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Real World Portal: The Physics Behind the Exploratorium's Giant Mirror

By Wesley Fenlon

Of the over 100 new exhibits at the new Exploratorium science and learning museum in San Francisco, a giant mirror is our favorite. Here's why this curved piece of metal looks like a portal into a parallel universe.

The ripples in carnival mirrors prepare us for what we're about to see when we look into them. Where the surface of the mirror bulges out or contracts inwards, so too does our image, stretching out our reflections into bloated torsos and oddly shrunken heads. No matter how you pose in front of a funhouse mirror, you're going to look weird and misshapen. By contrast, the San Francisco Exploratorium's giant curved mirror isn't so predictable--you can't tell how its metallic surface will distort your image. As a result it's one of the museum's most fun and striking exhibits.

The mirror's metallic surface is so massive and reflective that it fills your entire peripheral vision from a few feet away, drawing the eye inwards and making it difficult to focus on the mirror's edge. But it also produces an effect that you've probably never seen in another mirror: From the right perspective, reflections leap out of the mirror like 3D projections. It's far better than watching a 3D movie. Yeah, you lose the transforming robots and projectiles shooting out of a screen, but you gain a feeling of tangibility that no movie screen can produce.

"I think that one of the reasons that's a little surprising to us is that when we use mirrors or lenses we often project the images onto a two dimensional surface," said Thomas Humphrey, Ph.D, who introduced us to the exhibit. I talked to Humphrey about the mirror after his presentation, expecting a detailed physics lesson, and ended up getting a more experiential overview of how our eyes interact with the gigantic reflective surface. He continued:

"When you use a camera and a lens you project it into a [two dimensional] sensor array...or if you go to a non-3D movie, you project it onto a 2D screen. Your plasma screen at home is a 2D screen. So all the images we see, they're really, geometrically, two dimensional. And we use other features of the image--like one thing is in front of something else, that blocks it, obscures it--[to] tell us that one is in front. But this [mirror] shows us something that you pretty rarely see. It shows you that mirrors actually make 3D images, and when we put a screen up, we're just taking part of that 3D image, one slice of that 3D image, and showing it on the screen. The thing that's most common is that a mirror makes a 3D image, but we never see that because we're always slicing screens in there to see part of it. We're not allowing ourselves to see the whole thing. But actually what lenses and mirrors do fundamentally is make 3D images."

The 3D images this mirror produces happen to be upside down--but only sometimes. Just watch the video below.

Like a funhouse mirror, a concave or converging mirror will show you something different depending on your perspective relative to the surface. This happens because light hitting the curved surface of the mirror is reflected at a number of different angles depending on the curve.

This chart of concave mirrors shows how varied their reflections can be. At the mirror's focal point, no image will appear, because light rays reflected off the object travel in parallel. Physics Classroom elaborates on this effect: "the light rays neither converge nor diverge after reflecting off the mirror...Subsequently, the light rays will not converge on the object's side of the mirror to form a real image; nor can they be extended backwards on the opposite side of the mirror to intersect to form a virtual image...when the object is located at the focal point, there is no location in space at which an observer can sight from which all the reflected rays appear to be diverging. An image is not formed when the object is located at the focal point of a concave mirror."

From other positions and angles, the concave mirror produces different images. Some are magnified and inverted; others are upside-down, but not ginormously huge. Humphrey encourages people to check out a variety of poses in the mirror. As an exhibit, the mirror is all about discovery.

"Sometime the way I begin to talk about an object like that is to pull upon people's experience, and say you've already experienced mirrors like this before, and there are ways you can go looking for more of them," he said. "One thing I point out to people is when they're eating their cereal in the morning, or eating a bowl of soup, and you pull the spoon out and you clean the spoon and look into the spoon, you see a little miniature version of yourself. The spoon wasn't meant to be a mirror. But you see yourself in there. People say 'oh yeah, I've had that experience.' Well, the giant mirror is nothing more than a giant spoon."

And giant is important, because the mirror's size actually affects how we perceive the images its reflects. "The spoon is a very small angular view when you're looking at that, but this one is so big it goes way beyond your acute vision into your peripheral vision," Humphrey added. "Just by virtue of the size of the object the world looks different, so you see that 3D much more clearly, which you're not used to with mirrors."

The mirror offers one other fun trick thanks to its concave design: If you position yourself near someone's reflection, the mirror will reflect sound waves just like it does rays of light. A whisper towards a reflection's ear will arrive, loud and clear, in the real person's ear.

The mirror also has an interesting but mostly mysterious history predating its spot at the Exploratorium. Though you've probably seen plenty of large mirrors in your life, most of them are glass-fronted, not purely metal. This mirror has no glass protecting its reflective surface, making it especially vulnerable to scratches. It's in remarkably good shape, given how many homes it's had in the past.

"It's my understanding that it was originally made in Germany," Humphrey said. "It was used by NASA, actually, to be a flight flight trainer. They would project images into that mirror. If you're a pilot learning how to fly a plane you would see the sky and the clouds and skyscrapers and landing strips in front of you and you could practice without killing yourself...I think it was in some museum show, probably at the Air and Space Museum in Washington."

The mirror eventually made its way to the Chabot Space & Science Center in East Bay, where there wasn't much room for it; the Exploratorium paid to have it hauled across the Bay. "And now it's an exhibit," Humphrey said. "Like a lot of other things around here it's sort of a found object."

You can find the giant mirror at the Exploratorium on San Francisco's Pier 15. It's something you really have to see to believe.