Parallax error

Malay: Ralat paralaks · Chinese: 视差误差

Parallax error is a reading error that happens when you look at a scale from the wrong angle, from above or below, instead of level with the marking. Because your line of sight is slanted, the pointer or the meniscus appears to line up with a different graduation than it really does, so the value you record is too high or too low. Reading instruments correctly is an examinable skill in the SPM Chemistry papers (4541), and avoiding parallax error is the reason for the standard instruction to read at eye level.

For example, when reading a burette, a thermometer or a measuring cylinder, you must bring your eye level with the bottom of the meniscus or the top of the liquid column. If instead you read from above, you will record a volume smaller than the true value; from below, a volume larger than the true value. The same slanted-view problem affects reading a pointer on a scale that sits a little in front of its markings.

The confusion to avoid is thinking parallax error comes from a faulty instrument. It does not, the apparatus is fine; the error is caused by the position of your eye. It is also different from simply misreading the number: parallax is specifically about viewing angle. Correct it by lowering or raising your head until your eye is directly level with the reading. Because the same wrong angle gives the same wrong shift every time, repeating the reading will not remove a parallax error, only fixing your eye position will.

Within this chapter, parallax error connects the apparatus terms to the meniscus and to careful measurement in a fair test. Our teachers watch your eye position during practicals so this everyday error never costs you marks.