Focal call length from the optical center of a lens to the place where the parallel bunch of light beams gathers in one point. At the collecting lens focus real, and at disseminating it is under construction geometrically on continuations of beams, and is called imaginary. To find focal length of the collecting lens, fix it on a support, direct to it a bunch of parallel beams from a light source, and, moving it, achieve that on the screen the point appeared. Measure distance from the center of a lens to the screen. It will be equally focal. For the disseminating lens calculate focal length by formulas.
It is required to you
- the collecting and disseminating lenses, ruler, support
Instruction
1. Measurement focal you rasstoyaniyazakrepit the collecting (convex) lens on a support. Take a light source and direct to it. In this case, the beams falling on a lens can be considered parallel. Move a support pad until the indistinct spot of light, does not meet in one point. Measure distance from the center of a lens to this point, it also will be its focal length.
2. Calculation of focal length of a lens Place some subject before a lens, and receive its image. If the image arises on the screen on other side of a lens, its consider valid if on the same side – imaginary (the magnifying glass so works).
3. Measure distances from a subject and from its image to the optical center of a lens. In case the image turned out imaginary, consider it negative (when calculating before value put a minus sign).
4. To calculate focal length of a lens multiply distances from a subject to a lens and divide value into their sum of f=s·d/(s+d). If focus of a lens turned out negative, it means that it imaginary, and a lens – disseminating.
5. As any lens is limited to two spherical surfaces, their radiuses and also index of refraction of glass of which this lens was made often are given. In this case, to calculate focal length of a lens do the following sequence of actions: 1. Find the relation 1/R1 and 1/R2, radiuses of curvature of lenses. At the same time consider that if the lens concave, then its radius is considered negative. 2. Calculate the amount of the received values 1/R1+1/R2.3. From the index of refraction of glass of a lens take away number 1 (n-1).4. Increase the number received when calculating in Paragraph 2 by the number received in Paragraph 3.5. Divide number 1 into the number which turned out in calculation result in Paragraph 4. It will also be focal length of this lens.