The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus.hen a lens is used to form an image of some object, the distance from the object to the lens u, the distance from the lens to the image v, and the focal length f are related by. The focal length of a thin lens can be easily measured by using it to form an image of a distant light source on a screen.
1 For killing mold with bleach use a ratio of one cup of bleach per gallon of water (ie about 1 part bleach to 10 parts water). 2  Apply the solution to non-porous surfaces with mold growth either by using a spray bottle or by using a bucket and a sponge or cloth.
Borax is a good natural choice for cleaning mold. It is toxic if you swallow it, but otherwise, it does not emit dangerous fumes or chemicals when you use it. What makes it great to use on mold is that it disinfects the surfaces it cleans. You mix borax with water in a one cup to one gallon ratio. Use the mixture to scrub the mold until it comes loose from the surface. Then, use a rag or vacuum cleaner with a HEPA filter to clean up the mold that comes loose.