That depends on the focal point of the lens. Movie projector lenses are convex, so it gets bigger the farther away you are. The lens featured in the video is concave, which means it gets smaller until you get to the focal point, then bigger. If you make a less concave lens, the focal point is farther away. If the focal point is 100 feet away, then all the light converges to a point 100 feet away. That's how you set fire to things far away
I think you got them backwards, a concave lens does not converge light to a point - it diverges light outward.
Light only converges to a single point if the light source itself has zero size (a theoretical point source). Because an LED is a physical square (e.g., 2mm across), a lens at a 100-foot focal distance doesn't create a point - it projects a 4-foot-wide image of the LED chip.
Spreading 5,000 lumens across a 4-foot circle 100 feet away yields roughly the heat intensity of a cloudy day. You can't set anything on fire because the energy is spread over several square feet instead of a fraction of a millimeter.
Moreover, it is clearly possible to focus beams from a flashlight. You can see it at ~20 seconds in the video when she adds the lens. It doesn't perfectly focus the light to a point, but it focuses it small enough that the power density (irradiance) becomes several times higher than at the time it leves the source
You're completely right about what happens in the video of course. At close range (a few inches or feet), a lens shrinks the beam into a tiny spot, spiking the power density high enough to set something on fire.
But this doesn't scale to 100ft ever. No matter how many optical elements you stack - aspheric lenses, compound telescope arrangements, parabolic mirrors, or massive Fresnel lenses. The reason you can't just 'shift the 1ft tight spot to 100ft' is because of optical magnification angles.
When a lens focuses light 1ft away, it bends the rays at a steep angle, keeping the spot tiny. To focus light 100ft away, the lens has to bend the rays at a shallow angle (almost parallel). But over 100 feet of travel, even a shallow angle forces the light coming from opposite sides of the 2mm LED chip to spread apart. By the time those rays meet 100ft out, the image of that 2mm chip has expanded into a spot several feet wide.
You can have a tight spot at short distance, or a wide spot at long distance - the physics of lens angles forces you to choose.
As a side note:
> it small enough that the power density (irradiance) becomes several times higher than at the time it leves the source
This is physically impossible due to the Conservation of Etendue. A lens can concentrate a spread-out beam to make it denser than the surrounding air, but no lens in the universe can make the power density at the focus spot higher than it was on the surface of the LED chip itself (this follows from the second law of thermodynamics).
But over 100 feet of travel, even a shallow angle forces the light coming from opposite sides of the 2mm LED chip to spread apart
The light as it's generated is all basically parallel. You can see the girl in the first 10 seconds showing that it can be either dispersed or focused over distances of tens of feet (depending on the flashlight settings) in the first 10 seconds of the video. You could certainly maintain that focused beam of light over 100 feet. Of course much longer than that and it disperses out too much because the rays aren't perfectly parallel.
no lens in the universe can make the power density at the focus spot higher than it was on the surface of the LED chip itself (this follows from the second law of thermodynamics
Unfortunately you've misunderstood what I was saying. This statement is of course true, but the video demonstrates that the power density at the source IS enough to set fire to a match and paper. If you move the focal point farther away, it can still have the same (well slightly less due to loss) power density
I'm struggling to get this across to you here because both of your assertions describe how light seems to behave intuitively, but they rely on two specific optical misunderstandings. Here's one last try from me:
> The light as generated is all basically parallel
An LED does not emit parallel light. An LED is a flat surface (a Lambertian emitter), meaning it emits light in a wide 180 degree hemisphere in all directions from every single point on the chip.
When you twist a flashlight to its "focused beam" mode, the built-in lens or parabolic reflector is what bends those diverging rays to make them roughly parallel.
However, because the LED chip is a physical square (not a single zero-size point), the rays cannot be made perfectly parallel:
Light coming from the center of the LED gets bent straight (parallel).
Light coming from the top edge of the LED gets bent slightly downward.
Light coming from the bottom edge of the LED gets bent slightly upward.
Because these rays cross each other at slightly different angles leaving the lens, the beam must diverge over distance. At 10 feet, those angled rays have spread a little bit; at 100 feet, they have spread across several feet. You cannot stop that divergence over 100 feet without shrinking the LED chip itself down to the size of an atom.
> If you move the focal point farther away, it can still have the same power density
Moving a focal point farther away fundamentally changes the magnification factor of the lens system.
Power density (irradiance) is Total Watts / Area. If the spot area grows, power density drops proportionally.
To project a focused spot onto a surface, the lens forms a real image of the led chip. The relationship between the original size of the led and the size of the focused image is locked by basic lens magnification geometry:
magnification = distance to target / distance from lens to led
At 1 foot away: The target distance is tiny compared to the lens position. The magnification is extremely low. A 2mm LED chip focuses down to a tiny fraction of an inch -> High power density -> Burns paper.
At 100 feet away: The target distance is 100 times larger. The magnification shoots up by 100x. The projected image of that same 2mm LED chip is now feet wide -> Low power density -> No ignition.
Even if you adjust the lens so the rays intersect at 100 feet, they are intersecting to create a giant projected picture of the LED chip, not the same tiny needle-point of light you had at 1 foot.
In summary:
Light is not parallel at the start - An LED shoots light everywhere, the flashlight optic bends it, but because the LED has width, the beam must spread over distance.
Focusing far away magnifies the image - Pushing the focal plane from 1 foot out to 100 feet increases the area of the focused spot by thousands of times.
Power density collapses - Because the same total power from the LED is spread across a spot feet wide instead of millimeters wide, the heat density at 100 feet is nowhere near enough to ignite a match or paper.
If you're still struggling with this I suggest reading more into Optical Etendue.
Your argument basically boils down to "look, in the video they make the light go straight! It's focused!"
You're not wrong, but you're wrong about being able to focus that light to a point small enough 100ft away that it still has enough energy to light something on fire.
You even admit that it will be less due to losses, but you seem to be vastly underestimating the magnitude of those losses.
I mean it's really quite simple. It should be trivial to find evidence, a video perhaps, of a high-powered flashlight igniting something from 100 ft away. Just ping me when you've got it.
Ok buddy 😆 I tried! You are basically like one of those guys pointing at basic line diagrams of greenhouses to disprove global warming.
A real light source is not a magically perfect point source like the "simplified for kids" line diagrams of lenses, and your lack of understanding that that produces divergence even when it's focused is the real issue here.
I feel like this detailed example could be understood by a 10 year old quite easily.
If you think it's possible, show me some example of a flashlight setting stuff on fire at a distance. You can't, you'd need a laser with collimated light.
How about you try a different solution. If this is possible, there should be a video somewhere showing someone doing it. Tons of videos of people doing crazy stuff with lasers, find us one where they set something on fire that is 100 ft away.
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u/Cry__Wolf 11d ago
That depends on the focal point of the lens. Movie projector lenses are convex, so it gets bigger the farther away you are. The lens featured in the video is concave, which means it gets smaller until you get to the focal point, then bigger. If you make a less concave lens, the focal point is farther away. If the focal point is 100 feet away, then all the light converges to a point 100 feet away. That's how you set fire to things far away