Psychology Class and Candles...

[Sharp]

In today's psychology class we were discussing about the senses' minimum perception threshold (i.e. the minimum level of intestity of a stimulus for it to be detected by the body's sense organs).
Well, the teacher was making examples to make the thing clearer, and so we ended up with sight and, according to her, the human eye is sensible, at night, with clear sky, to a CANDLE 31 MILES (50 KM) AWAY! I couldn't believe it, so asked her to confirm the numbers again, and she did confirm it was 31 Miles! To my knowledge, the eye's sensibility to a candle is something well under 330ft (100 m, more or less).

One thing's sure, she's no flashaholic... /ubbthreads/images/graemlins/rolleyes.gif

Richard

 

[_mike_]

I would imagine if you were out in a desert somewhere with unlimited visibility and it was cloudy with no moon and no other ambient light, and the air was clear, you might be able to pick up a lit candle at 31 miles. Though I wonder how big a candle (wick) the instructor was planning on using? Might have to squint though. /ubbthreads/images/graemlins/wink.gif

 

[Sharp]

No way you can see a normal-sized candle (that's what she meant as example) 31 miles away... Not even an M6, I think...
Well, I guess I'll be to go through my teacher's ignorance in this topic.
Theoretically, I could've proved her wrong, by going only 10 mile away and flash a KL1 at her and she'd see nothing, but I didn't feel like proving her wrong today... /ubbthreads/images/graemlins/au.gif

Richard

 

[_mike_]

You should ask her if she has experienced this firsthand or if this is just something she has read. Man, that'll get her going. /ubbthreads/images/graemlins/evilgrin07.gif

 

[jtr1962]

A standard candle at 31 miles is 0.0000000004 (4x10^-10) cp. I highly doubt the eye could pick that up, even under ideal conditions. A good point of comparison would be how bright that would be relative to a magnitude 6 star, which is about the dimmest star anyone with normal vision can see. The sun is -26.8 on the magnitude scale. 5 orders of magnitude is a factor of 100 in brightness, so a 6 magnitude star is about 10^-13 as bright as the sun. The sun is roughly 100,000 cp, so a magnitude 6 star is 10^-8 cp, or about 25 times as bright as the candle. In other words, the candle is probably not visible from 31 miles, although it would be barely visible at about 6 miles. This is still a surprising result.

 

[Lynx_Arc]

College professors/teachers tend to believe whatever is written in books as fact without testing or thinking about it more often than they believe people that try to convince them otherwise. I have caught several mistakes in textbooks and pointed it out to teachers and half the time I am scolded as not knowing what I was talking about, the other half of the time I was said I was right but nothing became of it. I would tend to agree with jtr1962 using my own experience, most people cannot focus on anything that small that far away thus picking out something that would be akin to a pixel on a 7 billion by 4 billion monitor would be essentially impossible without cheating. Perhaps the person *seeing* this candle had a telescope or the candle was against a mirror or inside a lighthouse.

 

[Brock]

Being an avid boater it is amazing what you can see at a distance, but I can assure you not a candle at 30+ miles. Two problems, first at 31 miles it would have to be about what 100 stories up to clear the curvature of the earth? depending on how high you are. Secondly about the farthest we could see a street light in pitch black was about 20 miles, because of the fuzziness of the heat from the ground. If you went up in a bozeman’s chair about 30 feet and use bino's you could see much better.

I can tell you, you can see a photon at 1.5 miles. We used them for signaling, that is amazing. Again it was pitch black.

 

[Brock]

Oh, and everyone on our crew agreed that when locating navigational buoy’s if you looked directly at the light it was much harder to see, if you looked just to the side if it a bit it was more noticeable, we attributed this to burned out nerves in the retina???

 

[jtr1962]

Actually I think at 31 miles the candle would need to be about 1275 feet above the Earth's surface to be visible due to the curvature of the Earth (going by similar triangles-31/4000*31*5280=1269'). Even at 6 miles it would still need to be about 250 feet up. In fact, under most circumstances the curvature of the Earth would prevent the candle from being seen long before it was too dim. An average sized human holding a candle at 4 feet above the ground would disappear over the horizon at roughly a quarter of a mile (assuming that the viewer's eyes are 5 feet above the ground).

