step 5Acquire Focusing Lens
Now that you know the focal length of your Fresnel, it's time to get a diverging lens to bend the light into a beam. This will go right at the focal point, so you get as small a beam as possible.
Benefits of Creating a Beam:
In optics, the strength of a lens is measured by its focal length (stronger lenses have shorter ones). To cancel the converging effect of the Fresnel lens, we need to either diverge the light before it gets to the focal point (use a diverging lens with a negative focal length) or converge it after the light spreads out beyond the focal point (using a converging lenses like a magnifying glass).
Diagram 1
When two lenses are far apart, it's useful to think of light in terms of geometry and angles: the focusing lens has to be strong enough that its focal length is small so that the light spreading out from the Fresnel's focal point is completely captured by the second lens.
From basic geometry, we know that the second lens has to have at a ratio ratio of diameter to focal length at least as big as the Fresnel lens in order to capture all the light. This means if the second lens has a focal length fB , it has to have a diameter of at least
dB = fB (dA / fA )
where dA and fA are the diameter and focal length of your Fresnel (use the larger width since the Fresnel is not a circle).
Diagram 2
With a strong enough lens (the one I got had a focal length of 35mm), you put the lens 35mm (or whatever) past the Fresnel's focal length. The light will then be bent inward, forming a beam. Of course, this will only be approximate, so you'll have to move the lens back and forth until you find the correct distance.
Optics Applet
An excellent resource for basic optics is this Optics Applet I've found. You can't really use it to get real-world numbers, but it's very handy for planning and understanding how lenses interact. Place a "beam" on the x-axis, then a couple lenses (you can adjust the focal lengths by dragging the little white squares).
You can find lenses in lots of random places online, and the closer the focal length is to your measurement, the better. Also, bigger lenses are preferable because giant Fresnels typically don't create a very small focus spot (between 1 and 2 inches wide) so you'll need at least a 2" wide lens to capture all the light.
Where I bought my lens:
Surplus Shed
There are other places I'm sure, especially educational sites and the like...but it may be hard to find the exact lens you need. I should also mention that you want a glass lens, plastic simply won't do for this intensity of light.
Benefits of Creating a Beam:
- Objects don't have to be right at the focal point to burst into flames!
- The beam can be further manipulated - magnified, reflected, put through a prism, whatever floats your optical boat.
- Ridiculously intense light beams are like lasers - they're awesome .
In optics, the strength of a lens is measured by its focal length (stronger lenses have shorter ones). To cancel the converging effect of the Fresnel lens, we need to either diverge the light before it gets to the focal point (use a diverging lens with a negative focal length) or converge it after the light spreads out beyond the focal point (using a converging lenses like a magnifying glass).
Diagram 1
When two lenses are far apart, it's useful to think of light in terms of geometry and angles: the focusing lens has to be strong enough that its focal length is small so that the light spreading out from the Fresnel's focal point is completely captured by the second lens.
From basic geometry, we know that the second lens has to have at a ratio ratio of diameter to focal length at least as big as the Fresnel lens in order to capture all the light. This means if the second lens has a focal length fB , it has to have a diameter of at least
dB = fB (dA / fA )
where dA and fA are the diameter and focal length of your Fresnel (use the larger width since the Fresnel is not a circle).
Diagram 2
With a strong enough lens (the one I got had a focal length of 35mm), you put the lens 35mm (or whatever) past the Fresnel's focal length. The light will then be bent inward, forming a beam. Of course, this will only be approximate, so you'll have to move the lens back and forth until you find the correct distance.
Optics Applet
An excellent resource for basic optics is this Optics Applet I've found. You can't really use it to get real-world numbers, but it's very handy for planning and understanding how lenses interact. Place a "beam" on the x-axis, then a couple lenses (you can adjust the focal lengths by dragging the little white squares).
You can find lenses in lots of random places online, and the closer the focal length is to your measurement, the better. Also, bigger lenses are preferable because giant Fresnels typically don't create a very small focus spot (between 1 and 2 inches wide) so you'll need at least a 2" wide lens to capture all the light.
Where I bought my lens:
Surplus Shed
There are other places I'm sure, especially educational sites and the like...but it may be hard to find the exact lens you need. I should also mention that you want a glass lens, plastic simply won't do for this intensity of light.
T
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