While travelling on highways during night, one thing you might have noticed that the signboards along the highways are almost always visible. Moreover they appear to glow as your vehicle approaches the signboards.
Have you wondered what might have caused these signboards to glow during night?
Surely there is some kind of reflection of light as the light of your vehicle hit the surface of the sign board. However the glowing of signboards involves a very particular kind of reflection.
Actually these signs are coated with the reflective material such as glass beads or micro-prismatic plastic that reflect the light back to its source.
Hence the light from your vehicles hit on the signboard surface; the light is reflected to back to your vehicle enabling you to read these signs easily by making them appear to glow.
Retro-reflection
The reflection in which the light is reflected back to its source is known as retro-reflection. This can be understood as follows:
Take two mirrors and place it at 900 to each other.
Now if the incident beam hit upon the surface of any of the mirror then the reflected beam will return to its source by taking a parallel path to incident beam.
Signboards on highways glow at night due to a property called retro-reflectivity.
Therefore the coating with which the signboards are made have this property of retro-reflectivity, which cause the signboards appear to glow.
You can watch a video here to understand retro-reflectivity: