How Sunlight Reflected Off a Building Can Melt Objects

The "Walkie Talkie Building," a name Londoners have given a distinctively shaped skyscraper near Saint Paul's Cathedral, has been in the news this week after reflected sunlight from its mirrored facade melted the side mirrors and panels on a Jaguar XJ parked on a nearby street.

So how on Earth does a skyscraper melt a car?

In a nutshell, it does so by using the same principles a Boy Scout might use to start a fire with a magnifying glass—by concentrating a beam of sunlight on a point.

But at 20 Fenchurch Street, London's hottest new address, instead of a lens being used, it is the concave flank of a 37-story skyscraper covered with 355,000 square feet (33,000 square meters) of highly reflective south-facing glass. It is a coincidence of shapes and materials, say physicists, that is ideally suited to focusing a tremendous amount of solar energy on a small area and generating a lot of heat—enough to melt the plastic coatings on the side mirror of an expensive sports car, fry an egg, blister a bicycle seat, or burn a hole in a doormat, all of which are reported to have occurred in the hot spot beneath the Walkie Talkie.

"This is exactly the same principle that was used to light the Olympic torch last year," said Simon Foster, a physicist at London's Imperial College. "They used a reflective parabolic bowl to focus the sun's rays on a point and lit the torch."

Foster added, "Archimedes supposedly used this bit of physics to set fire to the Roman fleet more than 2,000 years ago. Funnily enough, only two months ago I was demonstrating this very thing to a group of schoolkids, using a cardboard parabola covered with tin foil, and showing them how you could fry eggs with the sun."

Sunlight is reflected off of the "Walkie Talkie" building.

Photograph by Leon Neal, AFP/Getty Images

Unintended Consequences of Building Design

What does seem to be new, however, is the idea of building a £200 million ($312 million) skyscraper with highly reflective concave sides.

"Plain flat surfaces produce no focusing, so it is unsurprising that most skyscrapers have stuck to that design," said Chris Shepherd, a physicist at the Institute of Physics in London. "Outward curves, like those on London's Gherkin Building, defocus the rays, but the concave surfaces of the new Walkie Talkie Building will focus the rays and produce the unfortunate consequences that are being reported." (Related: Las Vegas building burns tourists.)

And those are many. The effect lasts about two hours per day, during which heat detectors have measured surface temperatures as high as 199°F (93°C) on a black plastic case and 225°F (107°C) on the seat of a bicycle that had been left in the "hot spot." That's easily enough to warp PVC plastic, which has a melting point of about 212°F (100°C). The air temperatures along the affected part of the street have been estimated at up to 122°F (50°C).

"It was too hot for me to want to stay down there very long," said Foster.

Fortunately for the shops, businesses, motorists, and pedestrians in the area, the skyscraper-induced solar heat wave is likely to last only two to three weeks, by which time the sun will have shifted in the sky and no longer be so focused by the concave facade onto the street below—that is, until the same time next year.

By then, the building's owners hope to have found a cure.