Question

To minimize parallax error, the observer should place the object:
A. As near to the scale of the ruler as possible and the eye must be directly above the scale
B. As far to the scale of the ruler as possible and the eye must be directly above the scale
C. As near to the scale of the ruler as possible and the eye must be to the right of the scale
D. As near to the scale of the ruler as possible and the eye must be to the right of the scale

Answer 1

Hint: Parallax error is the shift in the position of the object due to the shift in the angle at which the object is observed. The closer you are to an object; the better will be your vision of that object.

Complete step by step answer:
We are given to find where the object must be placed by the observer to minimize parallax error. Consider a pencil whose length has to be measured using a scale. If the eye is not kept exactly above the scale then there are chances that we might get a wrong reading due to parallax error. In the above given case, the true length of the pencil is 2.6 cm (B). However due to parallax error, one might end up measuring its length to be as 2.5 cm (C) or as 2.7cm (A).

On the other hand, if the pencil is kept far away from the scale, the perceived length of the pencil will be less than its actual length (just like how an aeroplane in the sky looks small to us but the same aero plane as it approaches the ground looks bigger and bigger).
Hence it is important to place the object as near to the scale as possible and ensure that the eye is placed exactly above the scale.
The correct option is Option A.

Note:The apparent motion of the object due to parallax error is called ad parallax shift. Measuring parallax shifts is very important in the instruments. Parallax error is formed only when you view the object markings with an incorrect angle and zero error is the error formed when the instrument measures a reading when there should not be any reading. Be careful while working with types of errors.