Relationship Between Light Intensity of an Object and Distance From the Source of Light (page 2)

Making the measurement

1. Position the light bulb, so a few meters are in front of it without obstruction.
2. Turn off the room lights.
3. Pick a location such as about 25 cm from the light bulb and take a reading on the light meter. (This starting distance is arbitrary and depends on the sensitivity of the meter you are using.)
4. Record the distance (any choice of unit, inches, or meters can work, but be consistent throughout your investigation). Record the light intensity, as shown in Figure 81-2, in the units in which the light meter is calibrated (such as lumem/m² lm/m²).
5. If you are using the solar cell, orient it so it is perpendicular to the line between you and the source of light; the unit of measurement will be in amps. Be careful not to block the front of the solar cell with your fingers, which can compromise the accuracy of your reading.

Expected Results

The farther away you get, the less intense the light becomes.

The rate of drop-off is not linear. The farther away you get, the faster the light intensity falls off.

More specifically, the light intensity drops off as the inverse-square of the distance. This is shown graphically in Figure 81-3.

Why It Works

Light intensity is related to the distance from its source according to the equation:

I = I_o/r²

where I represents light intensity at distance, r, between the light source and the point of measurement for an initial intensity, I_o.

Other Things to Try

A similar inverse square law relationship can be found with a source of sound and a sound intensity meter.

The Point

Light intensity drops off as the inverse square of the distance from the source of light.

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