But do not just take our word for it! Why not see for yourself if light really behaves this way? This project shows you how you can use a light-sensitive resistor, called a photoresistor, which has an electrical resistance (measured in ohms (Ω)) that changes with exposure to light, and a digital multimeter to see if light intensity really does decrease according to the inverse square law. You will measure the resistance of the photoresistor at different distances from a light source. Using information from the photoresistor's datasheet, you can convert the resistance measurement to lux, the SI unit of illuminance (a measure of intensity that accounts for how different wavelengths are perceived by the human eye). You can then create a graph to see how illuminance changes with distance from the light source, and verify if it follows the inverse square law.

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You can find this page online at: https://www.sciencebuddies.org/science-fair-projects/project-ideas/Elec_p028/electricity-electronics/measure-intensity-of-light

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The figure shows directional light originating from a point source that covers a larger area the further away it is from the source. As the light travels it has a specific brightness and size at any given point. The inverse square law shows that when light travels twice the distance its area grows four times as large and the brightness decreases by four times. The rate a light grows in area and decreases in brightness is related to the distance it travels from another point squared.

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As you move away from a light source, the light gets dimmer. No doubt you have noticed this with reading lamps, streetlights, and so on. Figure 1 shows what is happening. The blue area, marked "S," represents a point source of light. Imagine the light from the star spreading out into empty space in all directions. Now imagine the light that falls on a square at some arbitrary distance from the star (r). Move away, doubling the distance from the star (2r). The light from the original square has now "spread out" over an area of 4 (= 22) squares. Thus, at twice the original distance, the intensity (power per square meter) of the light passing through a single square will be 1/4 of the original intensity. Going out still farther, tripling the original distance (3r), and the light from the original square now covers an area of 9 (= 32) squares. Thus, at three times the original distance, the intensity of the light passing through a single square will be 1/9 of the original intensity. This is what is meant by the inverse square law. As you move away from a point light source, the intensity of the light is proportional to 1/r2, the inverse square of the distance. Because the same geometry applies to many other physical phenomena (sound, gravity, electrostatic interactions), the inverse square law has significance for many problems in physics.

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