This is a corner reflector used by surveyors to measure distances precisely by bouncing lasers off them. [
Link to Wikipedia article]. Read up on how they work; their operation is relevant to total internal reflection and prisms in O&A-level physics.
This particular reflector was near bar named "The Colonial" above Little India Station on the underground NEL line. They can also be found on almost all the pillars that support the elevated sections of old East-west and North-south MRT lines.
The coolest thing about these reflectors is that the Apollo astronauts left a panel of them on the moon. [
Link to Wikipedia Article] I really think it is ridiculous that anyone should doubt that men have been to the moon, especially when observatories around the world bounce lasers off those reflectors to measure the distance to the moon. Don't try using your laser pointers; the power needed to reach the moon is in the neighbourhood of a hundred watts, and even then, only a few photons get reflected by the corner reflectors and make it back to earth!
