Just in case anyone out there is interested in understanding why the image seen in a plane mirror looks like it does, read on.
The short answer to this question is: because the angle of reflection is equal to the angle of incidence.
Because the Message Board rules do not allow the posting of attachments, and because I am not smart enough to be able to incorporate a diagram in this text, you will have to make a drawing that illustrates the answer. ( In any place where there is both light and an observer, every point of every object in the place acts as if it were an emitter of the light that reaches the eye of the observer. Just considering that light, the light path will follow a straight line between the point and the eye, so you can omit all the complications of the fact that light acts partially like a particle and partially like a wave and just think of rays of light. For reflection, refraction and transmission in a transparent medium, light acts mostly like a wave. For emission, adsorption and transmission in a vacuum, light acts mostly like a particle) Draw the side view of a stick figure with right arm raised and left arm by its side, facing a plane mirror. On the side view, draw the light ray that comes from the top of the figure’s head, hits the mirror and is reflected into the eye. Locate the point on the mirror so that the angle between the ray from the head to the mirror makes the same angle with the mirror as the angle that the ray from the mirror to the eye makes with the mirror. This will make the point of impingement on the mirror fall half way between the vertical distance between the head and the eye. The eye sees the image at this point on the mirror. Now do the same exercise with the light path from the raised hand to the eye, and also do the light path as seen in the top view. Do the same exercise with other parts of the figure. If you did all the points, you would draw the complete image seen in the mirror. And the image is indeed reversed front to back, as Martin Gardener, cited by Andrew T in the other thread on spoon reflections, pointed out, but neither top to bottom nor right to left. RM Mentok also posted a comment that a mirror does not reverse left to right. We three make a majority on this issue.
Notice that the image is both shorter and narrower than the object. The image is actually seen as if it were as far back of the mirror as the object is in front of it, so it looks smaller.
The plane mirror image is called a virtual image, apparently because the light from the object to the eye does not go though a focal point in getting to the eye.
For those who would now like to know why the angle of reflection is equal to the angle of incidence, I fear that the explanation takes a page of text and a diagram to explain. But this info can be found in almost every college freshman physics text. Look it up.
In another post on the other thread on spoon reflections, nitpick pointed out that the image changes back to right side up when you get close to it. Here is a little more detail on that. The appearance of the image depends on the relative position of the eye and the focal point for the spoon. The return to an upright image occurs when the eye gets between the focal point of the spoon and the surface of the spoon. This is a lot easier to see if you can find something like a gravy ladle that is spherical but less than a hemisphere. And easier yet if you can find a nicely polished wok. The focal point of the wok will be located out from the center of the wok a distance equal to half the radius of curvature of the wok. Actually, a spoon does not have a single focal point because the radius of curvature of the lengthwise dimension of the spoon bowl is larger than the radius of the transverse direction of the spoon bowl.
