Why do I see rainbows in car windows when I wear polarised lenses?

Sometimes, when you’re wearing Rocket sunglasses, which all come with polarised lenses to reduce glare, you may see “rainbows” appearing on the side or rear windows of other vehicles.

Don’t worry - there’s nothing wrong with your lenses or your eyes. The rainbows you see are actually strain patterns in the tempered glass windows.

When car side and rear windows are produced, they go through a process called tempering to prevent them from shattering into sharp pieces. During this process, the glass goes through uneven expansion and contraction because the surface of the glass heats and cools at a faster rate than the center of the glass. This type of stress on clear materials, such as glass, produces birefringence, which refracts (redirects) light at many different angles.

When you’re wearing non-polarised lenses, you don’t see a rainbow because your eyes absorb light waves of all angles. However, sunglasses with polarised lenses filter all light waves except vertical ones, allowing you to see the strain patterns in car side and rear windows.