I recently saw a lecture that said the following regarding the sides of a spoon as concave/convex mirrors:

  1. The concave inside of the spoon must be a positive focal length mirror because it would take normally incident rays and bend them towards the axis.

  2. Conversely, the convex outside of the spoon must be a negative focal length mirror because it bends rays away from the axis.

I immediately found this perplexing, since my understanding is that it precisely is the opposite. Wikipedia (seemingly) agrees with me and says the following:

If the lens is biconvex or plano-convex, a collimated beam of light passing through the lens converges to a spot (a focus) behind the lens. In this case, the lens is called a positive or converging lens. For a thin lens in air, the distance from the lens to the spot is the focal length of the lens, which is commonly represented by $f$ in diagrams and equations.

If the lens is biconcave or plano-concave, a collimated beam of light passing through the lens is diverged (spread); the lens is thus called a negative or diverging lens. The beam, after passing through the lens, appears to emanate from a particular point on the axis in front of the lens. For a thin lens in air, the distance from this point to the lens is the focal length, though it is negative with respect to the focal length of a converging lens.

Am I misunderstanding or misinterpreting something here?

I would greatly appreciate it if people could please take the time to clarify this.

  • 2
    $\begingroup$ You are mixing up mirrors and lenses. Look up concave and convex mirrors instead of lenses. $\endgroup$
    – Bill Watts
    Nov 24, 2019 at 8:12
  • $\begingroup$ @BillWatts I understand. Thanks for the clarification. $\endgroup$ Nov 24, 2019 at 8:17

1 Answer 1


Your first quote deals with a mirror lens, reflection, the second quote refers to light passing through a clear lens, refraction.


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