Final answer:
A white line is used as a reference mark in physics experiments, like a double-slit experiment, to accurately measure distances and calculate wavelengths or other properties of waves.
Step-by-step explanation:
A white line may be used as a reference (or slippage) mark between the instrument origin and either end of the line, or the middle of the line. This reference point can also be called "the origin." In physics, particularly in experiments involving optics or waves, such as those requiring the measurement of the distance from a slit to a wall, or calculations involving dark and bright bands in interference patterns, precise reference points are crucial for accurate data collection and analysis. For example, in a double-slit experiment, you might measure the distance from the center of the pattern to several of the dark bands, and from the slit to the same dark bands, using a white line as the reference mark to maintain consistency. This helps in determining the wavelengths of light or other properties of the wave.