- South Sudan (/suːˈdɑːn, -ˈdæn/), officially the Republic of South Sudan, is a country in East Africa which is bordered by Ethiopia, Sudan, the Central African Republic, the Democratic Republic of the Congo, Uganda and Kenya including the vast swamp region of the Sudd, formed by the White Nile and known locally as the Bahr al Jabal, meaning “Mountain Sea”. It is a landlocked country. The population is nearly 12.7 million people in 2024, and Juba is the capital and largest city. South Sudan gained independence from Sudan on 9 July 2011, making it the most recent sovereign state with widespread recognition as of 2024.
- 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.
