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Question
The length of a pencil is measured using a ruler as shown. What is the apparent length of the pencil seen by the observer and the actual length of the pencil?
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Solution
Due to parallax error, the starting and ending points are 0.5 cm and 4.2 cm, respectively.
Therefore the observed length is,
Observed Length = 4.2 - 0.5 = 3.7 cm = 37 mm (since 1 cm = 10 mm)
On the other hand, when measured correctly, the starting and ending points are 1 cm and 4 cm, respectively.
Therefore the actual length is,
Actual Length = 4 - 1 = 3 cm = 30 mm (since 1 cm = 10 mm)
That’s why we should always view from the angle that leads perpendicular to the marking on the scale.
Hence, the correct answer is option C.
Therefore the observed length is,
Observed Length = 4.2 - 0.5 = 3.7 cm = 37 mm (since 1 cm = 10 mm)
On the other hand, when measured correctly, the starting and ending points are 1 cm and 4 cm, respectively.
Therefore the actual length is,
Actual Length = 4 - 1 = 3 cm = 30 mm (since 1 cm = 10 mm)
That’s why we should always view from the angle that leads perpendicular to the marking on the scale.
Hence, the correct answer is option C.
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