Giant Fresnel Lens Deathray: An Experiment in Optics

Step 4 Measure the Focal Length

Rather than proceed with building the rest of the device, at this point you need to measure your lens' focal length. This is the distance from the lens to the spot of light it will produce. This distance will only be correct when the light rays hitting the lens are parallel to each other, and perpendicular to the lens. In other words, the light has to either be sunlight or two parallel laser beams, and hit the lens dead on. Unless you live at the equator, with the sun straight up, measuring the focal length is actually quite difficult. After a lot of frustration I decided to use lasers.

Materials Required:

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• 2 (or more) laser pointers
• A level
• Some flat ground
• A T-square
• A tape measure
• A large, rigid screen

We want to find the point in space where parallel light beams bending through the lens intersect. This is the focal point, and it will be straight out from the center of the lens.

Laser Setup

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To set up our parallel beams of light, put your two laser pointers on either side of a book or something so that they're parallel. The goal is for the lasers to be perpendicular to the lens, so make sure they're on a level surface. Turn them on and aim the whole setup straight at the lens.

Meanwhile, have someone hold the lens straight up, using a T-square to make the lens perfectly vertical. You'll get two weird diffraction patterns on the wall behind the lens.

Finding the Focus

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Now, with your tape measure extending out from the base of the lens, hold your screen up so the two lasers hit it. Move it back and forth until the two spots converge. When they do, see how far from the lens your screen is.

This may sound confusing, but the pictures should help. I recommend trying several times, maybe moving the lasers around, so you can see whether your results are consistent. For my lens, the focal length was about 40 inches (about 100cm) which is average for especially large Fresnels.

Comments

[Ev]

Here's how I found the focal point...  Get a trigger pump type of fluid sprayer, either a glass cleaner sprayer or you can buy empty plant misting devices at a hardware store. Put the lens in the sun and spray water behind the lens. You'll see the rays converge and diverge. I guess you could measure it with a metal tape. 

[spamattakky]

I really like the misting idea, would be great in demonstrations with my Fresnel lens cooker.

[forgoten]

I'm wondering, if you use a rear projection TV for the source of the lens, couldn't you just measure the distance of the lens to the projector? or would that not work? thank you for your consideration

[DrSimons]

Possibly, but projection TVs tend to have 3 different projectors - one for each color. This, combined with the fact that they also have a large mirror involved, makes it rather impractical I'm afraid.

[forgoten]

thanks for the feedback. i'm kinda sorry that i cant do it the lazy way :(