I recently bought a new jacket because my old one fell apart. I wanted a bright colour, easily seen by traffic when I walk to work on rainy, gray days (for obvious reasons). I also wanted my new jacket to be waterproof, again for those rainy days. I'm not much of a shopper, but I did shop around and the only jacket that I could find that met my criteria was black. So now I have a black jacket – the exact wrong colour for high visibility. As a teenager, I had a khaki jacket (army surplus) – a colour designed to blend into wilderness surroundings. I'd often wear this jacket camping. To be seen, I wore nuclear orange gloves that my grandmother had given me. From a distance often all that could be seen of me were the gloves. I actually loved the juxtaposition of my khaki jacket and nuclear gloves. Which brings me to how things appear to "stand out", using sharp contrasts like my gloves.
Distinctive shapes also stand out. For example, our brains are hard-wired to see faces, even in bizarre places like stucco walls and clouds. For this reason, soldiers often paint disruptive green patterns across their faces when they want to vanish in the woods. Straight lines where they shouldn't be also stick out as nature generally doesn't have straight edges. Ever looked at a satellite photo of a wilderness area and had the square shape of a cabin pop out? Movement sticks out. I can find escaped crickets (we keep critters that eat crickets) on out cricket coloured carpets because they move. So, if you are hiding from bad guys: stay still.
On my walk to work I want to be seen. My nuclear orange gloves vanished years ago so I can't rely on them. A retroreflector is an option which is just a good reflector set up to bounce the light right back where it came from irregardless of orientation. The shine from a cat's eye when light hits it is an example. One type of retroreflector is a corner reflector, which is three mirrors put together like the inside corner of a cube. Since many small versions of retroreflectors can be put together as a thin sheet and attached to a fabric, clothing can be made from them. On a dark, rainy day I could wear a retroreflector band around my wrist which would bounce the light of car headlights back towards the driver, warning the driver of my presence. So my problem is solved, I need to find retroreflector wrist bands: but where else are retroreflectors used?
Retroreflectors have made their way to the moon both on American (Apollo 11, 14 and 15) and Russian (Lunakhod 1 and 2) spacecraft as a way to determine the distance between the earth and moon. This is done by aiming a laser on earth at the retroreflector and measuring how long the light takes to return back. This method has found the average distance from the earth to the moon is about 385,000 km. All the retroreflectors on the moon are still in use. They are the only Apollo experiment still returning data from the moon (I don't know if there is Russian gear other than the retroreflectors still transmitting, but I doubt it) and has resulted in and improved knowledge of the moon's orbit.
On a tangent... somehow the Russian Lunokhod 1 rover got lost. On November 17, 1970, Luna 17 arrived at the moon and released the Lunokhod 1 rover to explore. This rover trundled over 10 km, taking samples of the lunar surface and transmitting pictures, until its power ran out at year later. Since a retroreflector was mounted on the rover, scientists were able to keep track of it with lasers from earth until 1974. Then they lost track of it (not sure why because it was no longer moving). Recently, NASA's Lunar Reconnaissance Orbiter spotted the rover's tracks (remember, the only movement on the moon has been us and there is no wind to cover tracks) and was able to pinpoint the rover's location. On 22 April 2010, a laser was bounced of its retroreflector once again.
So a retroreflector turns out to be an excellent way to be seen even from really far away. However, if you are really good with your optics, a retroreflector can be set up that will render one almost invisible.