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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.

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