6A02.15-Silhouette Shadows

6A02.15-3 6A02.15-6 6A02.15-5 6A02.15-4

Description: Demonstrates an umbra and penumbra using both point and extended sources.


  • Long table
  • DC power supply
  • 2 (14V) light bulbs
  • 12″ long fluorescent tube (120V AC)
  • Cardboard silhouette with magnet
  • Long pink cord
  • 2 SPST switches
  • Portable stand
  • 2 C clamps
  • Banana plug cables
  • (Optional) Moon globe

Setup Procedure:

  1. Connect two 14V light bulbs to the DC power supply and SPST switches.
  2. Using the C clamps, secure the switches to the table.
  3. Place the silhouette on a portable stand at the far side of the table.
  4. Position the three lights as shown in the above diagram.

Demonstration Procedure:

  1. Turn on one light bulb.
  2. Turn off the room lights.
  3. Note the sharp shadow, and how there is only a umbra or illuminated part.
  4. Have an assistant hold one end of the string at the light, and then make a path from the light to the edge of the shadow. Notice how it just touches the silhouette.
  5. Switch on both lights. Note that the shadow is still quite sharp.
  6. Also note that the umbra has been reduced in size, and there are now two penumbras.
  7. Turn off both lights and turn on the fluorescent light.
  8. Note the shadow is very blurry, and there is no umbra. You may once again trace the light paths using the string to show that there is no umbra.
  9. A variation of this is when a sphere is used in place of the silhouette to represent the umbra and penumbra made by the moon.