How Sunlight Reflected Off a Building Can Melt Objects

A new skyscraper in London exemplifies the phenomenon.

A man reacts to a shaft of intense sunlight reflected from the glass windows of the "Walkie Talkie" tower in central London.

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."