Intensity

The closer you are to a source of light, the brighter the light appears. If you want to read at night, you must sit near a lamp. At night, as you walk away from a street light, the area around you becomes darker. A street light doesn't give off less light when you move farther from it. It just provides you with less light the farther away you are. Photons travel outward from a light source in all directions. Near the light source, the photons spread through a small area, so the light is intense. Intensity is the rate at which a wave's energy flows through a given unit of area. You can think of intensity as brightness. Farther from the source, the photons spread over a larger area. The intensity of light decreases as photons travel farther from the source.

Figure 7 The closer you are to a surface when you spray paint it, the smaller the area the paint covers and the more intense the paint color looks. Using Models How does a can of spray paint help you model a change in light intensity?

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A can of spray paint can help you model a change in light intensity. Look at Figure 7. When the nozzle is close to a piece of paper, the paint forms a small, dense spot. When the nozzle is farther from the paper, the paint forms a larger, fainter spot because the paint is sprayed over a larger area. Like paint on paper, light intensity decreases as distance from the light source increases.

A wave model for light also explains how intensity decreases with distance from a source. As waves travel away from the source, they pass through a larger and larger area. Because the total energy does not change, the wave's intensity decreases.

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