Double Convex Lens, 200mm Focal Length, 3" (75mm) Diameter - Spherical, Optically Worked Glass Lens - Ground Edges, Polished - Great for Physics Classrooms - Eisco Labs

This lens is a good tool to get a person interested in the physics of light and, more generally, in science as a whole. Use it to perform the following odd science experiment:First, go to a place where you can see far into the distance (e.g., a long straight street or a large park, where the most distant objects that you can see are at least several hundred feet away). Now face toward those most distant objects, but make sure that you can also see objects closer to you. (Don't use the lens yet.) Now, keeping your eyes pointed in the same direction, move to the left. You will notice stationary objects apparently moving to the right relative to you. This is just the law of parallax. The closer an object is to you, the faster it seems to move to the right. The most distant objects hardly move at all relative to you. This is because the light rays from those distant objects are essentially parallel when they reach your eyes, so the law of parallax should have virtually no effect on them.Next, face toward an object that is at least ten feet away. Then hold the lens about two inches in front of one eye and close the other eye. Since the distance from your eye to the lens is less than the focal length of the lens, you will see the object right-side-up in the lens. Now, holding the lens still, move your head toward the left relative to the lens, keeping your eyes pointed in the same forward direction. You will see the object move to the left relative to your head. Weird, isn't it? Completely violates the law of parallax! But that's OK, since it's just a virtual image.Now, put the lens down and face toward an object that is about five feet away. Move toward the object. Its apparent size becomes larger. Then move farther away from the object. Its apparent size becomes smaller. This is just the inverse-square law. We instinctively use this law to determine when we are getting closer to an object or farther away from an object. Now look toward a distant object at least several hundred feet away. Move toward the object. Its apparent size does not change noticeably. Then move farther away from the object. Its apparent size does not change noticeably. That's because, since the distance from you to the object is so great, your motion does not change that distance percentagewise very much, so the inverse-square law should have virtually no effect on it.Next, face toward an object that is at least ten feet away. Then hold the lens right up to one eye and close the other eye. You will see the object right-side-up in the lens. Then, holding the lens still, move your head backward away from the lens. The object will appear to get larger in apparent size, giving the clear impression that it is getting closer and closer to you as you move away from it! Then, holding the lens still, move your head toward the lens. The object will appear to get smaller in apparent size, giving the clear impression that it is getting farther and farther away from you as you move toward it! Weird, isn't it? Completely violates the inverse-square law! But that's OK, since it's just a virtual image.

Works well for what I use it for. The edges of mine aren’t cut perfectly parallel but it’s fine as is.

Good quality glass lens. Perfect size to throw in a survival pack as a means to make fire, and it does that well.

This lense glass was heavier and thicker than I wanted, but it will do for now.

Love this strong magnifying, little easy-to-hold over my iPad lense. Helps me to instantly see stuff easier without a big handle magnifying lense. Very much like that it is glass over plastic because it does not scratch and can be cleaned without becoming permanently fogged.

Added this to my bug out bags fire kits. Small size and work quick on tinder. Worth the price for this use.

Great lens, great customer support (mine had some rough edges).

This is a great size and focal length / geometry for a magnifying glass. Mine came in great shape, well polished and not chipped or anything like that. As a 36 year-old I used it to burn paper with sunlight for the first time in my life. I guess some things are fun for all ages.