Mirror Equation

This equation links the focal length of a curved mirror to where an object is placed and where the image forms. It comes from the geometry of reflection for paraxial rays, so it works well when rays are close to the principal axis and the mirror is not used at very wide angles. If you know any two of the three quantities, you can solve for the third and predict whether the image will form closer to or farther from the mirror than the object.

Use it for concave or convex mirror problems in ray optics when you need to find image distance, object distance, or focal length. It is especially common in physics homework and lab questions involving mirrors, magnification, and image formation.

• Mixing up the sign convention for concave versus convex mirrors
• Using the formula with distances measured from the wrong reference point instead of the mirror pole
• Forgetting to invert the final fraction after solving for 1/d_i