Physics : Corner cube reflectors

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!

Physics : Diffraction through a privacy filter

I was wandering through the scraps and parts shop one day in the merry month of May and saw a stack of what appear to be privacy filters torn off old ATM machines. Knowing that they contain a mesh of fine fabric, I gambled that they would serve as a grating. I bought them for a song; they were 80 cents a piece because the owner didn't know what they were.

A trip home and a flash of the laser pointer later, the filters showed a promising diffraction pattern right off the bat. I set up the expriment in the manner shown below:

The first task at hand is to estimate the pitch of the grating.

At a distance of 50cm between grating and screen, I measured the distance between the zeroth and 7th maxima to be 2.05 cm. Using the formula nλ= dsinθ, with tanθ=2.05/50, and nλ=7X650nm, d works out to be 0.111 millimeters, which seems reasonable considering the grating is made of fabric fibers of some sort or other.

I also obtained an enveloped diffraction pattern with a red and green laser. Granted the lasers have different spot sizes and frequencies, they both show how the diffraction pattern on the screen is subjected to an envelope when multiple aperatures are involved. Estimating a beam spot size of approximately 1mm, we would expect the laser to pass through about 9 aperatures.

The diffraction pattern obtained with a red 650nm laser.

The diffraction pattern obtained with a green 532nm laser.

Physics : Diffraction by CD

Comparison of the wavelengths of green and red laser beams by diffraction from a Compact Disc.

The CD used is a regular stamped music CD. The specifications for the grating-like surface of the metal layer of the CD is 500nm track width and 1600nm track pitch.

Red Laser (650nm)

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Green Laser (532nm)

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