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
- Connect two 14V light bulbs to the DC power supply and SPST switches.
- Using the C clamps, secure the switches to the table.
- Place the silhouette on a portable stand at the far side of the table.
- Position the three lights as shown in the above diagram.
- Turn on one light bulb.
- Turn off the room lights.
- Note the sharp shadow, and how there is only a umbra or illuminated part.
- 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.
- Switch on both lights. Note that the shadow is still quite sharp.
- Also note that the umbra has been reduced in size, and there are now two penumbras.
- Turn off both lights and turn on the fluorescent light.
- 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.
- 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.