To minimize parallax error when reading a ruler, where should your eyes be?

• A. Perpendicular to the ruler
• B. In line with the ruler
• C. Behind the scale
• D. Above the ruler

Answer: (A)

Parallax error happens when you read a scale from an angle, so the mark you’re trying to read seems to shift as your head moves. To avoid this, view the ruler with your eye directly above the point you’re measuring, so your line of sight is perpendicular to the ruler’s surface. When the view is at a right angle, the mark and the line of sight align, and you see the true position without the apparent shift. Looking from the side, from above at an angle, or from behind the scale can make the reading seem to change as you move your head. So the best position is with your eyes perpendicular to the ruler.