An optical level is a surveying instrument used to establish a horizontal line of sight. It consists of a telescope mounted on a leveling base, allowing surveyors to measure differences in height over distances. By looking through the telescope, users can align the instrument with a reference point, ensuring accurate measurements in construction and land surveying. The optical level operates on the principle of light refraction, using a spirit level to ensure the instrument is perfectly horizontal. This tool is essential for tasks such as grading land, aligning structures, and creating accurate topographical maps, making it invaluable in fields like civil engineering and architecture.