The phenomenon of cat’s eyes on roads—those small reflective road studs used for lane markings—is explained by total internal reflection (TIR).
Here’s how it works:
- Glass or Plastic Dome: Each cat’s eye contains a small glass or plastic spherical or semi-spherical lens embedded in a reflective housing.
- Incident Light from Vehicle Headlights: When headlights shine on the cat’s eye, light enters the transparent dome at an angle. Inside, due to the high refractive index of the material compared to air, the light hits an internal surface at an angle greater than the critical angle.
- Total Internal Reflection Occurs: Instead of passing through, the light is totally internally reflected inside the dome and directed back toward the source—i.e., the approaching driver.
- Retroreflection: This TIR mechanism reflects light back in the direction it came from, making the cat’s eye appear to glow when illuminated, even from far away.
- All-Weather Visibility: Because TIR is highly efficient and doesn’t rely on coatings that might wear off, cat’s eyes remain visible even in wet or foggy conditions.
Thus, total internal reflection allows cat’s eyes to reflect light efficiently and enhance road safety by providing clear lane visibility at night or in poor weather.