WORLD SCIENCE

"Long before it's in the papers"
December 22, 2005

Mirror, mirror: I still don’t get it

Dec. 22, 2005
Courtesy University of Liverpool 
and World Science staff

Despite dealing with mirrors every day, most of us still don’t get how they work, a study has found.

Venus at Her Mirror by Diego Rodriguez de Silva y Velazquez (1599-1660). Despite being one of the most admired images in art history, the optics of the mirror portrayal make little sense. If we saw her face in the mirror, as above, she would be looking at our face.

It’s not just that people don’t grasp the complexities of how the atoms in the mirror bounce light back at us. It’s worse: many of us don’t even get basic facts about roughly which direction the light will reflect in, the study concluded. 

Marco Bertamini, a psychologist at the University of Liverpool, U.K., conducted experiments by covering a mirror on a wall. He invited participants to walk on a line alongside the mirror, and asked them to guess at which point at would be able to see their reflection. 

Results showed that people believe they can see themselves even they are level with the near edge of the mirror, he reported. That’s wrong: you must be directly in front of the mirror to see yourself, because light bounces off a mirror at the same angle it hits. That’s the basic fact about mirror reflections.

This means “The location of the viewer matters in terms of what is visible in a mirror,” Bertamini said. “A good example of this is what we call the Venus Effect, which relates to the many famous paintings of the goddess Venus, looking in a small mirror.”

“If you were to look at these paintings, you would assume that Venus is admiring her own face, because you see her face in the mirror. Your viewpoint, however, is rather different from hers; if you can see her in the mirror then she would see you in the mirror.” 

Participants were also asked to estimate the image size of their head as it appears on the surface of the mirror. They estimated it would be about the size of their real head. But they based their answer on the image that appeared to be behind the mirror—not the image that really was on its surface.

People failed to see that the image on the surface of the mirror is half the size of the observer because a mirror is always halfway between the observer and the image that appears “inside” the mirror. 

Bertamini added: “Mirrors make us see virtual objects that exist in a virtual world; they are windows onto this world. On the one hand we trust what we see, but on the other hand this is a world that we know has no physical existence. This is one of the reasons why throughout history people have been fascinated by mirrors.”

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