The belief of a broken mirror resulting in seven years of bad luck is most likely one of the most well known superstitions in the world. Just hearing the phrase makes anyone take a little extra care while moving and hanging mirrors. For many thousands of years people relied on highly polished metals as a reflective surface which held multiple purposes in daily life such as applying makeup, redirecting light, or even starting fires by focusing light into a fine point.
When the Romans created easier to break glass mirrors with a thin metal backing, mirror superstitions really went into overdrive. Cultures during the period considered any images seen in mirrors otherworldly and believed those images represented souls captured in another realm or dimension. If the mirror broke while someone was looking into it, the soul became trapped in many pieces, requiring either every single piece of the mirror to be ground into a fine dust or face seven years of bad luck while allowing the soul to mend. A soul is a direct reflection of who a person is; It's the light within and an energy binding life to their existence. What's very interesting about mirrors and bad luck stories is how believers refer to the soul becoming trapped inside another realm or dimension, almost alluding to a specific method for interacting or communicating with an alternate dimension. If it's true, that a broken mirror with a captured soul really does affect its reflective world physically, then it makes sense to have some sort of existing dimensional connection in order this to happen.