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2.2.3Unit 2 — Multicellular Organisms

Practical: Reflex Actions

5 min read 5 parts Lesson 8 of 39 Virtual lab

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Part 1 of 5Ruler Drop Test

2.2.3 Practical: Reflex Actions

This is a test of reaction time, which involves conscious thought, so it is voluntary rather than a true reflex.

1
One person holds a ruler vertically with the zero mark level with the thumb and finger of a partner, who must not touch it.
2
Without warning, the first person lets go of the ruler.
3
The partner catches the ruler as quickly as possible, and the distance fallen is recorded.
4
Repeat several times, calculate a mean and compare conditions such as left hand against right hand.
5
Convert the distance into a reaction time using a calculation or a conversion chart.

Part 2 of 5Investigating a Reflex

Pupil reflex
  • Compare the pupil in a dim room and after a person is exposed to normal room light;
    • it gets smaller in brighter light.
Blink reflex
  • A sudden puff or approach towards the eye causes an immediate involuntary blink.
Safety
  • Never shine bright lights directly into the eyes, and avoid touching the eye.
Observation
  • Reflexes are the same each time and cannot easily be stopped by deciding not to react.

Part 3 of 5Factors and Fair Testing

Factors and Fair Testing
Variables
  • Hand used, time of day, practice, distraction and caffeine could all change the reaction time.
Controls
  • Only one variable should be changed, and the starting position of the ruler must be the same.
Practice
  • Reaction times usually improve with practice, so it is useful to randomise the order of tests.
Repeats
  • Use at least five trials per condition and discard obvious errors to improve reliability.

Part 4 of 5Worked Example

Question

  • A ruler falls 0.15 m before it is caught.
  • Calculate the reaction time using t = √(2d ÷ g), taking g = 9.8 m/s².

  1. 2d = 2 × 0.15 = 0.30 m.
  2. 0.30 ÷ 9.8 = 0.0306 s².
  3. Square root of 0.0306 = 0.175 s.

Answer Reaction time = 0.175 s (to 3 decimal places).

Part 5 of 5Exam Checklist & Tips

2.2.3 – Key Points for the Exam

  • Reaction time The time between a stimulus and the response.
  • Reflex An automatic response that does not require a conscious decision.
  • Voluntary action An action controlled consciously by the brain.
  • Reliability How consistent results are when repeated.

Common Mistakes

  • Calling the ruler drop a reflex test; it measures reaction time.
  • Letting the partner see when the ruler is released, so the test is not fair.
  • Measuring from the bottom of the ruler instead of the position of the thumb.
  • Using only one trial, which gives an unreliable result.
Try it in the virtual labReaction time: ruler drop \n