Geometric Optics

Bulb in a Can


On a tabletop, a coffee can is balanced on a soup can. Placed in the center of the coffee can is a small candle.  Suspended above the coffee can is a plane mirror.

Q1: Treat the flame as a point source.  Determine the region of the tabletop that is lit by the reflection of the flame.  Draw and label the relevant light rays and lighted regions on a diagram of the situation.  Explain the reasoning used to draw the rays.

Q2: As the candle burns the flame get closer to the bottom of the coffee can.  How does this change the lighted areas of the table?  Explain using the diagram from question #1.

Q3: If the candle were moved from the center closer to the side of the can, how would the lighted regions on the table top change?


Arons, A.B. (1994). Homework and Test Questions for Introductory Physics Teaching. NY:Wiley. p162.

Seat Activities