Shows the relationship between the distance of a light source and its meaured intensity.
To try this
simulation:
Move the mouse up and down to move the light bulb back and forth from the light meter. The intensity of the light varies inversely with the square root of the source's distance.
What's Going On?
The light meter sensor measures the light that falls on a 1 centimeter square in the moveable screen. When you move the screen away from the sphere, less light hits the sensor than when the screen is close.
There is a catch! When you move the sensor twice as far away, you do not get half the light! This is because of the Inverse Square Law. This law states that if you move the light source twice the distance away (2 times), the light intensity is the inverse of the square of the distance.
The square of 2 is 4 (2 x 2 = 4). The inverse of 4 is 1/4. If the distance doubles, the light intensity is four times less.
There's More!
Imagine a light bulb hanging in space. Light shoots out from the bulb equally in all directions. Put a sphere (like a giant beach ball) 4 meters in diameter around the light bulb.
The area of the sphere is about 50 square meters (the formula for the area of a sphere is
Area = 4r²).
All of the light coming from the bulb is hitting the inside of the sphere. Let's call the
light intensity = 50 so each square meter gets 1 unit of light. In our experiment, pump up the sphere until it is 8 meters in diameter, or twice as big. The new area we get about 200 square meters.
The catch is, there is still the SAME amount of light coming from the bulb. The light intensity of 50 has to illuminate 200 square meters instead of 50 square meters.
If we divide 50 by 200 square meters we get 0.25 (or 1/4).
If we double the size of the sphere we get one fourth the intensity!
