As a scientific term, specifically in physics, the observer effect refers to changes that the act of observation will make on whatever (or whomever) is being observed. The example classically used is measuring a tire's pressure. The act of inserting a gauge lets out some air and thus alters the value that appears. At the microscopic level, the mere act of examining particles behaving changes the way they interact. In fact, recent research suggests that reality does not even exist until it is measured. Talk about the power of observation!
In a broader sense, we can say that the observer effect refers to the influence of the observer, who can be said to interact with whatever she is looking at. Consider that when you look at a person, a stranger even, and at a distance - say, from across the room or the street - it is inevitable that if you look long enough, the stranger will return your gaze, as if she feels your eyes on her.
Then there is the notion of the evil eye, that a person can look at you with malevolence and cause something bad to happen. Perhaps this is not so superstitious as it seems. Maybe it's more than a clever attempt by charlatans to sell you products to ward off the stranger's ill-intentioned gaze.
And if the evil eye is a demonstrable fact, then the opposite (and a much more benevolent alternative) is also true. If you regard others with loving kindness, you can bring blessings into their lives. Without a word or any action other than resting your gaze on their form, be it family, friend, total stranger or sworn foe. So subtle as to go unnoticed - unless you stare long enough, which will get the person to look right back at you and blow your cover. Which may not be too bad if the stranger in mind is a cutie pie. Though some may say that's simply flirting.
Regarding others with kindness and unconditional love in your heart and a smile in your eyes has a galvanizing force. You become walking sunshine, metaphorically speaking. Talk about making the world a brighter place!
It has been said that the manifest universe is the reflection of the unmanifest Reality, regarding Itself as if in a mirror. So if it is true that you are what you see, it is best to be kind.