Schlieren Optics

Demonstration of an optical technique that allows us to see small changes in the index of refraction in air. A point source of light is reflected from a concave mirror and focused onto the edge of a razor blade, which is mounted in front of the camera. Light refracted near the mirror and intercepted by the blade gives the illusion of a shadow.

Seen here are the heated gases from a candle flame and a hair dryer, helium gas, and sulfur hexafluoride gas. 

More information on our setup.

 Note that this version of the setup uses a white LED flashlight instead of an automotive light bulb.

 

Relativity Train

The Relativity Train is a realization of the famous Einstein thought experiments involving traveling trains carrying clocks and meter sticks. The demonstration is used to show how the preservation of the postulated constancy of physical laws and the speed of light in all inertial frames requires length contraction and time dilation in the train frame relative to the lab frame of reference. The demonstration is, of course, not a real experiment but rather a visual means of showing (without using any equations) how length contraction and time dilation are necessary consequences of Einstein's two assumptions.

 

Rotating Saddle

Mechanical analog of a Paul Trap particle confinement—a ball is trapped in a time-varying quadrupole gravitational potential. A large saddle shape (attached to a plywood disk) is mounted on a multi-purpose turntable. The saddle shape is essentially a quadrupole gravitational potential. Rotation of this potential subjects the ball to an alternating repulsive and attractive potential, much like the time-varying electric quadrupole potential of a Paul Trap used in trapping single ions or electrons. The plastic ball used here is about 25 cm in diameter and was purchased at a toy store. The saddle consists of many layers of fiberglass and was hand-made with help from Justin Georgi. The turntable is driven at about 110 rpm with a DC motor. We have observed this ball at this speed remaining stable for over 2 hours. Slow motion footage recorded at 240 fps with a Casio EX-FH25. Thanks to Rob for letting us use the camera.

 

Vortex Shedding (Flow Tank Version)

Wolfgang uses a pendulum partially immersed in a makeshift flow tank to show us the effect of vortex shedding on a small object.


When fluid flows around a cylindrical object, there is a range of flow velocities for which a von Karman vortex street is formed. The shedding of these vortices imparts a small, periodic force on the object. Here the object is a cylinder attached to a physical pendulum whose frequency of oscillation is adjustable. The end of the cylinder is submerged in flowing water. When the frequency of the pendulum is adjusted to match the frequency of vortex shedding, the cylinder swings transverse to the direction of flow with a peak-to-peak amplitude of a few centimeters.

Other Harvard demonstrations

A ping pong ball collides with a water balloon

A water-filled ping pong ball collides with a water balloon at approximately 25 m/s. The inelastic collision dramatically deforms the water balloon. The ping pong ball weighs 0.033 kg and is propelled by a Toro leaf blower; the water balloon is 3.36 kg and is suspended by a single string. The airflow from the leaf blower acts to slow down the rebound of the ball. Shot in 600 fps. Special thanks to Rob for letting us use the camera and setup.

Other Harvard demonstrations

Shoot-n-Drop

An apparatus that at once shoots a billiard ball horizontally and drops another one vertically from an equal height. Even though the two have different initial velocities, they both accelerate in the same direction and at the same rate due to Earth's gravity--this is confirmed by seeing and hearing both balls land simultaneously.

While we did our best to make sure the setup is level and the apparatus is precise, the video shows the balls actually land about 0.02 s apart from each other (the slow-motion part was done in 60 fps, and there seems to be a difference of about 1 frame). We consider this difference to be negligibly small. Normally the apparatus is positioned about 2 meters above the floor, and the difference in landing time is just as imperceptible. You will notice the "drop-ball" bounces towards the center of the picture. We think this is mostly because the concrete floor has small pockmarks and other local irregularities, which on average combine to form a level surface but individually can cause funny bounces. The ball may also have a very small horizontal velocity due to the way in which it is dropped.

Other Harvard demonstrations

Paramagnetism of Oxygen

Oxygen gas is condensed into liquid form and then poured between the poles of a strong magnet so we can observe its paramagnetic properties. We send O2 gas through a copper coil, which is then immersed in about 2 liters of (very cold--77 degrees Kelvin) liquid nitrogen. As the O2 travels through the coil it loses enough heat to change from a gas to a liquid, and that liquid is collected in a small pre-cooled dewar. Liquid nitrogen is poured between the poles of the permanent magnet, but since its diamagnetic properties lead to only a very weak interaction with the field, it just sloshes through as if it were water. The liquid oxygen, on the other hand, sticks between the poles of the magnet until it boils away. Because the oxygen molecule has an electronic structure that favors the non-cancellation of two of the electron spins, its net magnetic moment is free to point in the direction of an external magnetic field (just as a compass needle does). When enough of these moments are aligned, the material as a whole behaves like a single magnet. At room temperature only a small fraction of the moments are able to line up perfectly with the external field, but when oxygen is cooled and condensed into a liquid the effect is more noticeable. Nitrogen atoms are paramagnetic, but molecules are not.

Other Harvard demonstrations

Coffee Mug on String

Angular momentum helps save a red ceramic coffee mug from certain destruction. We used some string, two stolen mugs, and two Ticonderoga Dixon No. 2 pencils. Please do attempt.

Conductivity of Glass

Insulating glass becomes a conductor of electricity when heated with a blowtorch.

Two ceramic lightbulb sockets are wired in series to a household AC power cord. When two incandescent bulbs of the same Wattage rating are screwed into the sockets and the cord is plugged in, they both pass the same amount of current and so they both light with the same intensity. When one bulb is unscrewed, the circuit is broken and the other bulb goes out. If we can replace the missing bulb with a conductive material, the circuit will once more be complete and the remaining bulb will light again.

Conductivity of Solutions

We look at the conductivity of several solutions. Substances include tap water, distilled water, sodium chloride, hydrochloric acid, sodium hydroxide, sugar, vinegar, ethanol, and barium sulfate. The solutions are mixed to approximately the same ratios. The tester is a pair of stripped copper wires at line voltage in series with a 25W incandescent bulb. The probe is rinsed in distilled water between each test.

Harvard Natural Sciences Lecture Demonstrations

Falling Faster Than g

A hinged board rotates under the force of gravity and the free end accelerates at a rate faster than g. This board is 1 meter long and starts at an initial angle of just under 35 degrees. A steel ball bearing sits on a golf tee about 2 cm above the end of the board.

Other Harvard demonstrations

Synchronization of Metronomes

Five metronomes are set to 176 bpm and placed on a Foam Core board. When empty cans are placed underneath, the board is free to move from side to side and the metronomes are able to influence each other into synchronization. When the cans are removed the metronomes are no longer physically coupled and some of them begin to fall out of step.

Other Harvard demonstrations

Pendulum Waves

Fifteen uncoupled simple pendulums of monotonically increasing lengths dance together to produce visual traveling waves, standing waves, beating, and (seemingly) random motion.

Source: Harvard Natural Sciences Lecture Demonstrations

Other Harvard demonstrations