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ZingPath: Properties of Light

Light Intensity and Distance From the Source

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Properties of Light

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Lesson Focus

Light Intensity and Distance From the Source


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Students discover how changing the distance between an object and a light source affects the intensity of light received and the object’s temperature.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Explain that light intensity and temperature decrease as the distance from a light source \nincreases.
  • Explain that light intensity is inversely proportional to the square of the distance from the \nlight source.
  • Predict how the temperature of an object will change when the distance from a light source is changed.

Everything You'll Have Covered

The relationship between light intensity and distance from the source is an example of what is commonly referred to as an inverse-square relationship. The nverse refers to the fact that as the distance increases the light intensity decreases. The square refers to the fact that it is not a one-to-one relationship. Rather, as the distance changes, the intensity changes by a factor of one divided by the square of the distance. For example, if the distance increases to four times the original distance, the light intensity is 1/16 of the original intensity. Several other important properties have inverse-square relationships with distance, including sound intensity, gravitational force, and electrical forces.

The reason for the inverse-square relationship can be understood by envisioning a light bulb centered inside an inflating balloon. The light bulb gives off light at a constant rate, regardless of how large the balloon is. When the balloon is only slightly inflated, the light from the bulb is evenly distributed over a relatively small surface area. As the balloon inflates larger and larger, the same amount of light from the bulb must be distributed over a larger surface area. Since the surface area of a sphere is found using the formula, A = 4?r2, we can see that doubling the distance (r) from the balloon surface to the light bulb results in four times as much surface area. The same amount of light spread over four times as much area results in each unit of area receiving just one-fourth as much light intensity.

Light and heat share this inverse-square relationship because they are both forms of electromagnetic radiation. Light consists of radiation in the visible portion of the electromagnetic spectrum-that portion that the human eye is able to see. What we feel as heat is primarily radiation in the infrared portion of the spectrum. These waves have a slightly longer wavelength and lower energy than visible light.

Tutorial Details

Approximate Time 25 Minutes
Pre-requisite Concepts Students should be familiar with energy, kinetic energy, proportion, and temperature.
Course Physics
Type of Tutorial Experiment
Key Vocabulary angel, distance, Earth