6A20.41 Projected Filament with Mirror


Concave mirror reflections, ray optics


A concave mirror reflects a real, inverted image of a light bulb filament onto the wall. Covering parts of the filament or mirror with an index card illustrates the nature of image formation under a ray optics model. Covering part of the mirror dims the image while covering part of the filament obscures the corresponding part of the reflected image.



  • [1] U-shaped filament light bulb
  • [1] Concave mirror
  • [1] Index card
  • [1] Lab stand
  • [1] 90-degree clamp
  • [1] 3-finger clamp

Classroom Assembly

  1. Mount the light bulb to the lab stand using both a 3-finger clamp and a 90-degree clamp.


  1. Turn on the light bulb.
  2. Hold up a concave mirror facing the bulb and show that at a distance larger than the mirror's focal length, an inverted image of the filament appears on the wall.
  3. Ask the class what will happen if an index card covers part of the filament, and then do that experiment.
  4. Ask the class what will happen if an index card covers part of the mirror, and then do that experiment.


Additional Resources


  • PIRA 6A20.45


  • Don't attempt this at home!

Last revised

  • 2017


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If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.