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## Tuesday, December 22, 2009

### what is a parallax error?

Parallax error is the error that is most committed when readings are taken in physics. You can thus understand why it is important to avoid it at all cost. One must be aware of its existence at all time so that it can be avoided and as a result the true value of the reading is obtained.

The concept of parallax error is related to the term parallax.

Imagine that we have  in a room a pelican and a flamingo as shown in fig 1 below.

Fig 1

Now Garfield is moving about in the room and each time is is somewhere in the room, he looks at the two birds. At the point A he sees the flamingo on the left of the pelican whereas when he is at position C he would see the flamingo on the right of the pelican. Only when he would be at position B would he sees the two birds one behind the other.

He would discover that each time he is at a different position, he would find that the position of the flamingo relative to the pelican has changed.

You can also have this effect when you are in front of a clock. If you move from side to side you would find that the time that you can read from the clock is different.

So we can the understand that the parallax is the change in the apparent position of an object when the position of the observer changes.

Now let us look at the concept of parallax error. If you have placed a pencil on a metre rule and you are reading its length then just like in fig 1 above and in fig 2 below you can place you eye everywhere you want.

Fig 2

As you can see from fig 2 above I have chosen three position at which you can place your eye.

Clearly at these three position we can have the following reading

Reading at A  = 6.2 cm

Reading at B  = 5.8 cm

Reading at C = 5.5 cm

What you you think would be the correct reading?

The correct reading is would be obtained when the eye is placed at B.

Fig 3

Now there is a line from the tip of eye to the of the pencil  that continues up to the scale. this line is called the line of sight and the mark at which the line intersect the scale is the length of the pencil. This line of sight must be be at right angle to the scale. This is shown above in fig

If the line of sight and the scale are not ar right angle to each other then a parallax error is committed.

Similarly with a measuring cylinder the line of sight from the eye to the bottom of the meniscus must be at right angle to the scale as shown in fig 4 below. In this case the line of sight is horizontal and the scale vertical.

Fig 4

So as you can see above each time you are taking a reading you must ensure that the line of sight is perpendicular to the scale.

1. perfect and easy to understand! specially liked the definition you gave!

2. thank you very much.. it helped me a lot in my exam

3. now i can do my homework a LOT easier and faster!! thanks for the explanation. It made my day! :)

4. Love it!!!!

6. A very great idea....

7. I LOVE PHYSICS

9. Love your explanation really informative like listed above

10. Physics is an interesting course

11. I love Garfield :P
Nice explanation

12. U really brought light to a dark world.now i appreciate physics.

13. very easy to understand

14. I am interested in physic.

15. Nice and easy to understand! Thx !

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17. perfect! thanx a lott!!

18. this is really great I am gonna bookmark this rite away!

19. very nicely elaborated.........it has helped me alot

20. thank you so much!!

21. keep up the good work.. this site is very helpful 4 students :)

22. 9ice 1.It is educative,so keep it up

23. But how do you correct for the error? Say that figure 2 represents 3 observers viewing the pencil, with the ruler being the reference (the coordinate system). We want to *somehow* combine the readings of viewers A and C so that we get a reading that's equivalent to the correct reading (the reading taken by viewer B). What is the "combiner" algorithm? Something to do with triangulation, but what are the specifics?

24. But how do you correct for the error? Say that figure 2 represents 3 observers viewing the pencil, with the ruler being the reference (the coordinate system). We want to *somehow* combine the readings of viewers A and C so that we get a reading that's equivalent to the correct reading (the reading taken by viewer B). What is the "combiner" algorithm? Something to do with triangulation, but what are the specifics?

25. That explains the "what", but the real challenge is the "how". Specifically: how would you correct for the error? In figure 2, how would you combine the readings from A and C to get the same (correct) reading as the one you get from B? Something to do with triangulation, but what are the specifics of the "combiner" algorithm?

26. awesome explanation!

27. This was A great and simple way to describe Parallax error for my Physiology and Biomechanics, Sports Therapy Exam.

God Bless you for taking the time to do this.

28. Awesome explanation. My teacher'd be proud if I'd be able to explain the same to him :D

29. It was human