solar retinopathy - majority of serious cases involve sun gazers,eclipse viewing, photographers, etc. Even a single second is enough to eventually cause permanent damage to retina. Make an appointment to see an ophthalmologist, if you have black/dark spots in the visual field. A proeminent synptom is brow ache.
You know, light is absorbed by melanin of the RPE, and is converted to heat, and produces retinal damage in the RPE and surrounding layers of retina.