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Think Fast! Measure Your Reaction Time!

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Key Concepts
Stimulus, motor response, reaction time, signal transduction
Megan Arnett, PhD, Science Buddies


‘Think fast!’ someone says, as they toss a ball toward you. You catch the ball, but do you really ‘think’ about doing it? Probably not! Most likely you raise your hands to catch the ball without ever consciously thinking about it. How does this happen? Lucky for you, your brain is able to react to things in your environment without you having to consciously make a decision about how to react. There are many ways to measure the speed of this type of reaction, and in this activity you will be using a simple test to measure these reaction times.

Hand ready to grab a ruler being dropped to test reaction time
This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.


A person’s reaction time is the time that elapses between the moment that the person is given a sensory stimulus (something that initiates a functional activity) and when the person initiates a motor response to the stimulus. For example, if someone calls your name, the stimulus is the sound of their voice calling you. The reaction time is the amount of time between when you hear the sound, and when you turn your head to look at them. 

In some types of psychological testing, reaction time is used as an index of mental processing speed. These types of tests use reaction time as an indication of how quickly the subject can execute all of the mental operations required by the task. This speed of processing is used as a measurement of processing efficiency. These types of tests often use a press of a button as the required motor response, but other tests use eye movement, vocal response, or some other measurable motor behavior. Reaction time in these tests is at least in part a product of the speed of signal transduction between the sensory input and the motor output.

In this activity you will be using a simple test to measure reaction time in at least 2 volunteers.


  • Pencil or pen
  • Piece of paper
  • A ruler
  • At least 2 volunteers
  • A place for your volunteer to sit
  • A room or area where you can dim the lights


Use your pencil and paper to recreate the table below


Volunteer 1

Volunteer 2


Volunteer 1

(dim light)

Volunteer 2 (dim light)

Trial 1





Trial 2





Trial 3





Trial 4





Trial 5










Average Reaction Time






  1. Have your volunteer sit in a chair facing you.
  2. Stand in front of your volunteer and hold the ruler by the highest number, so that it hangs down approximately eye level to your volunteer.
  3. Have your volunteer hold out their hand at the very bottom of the ruler, ready to grab it when you drop it (but don’t let them touch the ruler!).
  4. Tell them they should grab the ruler as soon as you drop it.
  5. Drop the ruler.
  6. Note the number on the ruler where they caught the ruler. Write this number (in inches) in the ‘Volunteer 1’ column, next to Trial 1.
  7. Repeat steps 1-6, each time recording the point on the ruler where the volunteer caught it.
  8. Repeat the activity with your second volunteer. Record their distances in the ‘Volunteer 2’ column.
  9. With Volunteer 1, go into a room or area with low or dim lighting. Repeat steps 1-7. Record their distances in the ‘Volunteer 1 (dim lighting)’ column.
  10. With Volunteer 2, go into a room or area with low or dim lighting. Repeat steps 1-7. Record their distances in the ‘Volunteer 2 (dim lighting)’ column.
  11. Calculate the average distance for each volunteer in regular and dim lighting. Record this number in the ‘Average’ row for each column. The equation to calculate averages is: Average = (trial 1 + trial 2 + trial 3 + trial 4 + trial 5)/5
  12. Use the average distance you calculated in Step 9 and refer to the table below to find the average speed of reaction time for each volunteer. Record this value in the row ‘Average Reaction Time’ for each column.

    Distance (inches)

    Reaction Time (seconds)













  13. Extra: Try repeating this activity with more volunteers. Compare whether age or gender affects reaction speed. Do kids react faster than adults? Do girls react faster than boys?

Observations and Results

In this activity you tested the reaction time of at least two volunteers. Even though the volunteers were primed to catch the ruler, you probably found that they didn’t react instantaneously to you dropping the ruler. Instead, there was a short delay between the point when you dropped the ruler, and the point when your volunteer caught it. This delay is the result of the time it takes your volunteer’s brain to transmit information from their eyes, to their brain, to their hand.

The first signal in your volunteer’s brain was from their eyes, telling their brain that the ruler had dropped. This information came in through a sensory pathway, and had to be processed by your volunteer’s brain. Their brain then sent a signal from the motor cortex (the area of the brain that controls motor or muscle movements), telling their hand to grab the ruler. When you think about all of the steps involved, it’s amazing how quickly your volunteer’s eyes, brain and hand were able to react to the dropping ruler!

When you tried this activity in lower lighting, you most likely found that the reaction time for your volunteers was slightly slower. This is because in low light, it is more difficult for your eyes to identify the moment when you drop the ruler. Therefore, the processing time from eyes to brain to hand is slightly delayed.

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Additional Resources

Think Fast: Do Video Game Players Have Faster Reaction Times Than Non Players? by Science Buddies

Fast Food: Can Peppermint Improve Reaction Times? by Science Buddies

Science Activities for All Ages! from Science Buddies


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