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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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Light Intensity and Distance From the Source

Physical Science

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Learners 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 when the distance from a light source increases.
  • Explain that light intensity is inversely proportional to the square of the distance from the light 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 "inverse" refers to the fact that as the distance increases the light intensity decreases, and 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. Thus, doubling the distance decreases the light intensity to one-fourth of the original value. Several other important properties have inverse-square relationships with distance, including sound intensity, gravitational field strength, and electromagnetic field strength.

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 sensitive to. 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 Energy, kinetic energy, proportion, temperature
Course Physical Science
Type of Tutorial Experiment
Key Vocabulary angel, distance, Earth