Intensity and Distance

This demonstration will show that intensity from a light source decreases as distance from the source increases.

Time requirement: 15 minutes as a demonstration.



  1. Turn off the lights in the classroom.
  2. Stand a few feet away from a blackboard (or wall) and shine the flashlight on the board. Observe the intensity of the flashlight beam on the wall.
  3. While continuing to shine the flashlight on the board, move back a few feet. Once again, observe the intensity of the beam on the wall.
  4. Move back from the board a few more times, and observe the brightness of the flashlight beam on the board.

Observation questions

  1. What happens to the brightness of the beam on the board as the flashlight moves further and further away from the board?
  2. What happens to the diameter of the beam as the flashlight moves away from the board? Why does this happen?
  3. Is there any obvious relationship between the size and intensity of the beam on the blackboard?
  4. In reality, the flux from the Sun decreases with the square of the distance from the Sun. Jupiter is about five times farther away from the Sun than is Earth. How much more energy do we receive (per square foot or square meter) on Earth than is received at Jupiter?

Last updated: September 02, 1997
Joe Twicken /
Rob Wigand