Invisible bottle

Check out this trick involving a cup, a bottle, and glycerin.

This illusion is possible because glycerin refracts light in the same angle that plastic and glass does, giving the impression that there isn't any bottle at all inside the liquid. Outstanding, isn't it?

The concept behind the refraction is that when light travels form one medium to the other, it's direction changes. Every medium has a refractive index, and the refraction's angle is related to these index as in:

n is given by:

c is the speed of light in vacuum

v is the speed of light in the medium.

Pendulum

One of the earliest known uses of a pendulum was in the 1st. century seismometer device of Han Dynasty Chinese scientist Zhang Heng. Its function was to sway and activate one of a series of levers after being disturbed by the tremor of an earthquake far away. Released by a lever, a small ball would fall out of the urn-shaped device into one of eight metal toad's mouths below, at the eight points of the compass, signifying the direction the earthquake was located.
Nowadays the pendulum can be used to show the Earth spins, to measure the acceleration due to gravity which is important in determining the shape of the earth and the distribution of materials within it.





The oscillation period is given by:

l: string lenght; g: gravity

Experience

Each pendulum has different size of string that modified the oscillation's period. These diferences produce amazing effects sometimes looking like a snake, a DNA molecule.

Fire Gas Sound Waves

In this Veritasium's experiment there are a tube, gas, and fire with sound passing through and forming nodes, which variates the pressure and makes the fire's height oscillate. Check it out!

Water trick using sound waves and recording frequency

This is a very nice trick, where the water appears to be frozen in the air, sometimes moving up and sometimes moving down. This is due to the recording frequency, which is 24 frames per second, getting the same picture of  the waterfall constantly, giving the illusion of a frozen wave. At 25 hz, the water appears to fall at one frame per second, while it goes up at 23 hz. Amazing!

Resonance!

Amazing experience using a metal plate and a vibration mechanism. This work is made by the stationary waves, the salt stays at where there are destructive interference making the awesome images!