- Salix sitchensis (Wikipedia)
Salix sitchensis is a species of willow known by the common name Sitka willow. It is native to northwestern North America from Alaska to northern California to Montana. It is a common to abundant plant in many types of coastal and inland wetland habitat, such as marshes, riverbanks, swamps, coastal sand dunes, and mountain springs.
- Focal length (Wikipedia)
The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system’s optical power. A positive focal length indicates that a system converges light, while a negative focal length indicates that the system diverges light. A system with a shorter focal length bends the rays more sharply, bringing them to a focus in a shorter distance or diverging them more quickly. For the special case of a thin lens in air, a positive focal length is the distance over which initially collimated (parallel) rays are brought to a focus, or alternatively a negative focal length indicates how far in front of the lens a point source must be located to form a collimated beam. For more general optical systems, the focal length has no intuitive meaning; it is simply the inverse of the system’s optical power.