The parallax error is one that occurs when a measurement is made from different viewing angles. A simple example of what is meant by parallax is seen when we close the left eye in front of an object, and then immediately open the left eye and close the right eye: the object will appear to have moved to the right. But when we have both eyes open, we will see the object in the middle.
The same phenomenon can be repeated by positioning our eyes at different angles to an object, such as a simple point (image below). If we wanted to place this point on a ruler, we would make a parallax error when viewing it from the positions marked by the X’s in red.
From the X on the right the point will appear to be further to the left, so its location with respect to the ruler is closer to 3 than to 4. Meanwhile, the opposite occurs from the X on the left: now the point will appear to be further to the right, placing it closer to 5 on the ruler.
To avoid parallax error in such measurements, it is always necessary to place the eyes in the most vertical position possible with respect to the scale of the instrument or material.
In the image above we had a scale, the point being a needle, a pointer, the edge of an object, the meniscus of a liquid, etc. That is, we have a scale on which we want to make a reading. Very similar as it happens with the hands of the clock when we read what time it is.
In a laboratory we have scales that offer hundreds of instruments or materials made of glass or metal. When a student makes a measurement that includes direct scaling, he is subject to making parallax errors, which is random: he constantly varies the angle from which he observes or takes the measurement.
Sometimes because of the rush and wanting to write down a reading, the student does not realize that he is making the parallax error, since it does not only occur from left or right angles; also from above or below, and there may be even other angles. You should always take the time to fix your eyes in the most upright and frontal position when comparing an object against a scale.
That is to say, in the laboratory the eyes must be as far as possible in front of the object of our measurements. Otherwise, the accuracy will suffer.
Parallax error in chemistry
In chemistry laboratories we have in particular the parallax error in volume measurements. For this, there are glass materials such as the graduated cylinder or the burette, each with its own scales (think of a ruler but cylindrical and made of glass).
Depending on where and from what direction we want to see the meniscus, its apparent position on the scale will be different. Viewed from below, the meniscus will be lower (11.69 mL) than it is. But viewed from above the meniscus will now appear higher on the scale (11.76 mL).
The real volume (11.73 mL for our example) will be the one that we measure with the eyes (both) in a position as frontal and vertical as possible with respect to the glass material. As you will see in the examples section, there are many operations that generate parallax errors in a chemistry laboratory.
Parallax error in physics
In physics laboratories, on the other hand, volumes can also be measured, but other instruments are of greater importance in their practices.
For example, it is necessary to determine the dimensions, such as lengths and width of an object, using the micrometer (palmer) or the vernier. These instruments have their own scales, which are once again subject to parallax error.
Likewise, if these instruments are analog, we will have ammeters (current meter), ohmmeters (electrical resistance meter), barometers (atmospheric pressure meter), as well as many others, each with scales sensitive to parallax error.
In them, a needle or pointer will move along the scale, whose tip will indicate the appropriate reading as long as we fix our eyes as close and frontal as possible.
In the world of astronomy, parallax has a very special function: to measure distances between celestial bodies, such as planets and stars.
If you place a finger in front of your nose and close and open one eye at a time, you will notice how the finger appears from one side to the other. But this effect diminishes if it is repeated with the finger as far away from the face as possible. The closer a body is, the greater its parallax; and therefore the error that can be made by observing it is greater.
This is the basis for the use of parallax to determine how far away celestial bodies are: comparing the relative positions of two stars from different angles.
Examples of parallax errors
If we read the time by looking at the wall clock from the left or right, we will get an incorrect time for several minutes. To see the correct time, the hands of the clock must be seen from the front.
Inside the thermometer there is a rising or falling liquid whose meniscus equals a temperature scale. If we view this meniscus from above or below, we will get incorrect temperatures. The thermometer must be viewed from the front and as vertical as possible.
Whoever drives can know precisely what the speed of the car is. However, the copilot (on the right) will see that the needle points at a speed apparently higher than the actual speed.
Preparation of solutions
When a solution is to be gauged, it must be done with the eyes just in front of the gauging line, because otherwise a parallax error equal to that of the thermometer will be repeated.
Analog blood pressure meter
Very similar to the case of the clock, or that of a stopwatch, is that of the blood pressure meter, whose needle must be seen from the front or it can be concluded that the pressure is lower or higher than the real one.
In the granatary scales you have to read the readings from the front, because the same parallax error that occurs in, for example, school rules would be committed. Reading from the front the true mass of the object in question is obtained.