Question

Experiment 2: Inverse Square Law

The inverse square law is a concept that comes up in several areas of physics. This experiment does not demonstrate how gravity "works,” but it aims to give you a better picture of how the force of gravity diminishes greatly as distance increases. In Figure 6, gravity is represented by the amount of light spread out over an area, A.

Figure 6: Inverse square law. "A" represents the original area at distance, d. Note the number of arrows passing through each area at different distances.

Materials AA Batteries Flashlight 4 oz. Styrofoam® Cup Tape Measure	*Pen or Pencil *You Must Provide

Procedure

1. Turn the Styrofoam® cup upside down and place the face of the flashlight on the center of the bottom of the cup. Use a pen or pencil to poke holes around the face of the flashlight until there are enough holes to easily punch out a hole the size of the flashlight face.
2. Insert the flashlight to the cup through the hole so that light shines up through the bottom.
3. In a dark room, turn on the flashlight and hold the end of the cup five cm from the wall.
4. Use the tape measure to measure the diameter of the circle of light. Record the diameter in Table 2 (if you wish to try distances more than about one meter, get a partner or parent to help with your measurements).
5. Repeat Steps 3 - 4 for three more distances.

Table 2: Distance vs. Light Data
Distance from Wall (cm)	Diameter of Light (cm)	Area (cm2)

Post-Lab Questions

1. How did holding the light source at different distances from the wall change the area of light you saw?

2. What did you observe about the intensity (brightness) of the light at different distances?

5. How does the area and intensity of the light in this experiment relate to the effects of gravity over increasing distance?

Answer 1

The equation for intensity of light at a given point is given as:

I = S / 4r², where S is the source strength, and r is the distance from source where the illumination is to be found.

This means that intensity of light is varying at rate which is inversely proportional to the square of distance.

The force of gravity also works in the same way, as

F = G M₁M₂ / R² .

We can thus relate that both intensity of light and the gravitational force decrease with increase in the distance.