- East Asia is a region of Asia, which is defined in both geographical and ethno-cultural terms. The modern states of East Asia include China, Japan, Mongolia, North Korea, South Korea, and Taiwan. Hong Kong and Macau, two small coastal cities located in the south of China, are autonomous regions under Chinese sovereignty. The economies of Japan, South Korea, China, Taiwan, Hong Kong, and Macau are some of the world’s largest and most prosperous economies. East Asia borders Siberia and the Russian Far East to the north, Southeast Asia to the south, South Asia to the southwest, and Central Asia to the west. To the east is the Pacific Ocean and to the southeast is Micronesia (a Pacific Ocean island group that is classified as part of Oceania).
- 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.