There are two types of lenses – collecting (convex) and disseminating (concave). Focal length of a lens – distance from a lens to the point which is the image of an infinitely far object. In other words, it is a point in which parallel rays of light after passing through a lens are crossed.
It is required to you
- Prepare a lens, the sheet of paper, a centimetric ruler (25-50 cm), a light source (the lit candle, a small lamp, a small desk lamp).
Instruction
1. The first way – the simplest. Come to the place lit with the sun. By means of a lens focus sunshine on the sheet of paper. Changing distance between a lens and paper, achieve the smallest size of the received speck. As a rule, at the same time paper begins to char. The distance between a lens and the sheet of paper at this moment will correspond to focal length of a lens.
2. The second way - classical. Establish a light source on the table edge. On other edge, at distance of 50-80 cm, put the improvised screen. Make it of a pile of books or a small box and the sheet of paper fixed vertically. Moving a lens, achieve the accurate (turned) image of a light source on the screen. Measure distances from a lens to the screen and from a lens to a light source. Now calculation. Multiply the received distances and divide into distance from the screen to a light source. The received number will also be focal length of a lens.
3. Everything is a little more difficult for the disseminating lens. Use the same equipment, as for the second way with the collecting lens. The disseminating lens arrange between the screen and the collecting lens. Move lenses for obtaining the sharp image of a light source. Enshrine the collecting lens in this provision not movably. Measure distance from the screen to the disseminating lens. Note chalk or a pencil location of a rassevayushchy lens and remove it. Bring closer the screen to the collecting lens until receive the sharp image of a light source on the screen. Measure distance from the screen to that place where there was a disseminating lens. Multiply the received distances and divide into their difference (from bigger to subtract smaller). The result is ready.