Requires more than 15 minutes or more and can be awkward if your class immediately follows another.
Telescope Resolution

What it shows

A telescope (with video output) at the front of the lecture hall is focused on two point light sources at the rear of the hall. Although the light sources are only 1/2 mm apart, they are readily resolved. The Rayleigh limit of resolution can be clearly shown by reducing the telescope aperture to the point where the two light sources can barely be resolved, similar to the following images (from: Cagnet/Francon/Thrierr, Atlas of Optical Phenomena). At the Rayleigh limit the centers of both point sources coincide with the the first minimum of the other source....

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Vortex Shedding in Air Friday, April 8, 2016

A thin wire, moving through the air, is made to vibrate in the audio range at the vortex shedding frequency.

What it shows:  When air flows around an object, there is a range of flow velocities for which a von Karman vortex street is formed. The shedding of these vortices imparts a periodic force on the object. The force is quite small and not enough to accelerate the object to any significant amount, especially if the object is relatively massive. If the situation is such that the object can vibrate about a fixed position, we have the possibility of simple...

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Thermocouple Brownie

A brownie pan with two food safe thermocouples, one in the brownie batter and one in the air next to the pan, is put in a pre-heated oven, and the temperature profiles recorded and displayed.

Clean copper wire is used to make an armature for the thermcouple wires. Crimp the center of a 20 cm piece of 14 ga wire on the side of the baking pan. Bend loops at the ends of the copper wire to hold the thermocouple wire.

The oven temperature thermocouple should be about five centimeters away from the pan, at the same level as the center of the pan. The brownie thermocouple is...

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Samples of Elements

First day of Gen Chem: Metals and non-metals; solids, liquid and gas elements; compound of elements.

Copper, sulfur, lead, iron, antimony, iodine, carbon as powder and graphite sample, mercury, copper iodate, oxygen balloon.

Seismic Wave Refraction

Total internal refraction of laser beam through sugar solution with refractive index gradient; representative of seismic waves through Earth's mantle.

What it shows:

As a simulation of atmospheric refraction, this demonstration shows the gradual and continuous bending of light due to a gradient in the optical density of the medium. In this case the variable refracting medium is a tank of sugar water with a vertical gradient in the concentration of sugar and a HeNe laser provides the light beam. It can be used as a model of mirage formation (except that the...

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Mercator Projection

What It Shows

As shown in the drawing, the Mercator projection is a cylindrical map projection of the spherical globe. The meridians and parallels of latitude on the globe end up appearing as lines crossing at right angles in the projection. Areas on the globe far from the equator appear to be much larger on the projection. It's not trivial to visualize this projection and the intention of this demonstration is to shed some light on it (literally).


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The Surface Treatment of Glass

What it shows:

The strength of a material in tension or compression will be affected by discontinuities in its surface structure. This can be demonstrated for glass using microscope slides, and the comparison of failure stress before and after the removal of surface scratches.

How it works:

The slide rests between two custom built test beds (figure 1), the upper bed supporting the load. We use slotted 1kg and 0.5kg masses placed carefully in their holder, and allowing a short time between additions. We find the breaking...

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Supersaturation and Crystallization

What it shows:

A supersaturated solution is unstable, and by seeding it you can trigger rapid crystallization.

How it works:

Sodium acetate can dissolve in water in great quantities at high temperature, and if you let the solution cool carefully to around room temperature, you have a clear supersaturated solution. Disturbing this unstable equilibrium by dropping a small crystal of sodium acetate into the solution makes the whole thing solidify; the sodium acetate crystals growing radially outwards from the impact point of the...

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Dilatancy of Deformation

What it shows:

When sand in a balloon, just as atoms in a lattice, are close packed, they occupy the least possible volume. Any deformation, even compression, deforms this close-packed arrangement causing an increase in volume.

How it works:

The balloon is filled with sand, and black ink added allowed to percolate down and fill the air gaps. A capillary tube sticking out of the balloon indicates the ink level. When the balloon is squeezed the sand, which had settled down to a closely packed arrangement, is dislocated. Larger gaps...

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Crystal Growth & Recession

What it shows:

By providing a cold boundary, you can get water to crystalize as advancing needles of ice.

How it works:

This cold boundary can be provided by a petri dish of alcohol. Adding dry ice to this produces an endothermic reaction that lowers the temperature below 0°C. By placing a smaller petri dish containing distilled water within the alcohol dish (figure 1), the water freezes from the outer edge inwards. In front of a "thick" wall of ice shoots a monolayer of needles. The advance or recession can be...

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Supercooling of Water

Pure water cooled to below 273K without freezing; seeded to spontaneously crystallize.

What it shows:

A liquid can be taken to a temperature below its freezing point if it is cooled slowly and there are no nucleation sites for crystallization to begin. In this demonstration you can create a flask of liquid water at below 0°C that, when 'seeded' by the introduction of a nucleation site (in this case dry ice) will be instantaneously frozen.

How it works:

This is pretty much described in Setting it Up.


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Bouncing Photon

A photon (modeled by a bouncing ping-pong ball) is observed from two reference frames and provides the motivation for time dilation.