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6B10.20 - Inverse Square Law with Crookes Radiometer

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​6B10.20 - Inverse Square Law with Crookes Radiometer

Title6B10.20 - Inverse Square Law with Crookes Radiometer

To demonstrate the Inverse Square Law using a Crookes radiometer to show that the radient flux on the radiometer decreases by a factor of 4 when the distance between the radiometer and light source doubles.​

Assembly Instructions

​Attach a 100W light bulb to one of the timers and set the timer to 30 seconds (or plug light into on/off switch and provide stop watch).  Place the Crookes radiometer on one side of the table.  Using a meter stick, place the light bulb 10 inches from the radiometer.  Use a camera to image the Crookes radiometer so the class can see the spinning vanes with the reference mark visible.

Setup Time5
Operation Time7
Preview Time10
Operation Instructions

With the camera ported so the students can see the radiometer, make sure the light bulb is 10" from the radiometer.  Turn on the light bulb and start the timer.  Count the number of rotations in 30 seconds of the radiometer in response to the radient flux from the light bulb.  Then turn the light off and move the light bulb to 20" from the radiometer and repeat.  Compare the # of rotations to the distance the source is from the detector and you should observe the inverse-square law.  If the distance between is doubled, the intensity falls by a factor of 1/4​

Demo on DimeNo
PIRA 200No
Export Instructions (if different)
Category6 Optics
Subcategory6B - Photometry
Keywordsinverse-square, radiometer
Construction Information
Crookes' radiometer
light bulb - 100W
meter stick
timer - start/stop
camera - b&w
blocking plate
switch - on/off with power outlet
stop watch