 

[Orion]

For lack of a more scientific explanation: The eye has more sensitive rods/cones just off the center of the eye which is why you can see lights better if you look just a little away from them.

 

[Lynx_Arc]

If I remember correctly the optic nerve fouls things up in that area of the eye for that type of sight, if you focus off to the side of where light would fall directly over where the optic nerve connects you will hit more rods/cones. I would have to look this up to verify it though.

 

[Kiessling]

In the center of the eye (focal point in daylight) we are more sensitive to colors and not brightness for better sight during the day which makes sense. Next to this area there come areas that are more sensitive to black and white and have a better light-sensitivity overall ... so with those you can see better in the dark (where you cannot see any colors anyway when it's dark enough).
This is the reason why very dim stars disappear when you focus on them and reappear when you focus on something next to them ... by focussing on the dim object you are using the central area of the eye that is less sensitive to light and prefers colors.

bernhard

edited for horrible spelling

 

[Sharp]

For what I know, the limit for eyes' sensibility is a 3kW HID light 160km away, provided you're on a mountain top, and seeing the light from above, to overcome the earth's curvature problem...

Richard

 

[Lynx_Arc]

hey it could be the candle that was 31 miles away was the one currently used to measure candlepower ratings on spotlights today. That would explain it completely.

 

[gadget_lover]

I have to wonder if that 31 mile figure was supposed to be in pitch black,(no other light sources at all) in a vacuum. If so it would be an example of trying to make a point by making up an impossible real world example by extrapolating from data about rod and cone sesitivity.

In other words, I suspect that they found that under ideal labratory conditions the rods (or was that cones?) would generate a signal when exposed to a light source that was equivilent to the aformentioned theoretical candle. These analogies frequently disregard the physical properties that would constrain the real life experience.

Similar analogies include the the 'X number of drops in a swiming pool' for dilution and "X units laid end to end will reach all the way to....".

Looking down from 40,000 feet at the landscape, it's amazing how many lights you can see 30, 40 or more miles away.

Daniel

 

[Sharp]

Daniel,
most probably that's the right case. However, she messed that one up too, maybe because she herself didn't get the example she probably read, by saying that it would be as if she'd see a candle from her house, placed on the beach, which should be 30+ miles away from her house. I simply feel sorry for her... /ubbthreads/images/graemlins/ohgeez.gif
Anyway, this shows the abominal ignorance of the "unenlightened" regarding light, lights, and their properties /ubbthreads/images/graemlins/tongue.gif...

Richard

 

[Lynx_Arc]

She isn't alone, people used to believe everything orbited the earth at one time... it was called the dark ages.. perhaps this was the time you could see such a candle 30 miles away?

 

[jtr1962]

[ QUOTE ]
jtr1962 said:
Even at 6 miles it would still need to be about 250 feet up. In fact, under most circumstances the curvature of the Earth would prevent the candle from being seen long before it was too dim. An average sized human holding a candle at 4 feet above the ground would disappear over the horizon at roughly a quarter of a mile (assuming that the viewer's eyes are 5 feet above the ground).

[/ QUOTE ]
Actually, I made a mistake (although the 31 mile figure is correct). At 6 miles the candle (or observer) would need to be about 50 feet high or it would not be visible due to the Earth's curvature. And a human carrying a candle would disappear over the horizon after about 2.5 miles (not a quarter of a mile). I thought about it for a while and realized that intuitively my original figures where wrong. I then corrected my calculations. Remember though that even at 2.5 miles it would need to be pitch black to discern a candle. 6 miles might be possible on the dark side of the moon. And a 20,000 mcd white LED would be visible to maybe 25 miles under the same conditions, provided it was high enough so that it didn't disappear over the horizon.

 

[Lynx_Arc]

uhoh.... I can see it now... instead of a billion candlepower spotlight it is a 55 mile spotlight then a 2 million mile spotlight... ad nauseum.

 

[JonSidneyB]

Ummm, on the moon it would not work as well....while the lack of atmosphere would help, the curve is sharper.