Final answer:
If target length, or length and width, are provided, the observer must also give the proper length, which is the distance measured by an observer who is stationary relative to the measured points. Length contraction occurs when the measured length is shortened because the object is moving relative to the observer's frame. These concepts are part of relativistic physics and special relativity.
Step-by-step explanation:
If target length, or length and width, are given, the observer must also give the proper length (Lo). The proper length is the distance between two points measured by an observer who is at rest relative to both of the points. In contrast, length contraction (L) is the shortening of the measured length of an object moving relative to the observer's frame. This concept is a fundamental part of relativistic physics, specifically in the theory of special relativity proposed by Albert Einstein. All observers agree upon relative speed. However, distance can depend on an observer's motion. For example, when an object is in motion relative to an observer, the observed length can be shorter than the proper length due to the phenomenon of length contraction. This is because the length of the object is measured differently by observers in different reference frames due to the relativistic effects at high speeds.