In order to remove the error of parallax when taking measurements with a metre rule, the eye should be focused

Analyze the question: The question asks how to position your eye to avoid parallax error when reading a metre rule.

Concept: Parallax error happens when the line of sight to a scale is not perpendicular to it. Because the pointer or object being measured sits slightly above the markings (the rule has thickness), viewing from an angle makes the reading appear to shift against the scale. The result is a reading that is too high or too low.

Solution:

1. If you look slantingly from the left, the object appears to line up with a mark further along the scale than it really does.
2. If you look slantingly from the right, it appears to line up with a mark on the opposite side. Both cases give wrong readings.
3. If you look directly above the mark, with your line of sight at 90° to the rule, the object and the marking line up correctly. There is no apparent shift, so no parallax error.

So the eye must be placed directly over the marking, looking vertically downwards, with the line of sight perpendicular to the scale.

• A and B are wrong because slanted viewing is what causes parallax.
• D is wrong because looking "vertically upwards" would mean viewing from beneath the rule, which is not a practical or standard way to read it.

Answer: C. vertically downwards on the markings

Exam tip: Remember "Perpendicular means Parallax-free." Whenever you read a metre rule, burette, thermometer or ammeter scale, put your eye directly in line with the mark at 90° to the scale. This also applies to Chemistry practicals, so it is worth remembering.