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Sun vs. Artificial Light

...JR

Scholar
J
Joined
Nov 21, 2005
Messages
82
My question is simple:

What are the major differences between the photons emmited by the sun and those emmited by say a lamp? I know that if you go outside and place a magnifying lens over say a small stack of dried leaves, you can set them on fire by focusing the rays into one point on the pile. Is this possible with the light from a common lamp and if not why?
 
Well, you actually have 2 questions ...

The first -- the difference in photons is quantity and energy distribution. The sun will have photons covering the entire spectrum of light, whereas a lamp is usually limited to a select range, usually denoted by the lamp's color temperature (in Kelvins). Also, the sun generates many more photons than any lamp on Earth. The actual surface that emits photons with an incandescent lamp is very small -- a thin filament of wire, usually. And a fluorescent tube, although much greater in area, is so weak in brightness that one can stare directly at it with no harm. The sun, however, emits photons over an area as seen from Earth, much greater in apparent area than most lamps seen even at an arm's length away.

The second -- but it would not be impossible to build a lamp that generates enough photons that if concentrated by a lens could ignite a piece of paper. Just put your hand under an infrared lamp and see how much heat that generates; then imagine concentrating all that heat energy to a point.
 
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Just thinking said:
Well, you actually have 2 questions ...

The first -- the difference in photons is quantity and energy distribution. The sun will have photons covering the entire spectrum of light, whereas a lamp is usually limited to a select range, usually denoted by the lamp's color temperature (in Kelvins). Also, the sun generates many more photons than any lamp on Earth. The actual surface that emits photons with an incandescent lamp is very small -- a thin filament of wire, usually. And a fluorescent tube, although much greater in area, is so weak in brightness that one can stare directly at it with no harm. The sun, however, emits photons over an area as seen from Earth, much greater in apparent area than most lamps seen even at an arm's length away.

The second -- but it would not be impossible to build a lamp that generates enough photons that if concentrated by a lens could ignite a piece of paper. Just put your hand under an infrared lamp and see how much heat that generates; then imagine concentrating all that heat energy to a point.
Click to expand...

excellent, just what i was looking for and more :)
 
Just to add - biological matter usually absorbs infrared much better than visible light, so it is these frequencies that cause the leaves to catch fire. For an incandescent lamp, Just Thinking's post applies. Flourescent lamps give out almost all their energy as visible light with very little infrared, so even if you made one that sent as much power to the leaves as the Sun, you would still not be able to set fire to them.
 
Just thinking said:
The second -- but it would not be impossible to build a lamp that generates enough photons that if concentrated by a lens could ignite a piece of paper. Just put your hand under an infrared lamp and see how much heat that generates; then imagine concentrating all that heat energy to a point.
Click to expand...

Oh, you can do a lot better than burning paper with high-power lamps. There are commercial furnaces you can buy which use halogen lamps to reach over 3800 deg. F. That's quite a bit hotter than you need to burn paper, and a lot hotter than you need to melt steel, too.
 
Ziggurat said:
Oh, you can do a lot better than burning paper with high-power lamps. There are commercial furnaces you can buy which use halogen lamps to reach over 3800 deg. F. That's quite a bit hotter than you need to burn paper, and a lot hotter than you need to melt steel, too.
Click to expand...

As an undergrad research project I actually cobbled together a setup that used two projector lamps with built-in parabolic reflectors to melt analogs of chondrule precursors placed at their foci. No magnifying glass needed! Yep, a filament'll do it.
 
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Buckaroo said:
As an undergrad research project I actually cobbled together a setup that used two projector lamps with built-in parabolic reflectors to melt analogs of chondrule precursors placed at their foci. No magnifying glass needed! Yep, a filament'll do it.
Click to expand...

Parabolic reflectors are what the commercial furnaces use too. You get much more solid angle coverage with a mirror than you can with a lens.
 
When Burning leaves you will notice you get a tiny image of the sun as the visble portion of the light. comes into focus. The focal point of the infrared will not be quite at the same spot. Try raising the magnifying glass a little bit an you will find the power is increased.

Warning. Black newspaper print gets hot enough to cause an open flame! So be careful. Also wear sunglasses and put some sunscreen on the back of your neck or you'll get headaches! :) and don't look at the sun!

My fave was to burn those rolls of caps you get for toy guns or melt toy soldiers.

(OK I zaped a few ants but I felt guilty straight away, that part of my boyhood lasted about 2 mins).
 
Orangutan said:
When Burning leaves you will notice you get a tiny image of the sun as the visble portion of the light. comes into focus. The focal point of the infrared will not be quite at the same spot. Try raising the magnifying glass a little bit an you will find the power is increased.

Warning. Black newspaper print gets hot enough to cause an open flame! So be careful. Also wear sunglasses and put some sunscreen on the back of your neck or you'll get headaches! :) and don't look at the sun!

My fave was to burn those rolls of caps you get for toy guns or melt toy soldiers.

