Plane mirror

One essential knowledge statement generates the whole case. EK 13.2.A.3: the focal point of a plane mirror is an infinite distance from the mirror. Put $f \to \infty$, so $1/f = 0$, into the mirror equation:

Three readings fall out of that one line.

• Virtual, because $s_i$ is negative, meaning the reflected rays only appear to have originated behind the mirror (13.2.A.6). Nothing lands on a card held there.
• Same size, because $\lvert M \rvert = \lvert s_i/s_o \rvert = 1$.
• Equally far behind, which is EK 13.2.A.7.ii stated independently: the distance between the image formed and a plane mirror is equal to the distance between the object and the plane mirror. The equation reproducing a separately printed statement is a good sign you have the signs right.

Upright is not the same as unreversed. In the language of Unit 13 an image is upright or inverted (13.2.A.9.ii), and yours is upright. The everyday sense in which a reflection looks backwards is a front to back reversal along the direction you face the mirror, and it is not the orientation the equation is talking about.

It is examinable. The Topic 13.2 boundary statement limits the study of mirrors to plane mirrors, convex spherical mirrors, and concave spherical mirrors, so the flat case is one of exactly the three you are responsible for.