Mastering Parallax Error
CSEC Physics: Measurement & Accuracy
Essential Understanding: Parallax error is a common source of inaccuracy in physics experiments. It occurs when the position of the eye is not perpendicular to the scale being read, causing an incorrect reading. Mastering how to avoid parallax error is crucial for obtaining accurate measurements in your CSEC Physics practical exams and SBA projects.
CSEC Essential Skill: Parallax error is a systematic error that happens when the measurement of an object’s position is biased because the observer’s eye is not aligned properly with the measurement scale or indicator. Mastering this concept will help you achieve better accuracy in all your practical work.
What is Parallax Error?
Parallax error occurs when the position of the eye is not perpendicular to the scale being read. This creates an apparent shift in the position of the pointer or indicator against the scale markings, leading to inaccurate readings.
Interactive Parallax Simulation
Objective: Explore how eye position affects measurement accuracy. Move your eye position and observe how the apparent reading changes.
Eye Position
Reading
Status
Understanding the Effect
When your eye is at different positions relative to the scale, the apparent position of the pointer changes. This creates a reading error that can be positive (reading higher than actual) or negative (reading lower than actual).
Analysis: This chart shows how reading error varies with eye position. When viewing from above (positive angles), readings appear higher. When viewing from below (negative angles), readings appear lower. The error is zero only when viewing perpendicular to the scale.
Common Sources of Parallax Error
Thermometers
Reading the mercury or alcohol column from above or below the meniscus level causes systematic error in temperature readings.
Fix: Position eye at the same level as the meniscus.
Measuring Cylinders
Without eye-level positioning, the bottom of the meniscus appears at a different scale marking than its actual position.
Fix: Eye level with the bottom of the meniscus.
Analogue Meters
Reading ammeters, voltmeters, or galvanometers from the side causes the pointer to appear at the wrong scale value.
Fix: Use mirrored scales or view directly.
How to Avoid Parallax Error
Always position your eye so that the line of sight is perpendicular to the scale at the point of reading. For liquid columns, your eye must be level with the meniscus.
Position your eye so that the pointer covers its own reflection. This ensures you are viewing straight-on, eliminating parallax.
Select instruments where the pointer is as close as possible to the scale. The smaller the gap, the smaller the potential parallax error.
Digital displays inherently eliminate parallax error because the value is displayed directly without a pointer-scale system.
CSEC Insight: In your practical exams, examiners will watch your technique. Always demonstrate that you are aware of parallax by consciously positioning your eye correctly and mentioning it as a precaution in your report. This shows good experimental skill and can earn you marks.
Parallax Error vs. Other Errors
| Type of Error | Cause | How to Reduce |
|---|---|---|
| Parallax Error | Eye not perpendicular to scale | Use eye-level technique, mirrored scales |
| Zero Error | Instrument does not read zero when it should | Calibrate or adjust zero before use; apply correction |
| Random Error | Unpredictable variations (e.g., reading fluctuations) | Take multiple readings and calculate mean |
| Reaction Time Error | Delay in starting/stopping a timer | Use larger time intervals; repeat measurements |
A student reads the volume of liquid in a burette as shown. The correct reading is at the bottom of the meniscus with the eye at level B.
Position A
Eye too high
24.7 mL
Reading too high
Position B
Eye level
24.5 mL
Correct reading
Position C
Eye too low
24.3 mL
Reading too low
Result: Parallax error here is ±0.2 mL. Always read at eye level!
CSEC Practice Arena
🎯 Quick Summary for CSEC
- Parallax error = eye not perpendicular to scale, causing apparent pointer shift
- To avoid: Always position eye at same level as reading; use mirrored scales; minimize pointer-scale gap
- Common in: Thermometers, burettes, ammeters, meter rules, optics pin experiments
- For water: Read at the bottom of the meniscus; for mercury, read at the top
- In your report: State “Parallax error was minimized by…” as a standard precaution
- Exam technique: Make it obvious you are positioning your eye correctly. Examiners look for this!