(OK I zaped a few ants but I felt guilty straight away, that part of my boyhood lasted about 2 mins).
Click to expand...
Ant murderer! :)

Just an optics nitpick: The focal point in this case is closer to the lens than the image (of the Sun). I am not sure about the focal point in infra red, but it should be close to the one for visible light (but not exactly at the same distance, as you point out.)
 
Ririon said:
Ant murderer! :)

Just an optics nitpick: The focal point in this case is closer to the lens than the image (of the Sun). I am not sure about the focal point in infra red, but it should be close to the one for visible light (but not exactly at the same distance, as you point out.)
Click to expand...

I may not have made myself clear, I meant that you find the focal length for visible light by the usual method of making the bright dot as small as you can. (IT's actuall a little image of the sun at that point, if you have clouds near by they pop into focus too if its bright enough.) And the you move the magnifying glass up a little bit. As infra red is just above the visable spectrum you don't have to go far.

This image might help Wikipeida image.
http://en.wikipedia.org/wiki/Image:Lens6a.svg
Image:Lens6a.svg


O.
 
I had a rectangular Fresnel lens that used to be mounted in front of an old TV set in order to make the screen appear larger -- very old B&W set. Anyway, it was so large that I remember having to hold it with my arms outstretched due to its size (and me being very young). It did, however act like a magnifying lens, and one day I thought it might be something to watch as I tried to fry some bug. Well, I saw a bee in a flower and lifted the lens over to focus the light on the bee. All I did was pan across the plant -- the flower was instantly scorched -- no sign of the bee. I quickly decided this was too dangerous to keep playing with.
 
Orangutan said:
I may not have made myself clear, I meant that you find the focal length for visible light by the usual method of making the bright dot as small as you can. (IT's actuall a little image of the sun at that point, if you have clouds near by they pop into focus too if its bright enough.) And the you move the magnifying glass up a little bit. As infra red is just above the visable spectrum you don't have to go far.

This image might help Wikipeida image.
http://en.wikipedia.org/wiki/Image:Lens6a.svg
[qimg]http://en.wikipedia.org/wiki/Image:Lens6a.svg[/qimg]

O.
Click to expand...

You are quite correct apart from a couple of details:

You do not get an image of the sun at the focal point. The image of the sun and the clouds is where the little green arrow is in this figure. The focal point is at the distance f in the figure:

550px-Lens3.svg.png


From:
http://en.wikipedia.org/wiki/Lens_(optics)#Imaging_properties
 
Ririon, not entirely true. For the sun (and even clouds), you can assume S1 to be infinite. In which case S2=f. At least for most practical purposes.
 
elgarak said:
Ririon, not entirely true. For the sun (and even clouds), you can assume S1 to be infinite. In which case S2=f. At least for most practical purposes.
Click to expand...

Yes ... for distant objects the image will form at the focal point; distant being even 50 feet away or more.
 
Orangutan said:
(OK I zaped a few ants but I felt guilty straight away, that part of my boyhood lasted about 2 mins).
Click to expand...

2 minutes! That's pretty good morality. My twisted fascination lasted a full day when I discovered this bug killing method.

The smell of burning ant flesh smells the best. Other bugs smell terrible. It was probably the dislike of the odor that kept me from becoming the next psycho serial killer.

The sickest thing I did was fry a daddy-long-legs (Well I'll be darned. It's not really a spider. I didn't know that.). It was a mercy killing after I tore all its legs off. :boxedin:
 
Bruce said:
The sickest thing I did was fry a daddy-long-legs (Well I'll be darned. It's not really a spider. I didn't know that.). It was a mercy killing after I tore all its legs off. :boxedin:
Click to expand...

The poor spide... um... not spider. I knew that it was not a spider even when I was young.

By the way, the Daddy Long Legs is very poisonous. However, it cannot bite, so it is not a problem. But... if you wanted to so someone in, just find a bunch of Daddy Long Legs and place them with some liquid in a blender. Serve and wait.
 
A few days ago, I bought some compact fluorescent light bulbs---the kind that have the same base as regular incandescent bulbs. The color of things doesn't quite look right under their light.

Looking at the reflection of one in a CD, I could see fairly easily that it has a discrete spectrum of five colors, rather than a continuous spectrum as sunlight and incandescent bulbs have.

I have an LED flashlight that puts out a bluish white light. Its spectrum appears to be continuous.
 
Ririon said:
You are quite correct apart from a couple of details:

You do not get an image of the sun at the focal point. The image of the sun and the clouds is where the little green arrow is in this figure. The focal point is at the distance f in the figure:

[qimg]http://upload.wikimedia.org/wikipedia/en/thumb/7/71/Lens3.svg/550px-Lens3.svg.png[/qimg]

From:
http://en.wikipedia.org/wiki/Lens_(optics)#Imaging_properties
Click to expand...

Ah! I see, Thanks for correcting me. You see you learn stuff here.
:)
 
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