Concave Spherical Mirror: Observational Experiment


To construct the idea of a focal point and focal plain of a concave mirror.

Prior Knowledge

  • The law of reflection.
  • Geometry of a sphere. The concept of a normal line.

Description of the Experiment

The mirror that you see in this experiment is a segment of a sphere. You can see how big this sphere is by tracing the missing parts of it. Observe the experiment and record all patterns. Then explain the patterns using your knowledge of the law of reflection.

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  • Record your observations. It is better if you draw pictures similar to the experimental outcomes on a piece of paper.
  • Explain why all reflected beams come through the same point when the beams are horizontal.
  • Where is this point (called a focal point) in relation to the center of the sphere from which the mirror was cut? Where should it be according to the law of reflection?
  • Where are the points through which all reflected beams pass when they are not horizontal but parallel to each other? (Describe the location fo the points relative to the focal point.)
  • Explain how one can predict a path of any ray incident on a concave mirror if she knows the location of a focal point?