Who hasn’t seen the kids walking to school with their cute little warning stripes all over their jackets and backpacks. But why do gadgets like warning stripes or safety vests even light up, no matter from which direction the headlights come from? Obviously, there’s no mirror tied to the backpack or the vest but what is it? Here’s an idea on how students can explore the optics of safety vests and find out about a phenomenon called retro-reflection.
Physically speaking, retro-reflection describes the way light is reflected right back at the light source, no matter the angle. If you recall your physics lessons in high school, this is only the case when the light shines upon a mirror when held exactly perpendicular to the mirror. So how come, these materials used for safety vests don’t reflect light the same way?
Students can be prompted to design their own safety vest with aluminum foil. It has the perfect reflective characteristics that can be used to produce individually designed safety vests for the students. This takes them some time and is a great way of then using that self-made product to compare with the real thing. Where exactly is the difference? Engaging questions could be “why wear a safety vest at all?” and let them describe the function of both the color (being seen in broad daylight) and the reflective stripes (being seen during the night). This ultimately leads to the question of why the stripes are so dark during the day. Wouldn’t that make them less visible compared to the bright neon color of the vest?
This almost certainly leads to the experiments in the dark. With flashlights in a darkened room, students are able to fully explore the phenomenon and compare it to their home-made vest. Spot on!
Especially for bigger distances and lights closer to the eye, the effects of the safety vest’s reflective stripes becomes clear and well differentiated to the home-made aluminum vest. Students become certain, that the explanation can’t be simple reflection of the light. Letting students draw the lightpath might lead them into the right direction of understanding the textures of these retro-reflective materials.
One way to produce retro reflection is with layer of microscopically small spheres that are embedded into the stripes. These are as tiny as the hair can be thick, which means they are barely visible with the eyes. The reflection of light within these tiny spheres happens in such a way that they go nearly back in the same direction that they came from.
And by the way: cat’s eyes are also retro-reflectors. That’s why we see them so well in the dark when they appear in the headlights.