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.
Simply spray it on, wait for a duration of around 10 to 15 minutes, and then scrub the mold until the surface comes clean and clear of mold. You may want to leave the bleach solution set for a few minutes longer to loosen up those hard to scrub moldy areas before trying to re-clean and remove the mold.