The mirror plays an especially important function in advertising. It is usually used to allow us to see how we might appear if we used the product in question. Perhaps the most famous association we have of the mirror in advertising is from the long running Clairol campaign that used the tagline, "YOU, ONLY BETTER." At its simplest level, this pretty much sums up the role of the mirror in ads. It offers us a depiction of ourselves as we desire to be.
Judith Williamson uses the "mirror" as a metaphor for the social reflection of the self. But as she does so, she acknowledges the relationship between the mirror and the self gets more complex than what we have explored thus far. So, let's walk through this more carefully now. First off, the mirror provides a device for hailing or addressing the viewer as a subject who is aware of themself as having an identity. The best way to illustrate this is to think of those ads which use the "mirror" metaphor to address a negative identity -- a persona that we most likely do not want to be. The best example of this usage is in anti-drug ads -- so-called public service ads because they do not sell a commodity. These ads present us with negative stereotypes. Now contrast this negative use of the mirror with its more typical advertising usage in this Kellogg's Special K ad.
In ads like this one for Special K, the mirror doesn't just reflect a desirable social self, it is also a story-telling device. We call the story it tells one of "envy and desire." The body image shown in the mirror hails young women who simultaneously envy and desire this more "perfect" body image. This fits very closely to what Jacques Lacan has written in article called "The Mirror Phase:"