Скачать или смотреть 🔎 Easily find Focal Length of a Convex () Lens experimentally

() = Convex
)( = Concave

Convex = Converging
Concave = Diverging

Convex lenses focus light into a point.
Concave lenses spread light out (into a fan).

Focal length is simply the distance between the center of a lens and the point of light that it causes.

Accuracy: the crudeness of this procedure leaves much to be desired. What appears 'in focus' will not only be precarious, but it's also somewhat subjective. With that in mind, consider that finding focus is much like using a stereo microscope: focus is relative to a specific plane, and not to the conceptually visible three-dimensional pattern that the mind discerns as a 'clear image.' If you can hold this thought while you're attempting to measure focal distance, keeping in mind to try to clarify the light source itself and not the bulb in which it is contained, you will improve the accuracy of your measurement. Focus on the filament, not the bulb. As for increasing precision, you would do well to raise and lower the lens using an adjustable apparatus, like a chemistry ring-stand.

Further refinements for precision: a lower watt bulb will help, as will placing the point light source behind a screen (a piece of wood with a hole in it) to reduce stray light. Ideally, there will be no other light sources in the room. For the screen onto which the light will be focused, find a smooth surface. Alignment also matters; try to keep the light source, the lens, and the focus point screen all positioned in parallel planes. Oh, and clean both your lens and your light-bulb!

LENS DIAMETER: BIGGER IS ALWAYS BETTER.
Of course lens size matters. Please don't pick at that detail. You'd be missing the point. Larger lenses gather more light to focus. I shouldn't even need to say this. But a lens is going to be fit to a specific purpose, and that means focus distance given a specified size restraint. Ideally, in any apparatus that makes use of light—whether in terms of resolving an image to the highest possible degree or to the purpose of focusing gathered light's energy—the bigger the lens the better, and the longer the exposure time, the more pronounced its affect. Bigger is better, but we have to make reasonable compromises in engineering and design. If we were to use the largest possible lens to gather all the light that ever was, we would have all of the information necessary to resolve the ultimate and perfect picture of what the universe is. Until then, every other lens that's smaller than that will be a compromise, so shut up when I tell you that size isn't the most relevant number relative to purpose. In the practical sense, only focal distance matters.