The humble bicycle reflector works on the principle of corner cube retro-reflection. Incident light bounces off the orthogonally placed, reflective plastic surfaces and emerges in the opposite direction, parallel to the incident ray. Its totally and internally reflected. I have shown this for you in the following image. Additionally, you can play around with this cool applet from a Taiwanese education website to simulate corner reflection.
I call it humble because it is cheap and sustainable. Think about it. You hardly need an external power source to make this thing work. As long its clean and there is a good source of light shining on it directly behind, it signals back its presence faithfully. And if you have three wide paneled reflectors, you can extend the range of the light horizontally.
This simple concept was so beautiful to scientists that an array of 100 of these corner cubes were left on the moon as part of the Apollo Moon mission of 1969. The idea? By shooting laser pulses from an earth based telescope, you can measure the round-trip travel time and hence the earth-moon distance to an accuracy of mere centimeters!
Most people may think all Neil Armstrong left on the moon was a U.S flag and his footprints. Not quite. In addition, there's also these corner cubes and it has served scientists with lunar laser ranging for over 35 years! If you want to learn more on lunar laser ranging, read this.
Speaking of which, I hear now they have found water on the moon's surface after the whole "bombing mission" last month. I think in a few years, the Giro d'Italia should dream bigger and hold its first stage on the moon instead of sticking to earth. Hey, there's water for aid stations!
P.S : Don't forget to catch the Leonid Meteor shower on Nov 17! I'm going to be headed out in the wee hours of Tuesday morning on my bicycle to see if I can spot any.
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