The Hidden Waves at the Beach

Electromagnetism researcher Nader Engheta explains the physics behind invisible waves washing over beachgoers at the shore.

For most beachgoers, the word “waves” conjures the swells rolling in from the horizon, breaking white against the sand.

“But those are only some of the ones we can see,” says Nader Engheta, the H. Nedwill Ramsey Professor in the Department of Electrical and Systems Engineering in the School of Engineering and Applied Science. “The ocean is the most obvious example, but it is far from the only wave at the shore.”

Nader Engheta is the H. Nedwill Ramsey Professor in the Department of Electrical and Systems Engineering in the School of Engineering and Applied Science, with secondary appointments in Penn Engineering’s Departments of Bioengineering and Materials Science and Engineering and in the Department of Physics and Astronomy in the School of Arts & Sciences.

A Beach Full of Invisible Waves

“The crashing we hear is an acoustic wave, which requires a medium like air or water for propagation,” explains Engheta, who also has secondary appointments in Penn Engineering’s Departments of Bioengineering and Materials Science and Engineering and in the Department of Physics and Astronomy in the School of Arts & Sciences.

“Sunlight is a wave as well: an electromagnetic one, streaming 93 million miles to reach us. Unlike other waves, light doesn’t need a medium,” he says.

What sets electromagnetic waves apart is frequency: how many times per second the wave cycles. Human eyes register just one narrow band of those frequencies, the visible spectrum, while the rest streams past unnoticed — which is also why the same wave can breeze through one material and bounce off another.

Engheta demonstrates this in lectures by having students picture their phone’s flashlight aimed at a wall. The light doesn’t go through the wall, whereas an incoming call will. “Same kind of wave, different frequency,” he says. “The wall blocks visible light but lets longer, lower-frequency signals slip through.”

When a light wave strikes skin, the sand, or a beach umbrella, it jostles the charged particles inside, setting them into motion. That motion quickly randomizes, Engheta says, “and that randomized motion of charged particles is what we call heat.”

The same principle powers a microwave oven. “The microwave never delivers heat; it just delivers energy that becomes heat once matter gets in the way,” he says. “Your skin at the beach works the exact same way!”

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