Windshields are rarely gush-worthy, but the one on Tesla’s Model X truly is: It swoops up and over your head, offering an unfettered panoramic view of the road. But that windshield is also causing trouble. As Jalopnik recently highlighted, some drivers have complained the glass causes double vision at night. What’s happening?

In one word, physics. And pretty simple physics. I promise. You don’t even need any math to understand.

Imagine looking through a vertical pane of glass, like a window. Whenever light passes through two different materials—like going from air to glass or glass to air—some light passes through the second material and some reflects off. When light hits a window straight on, most of it goes right through the glass and very little reflects.

But if it hits at an angle—like on a sloped windshield—some the light will reflect off the glass-air boundary, which goes back and hits the air-glass boundary, and so on.

“Those multiple reflections keep rattling around in there like a ping pong ball,” says Wayne Knox, an optics researcher at the University of Rochester, who was so excited he made me a whole Powerpoint presentation diagramming the reflections.

Some light is lost with each reflection, so the images get dimmer and dimmer and dimmer, until you can’t see them anymore. That’s why drivers might see two or even three but not infinite traffic lights through their windshields.

The angle of a piece of glass affects whether light is reflected inside the glass. And yes, smarty pants, this is a simplified diagram that does not show refraction. The angle of a piece of glass affects whether light is reflected inside the glass to cause ghosting. And yes, smart alecks, this is a simplified diagram that does not show refraction. WIRED

The phenomenon is common enough to have a name: ghosting. And drivers have complained about it with cars as varied as the Toyota Prius, Dodge Durango, and Chevrolet Camaro. Now the engineers at Tesla are surely no strangers to Optics 101, and ghosting doesn’t happen with every windshield. So something else has to be going in—something particular to the Model X or to the specific cars whose drivers complained about ghosting.

Here we leave the laws of physics for the land of conjecture. “Reflections (ghosting) occurs in all laminated glass to varying degrees,” a Tesla spokesperson said in a statement. “We have received only a small number of questions from Model X customers about the windshield and have taken action to address these unique cases.”

What the ghosting looks like from inside a Tesla Model X. What the ghosting looks like from inside a Tesla Model X. Jeff McClure

Let’s begin with laminated glass, which is standard in cars these days. “Laminated glass is basically a glass sandwich,” says Debra Levy, president of the Auto Glass Safety Council. In between two layers of glass is a layer of durable plastic, which keeps the whole windshield intact in case the glass shatters. Now remember how light can reflect at the interface between any two materials? With laminated glass, you now have two additional interfaces: air-glass, glass-plastic, plastic-glass, and glass-air. If you go back to the ping-pong ball analogy, “that can increase the amount of rattling around,” says Knox.

To make it even more complicated, windshields can also come with various coatings to repel water, keep out sunlight, and more. Every coating adds another interface to the windshield that light can reflect off. Depending on the physical properties of the coating and the laminated glass, that could cause more reflections to rattle around.

And then there is the shape of the windshield itself. Back in the 1990s, Thomas Brown, an optics researcher at the University of Rochester, remembers going out to inspect the windows of an air traffic control tower in west Texas. In this control tower, the windows were tilted toward the ground, which allowed the staff to see more of the tarmac. But the windows were also installed incorrectly, and the weight of the glass caused them to sag in the middle. This curvature caused ghosting, which Brown and his team diagnosed by literally shooting a laser out of the tower at night.

Brown suspects the curvature toward the top of the Model X’s windshield may also play a role. Anecdotally at least, taller passengers seem to have more of a problem with double vision. A piece of glass both curved and angled piece of glass could be more prone to ghosting. “With the reservation that I haven’t test driven a Tesla,” he added as we were getting off the phone. “Just let me know when I can test drive one.”

The reports of ghosting with the Tesla Model X are rare enough that they are probably the result of some specific combination of factors: light conditions, the driver’s height, or possible irregularities in a windshield. Jeff McClure, a Model X driver who wrote about the problem on the Tesla Motors Club forum, says he has been in talks with the local service center to replace his windshield.

The reason it’s so hard to make an exact diagnosis is that windshields are pretty damn complicated these days. “There are hundreds upon hundreds of different properties that make up the glass,” says Levy. “The windshield used to be vertical, and it was just to keep the bugs and wind out of your face. Today the windshield is an integral part of the car’s safety system.” With its vertical windshield, the Ford Model T didn’t have ghosting, but by our modern standards, it sure had a lot of other safety problems.


Why Tesla’s Model X Is Giving Some Drivers Double Vision