What type of error must be avoided by looking perpendicular to the ruler in readings?

• A. Parallax error
• B. Systematic error
• C. Random error
• D. Zero error

Answer: (A)

Parallax error happens when the reading on a ruler is viewed from an angle rather than straight on. If your eye isn’t in line with the scale, the mark can look slightly shifted, so you might read a value that’s too high or too low. Looking perpendicular to the ruler puts your line of sight directly through the plane of the scale, so the true position lines up with your eye and the reading is accurate. This error is specifically about how we view the scale, not about the instrument being biased (systematic error), random fluctuations (random error), or the zero point not starting at zero (zero error).