Why You See Rainbows Through Car Windows
Why Polarized Lenses Create Crazy Patterns in Car Windows
You don't need to be crazy if you notice your car windows suddenly covered in a variety of colors. It can't be attributed to bad window tinting. It doesn't necessarily mean your lenses are bad.
It's just a matter of physics that you are experiencing.
Tempered glass is used on many rear and side car windows. It allows the glass to shatter but not cause sharp edges. The glass is first heated to temper it, then quickly cooled to room temperatures. The glass's surface cools faster than its center and contracts, creating compressive stresses. However, the center of glass expands due to temperature and produces tensile stress.
Stressed birefringence is what you see when you look out of your car's window. Birefringence is when optically clear materials are stressed. This basically means that it changes the polarization.
You may see a strain pattern in the rearview mirrors. This is due to the tempering process.
The glass is partially polarizing light along a horizontal axis, so you can see the lines and dots. Because your polarized lenses are vertically oriented, the light is blocked. This is why dark lines or dots appear.
It is basically like having two different polarized filter that are perpendicular to each other. The one traps light moving vertically and the other traps light traveling horizontally.
This pattern can be seen without the use of polarized lenses. There are many factors that can cause light's polarization. If light is already polarized and it meets the window, it's effectively passing through a second filter. Because polarized lenses can polarize light, the dots will appear more clearly. Only a small portion of light is polarized by the car's window.
Because manufacturers use different tempering methods, the pattern may not be consistent on all cars. Some vehicles may have a pattern that looks like diamonds while others might have dots or squares. It all depends on what heating pattern was used during tempering.
The rainbows seen in car windows is also a result of stressed birefringence.
The rear and side windows allow incident light to pass through. Tempering causes polarization to be rotated by the stresses. The wavelengths determine how much rotation is required, which results in different colors. You can see the stress in the car windows caused by tempering.
This phenomenon is similar to a prism, or a raindrop, which causes light to bend or refract and thus shows the various colors of light. Because your sunglasses block certain wavelengths of light, you can only see the rainbows through the car windows.
Stressed birefringence can also be observed in plastic cups and utensils. You can see the same rainbow pattern in a cup of plastic if you wear polarized lenses. Photoelasticity is a method used by structural engineers and scientists to determine the material's stress distribution.
The reason the rainbow effect in windows with tinted windows is stronger is because the window tinting film is often made of polyester, which is a type of plastic. The stresses within the plastic film can cause rainbow patterns in tinted car windows in the same way as you can see in a spoon or cup.
You won't find any patterns or rainbows in windshields because windshields are made from laminated safety glass and not tempered.