What is the difference between the way a mirror reflects light vs a retroreflective safety vest?


What is the difference between the way a mirror reflects light vs a retroreflective safety vest?

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Hi /u/Johnnysmooth1996!

A mirror has a very smooth surface, therefore the light rays are reflected approximately by a single surface. Therefore, they keep their relative position and orientation, which leads to a clear image in the reflection.

Reflective safety vests, on the other hand, scatter the incoming light rays in different directions, so as to be visible from many different angles when illuminated from one direction. As a consequence of this scattering, the reflected image is not preserved.

With a mirror, every single photon of light that hits it bounces back at the same angle at which it hit the mirror (we say the angle of incidence is equal to the angle of reflection). This means if all the light off of one object was travelling parallel to each other, it would continue travelling parallel after bouncing off the mirror. This is what allows it to preserve the image.

With something like a retroreflective vest, there’s something else happening on top of that. A retroreflector is a special shape consisting of multiple mirrors. The light that comes in will always be reflected on the exact same trajectory back towards the source, albeit just a little offset.

For example, there are retroreflector panels on the moon that were left by the Apollo astronauts. If you were to shoot a laser at them from the north pole, the light would return to the north pole. If you shot a laser at it from the south pole, the light would return to the south pole.

Now a retroreflective vest won’t be a perfect retroreflector, since their job is to get light to a driver from a car, but the basic principal is the same — redirect all the incoming light back to where it came from… and up a little bit into the driver.

A retro reflector is really just a series of microscopic mirrors alligned in such a way that any light entering the inside of the mirror will be reflected back towards it’s point of origin. This is usually done with microcrystals the inside walls of which act like mirrors.