[★★★★]

Wow!

Microscope Resolution Tuesday, December 6, 2016

What it shows:  The wave nature of light limits our ability to see the very small. Application of the Rayleigh limit of resolution tells us that the size of the smallest objects one can resolve under a microscope is approximately equal to the wavelength of light. The optical limits of a microscope are demonstrated as one attempts to resolve 1 μm diameter spheres (about twice the wavelength of light) — one sees spots of light surrounded by diffraction rings rather than sharply defined spheres, similar to the 3rd image (from: Cagnet/Francon/Thrierr, Atlas of Optical...

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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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Cannon Ball Boat Puzzler

What it shows

Does the level of the ocean rise or fall when a cannon ball is tossed overboard? A question of displacement.

How it works

A difficult effect to see at sea, but it becomes clear by taking some parameters to extremes. Reducing the ocean to 12L in volume, and the boat to practically no mass by using a plastic bowl, a cannon ball of 1kg mass suddenly becomes substantial. With the cannon ball in the boat, its weight is distributed throughout the boat; the lowered density increases the amount of water displaced (by the fraction of the boat submerged), raising...

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NMR model

What it shows:  Using the classical description of the motion of a spin in an external magnetic field, the demonstration helps visualize NMR in the time domain. The nuclear magnet and its classical vector model are represented by a spinning ball with magnets attached. A rotating mass is characterized by its angular momentum L, which is the analog of the magnetic moment mu, which characterizes a rotating charge distribution. The spinning ball mimics protons in that it has both angular momentum as well as an "intrinsic" magnetic moment. The torque...

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Air Table Center-of-Mass Motion Monday, May 2, 2016

What it shows:  Two bodies, rotating about each other, rotate about their common center-of-mass (COM). The COM exhibits uniform motion (or none at all) regardless of what the two bodies are doing.

How it works:  The "bodies" are 4-1/2" diameter acrylic disks that float on a cushion of air on a large air table.1 Presently we have three versions ready to go. (1) The first version has two disks connected by means of a 12"- long plastic ruler. A large "dot" at the center of the ruler marks the COM. The disks can be made to simply...

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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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Pulse Reflections in a Coax Cable Thursday, February 25, 2016

What it shows:  A voltage pulse, injected into a long coaxial cable, will travel down the length of the cable and undergo a reflection at the other end. The nature of that reflection depends on how the cable is terminated at the other end. Shorting the cable at the far end produces an inverted reflection. With no termination (an "open" end), the reflected pulse is not inverted. When the impedance of the termination matches that of the cable, there is no reflection.

Knowing the length of the cable and noting the amount of time it takes the pulse to come...

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Inverse Square Law

[XL | t++ | ***]  inverse square law, luminosity

What it shows:  The intensity of light from a point source decreases as 1/r2, where r is the distance from the source.

How it works:  For the point light source, we use a 1500 watt clear light bulb. The detector is a small solar panel.1 The output current is directly proportional to the intensity of the light falling on the panel and the current is displayed on an analog milliameter. (The current can also be measured by a digital meter or computer.) Measured...

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Reverse Sprinkler Friday, December 18, 2015:

What it Shows

Inspired by Richard Feynman's story in his 1985 book (pp 63-65), Surely You're Joking Mr. Feynman, the demonstration answers the question "which direction does a lawn sprinkler spin if water enters the nozzle rather than being expelled from the nozzle?" The reverse sprinkler spins in the opposite direction of a "normal" sprinkler. "Dissipative effects" has been the hand-waving reason for the past 30 years, but the real reason why it spins in the reverse direction is far from obvious (see Comments, below). It turns out that a sprinkler designed to be "truly...

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Reaction of Sodium and Water

Sodium undergoes a reaction with water.

A liter of warm water in large pyrex vessel, covered with fine mesh stainless steel screen, is on a stool close by in-floor vent hood.  Add a few drops from the phenolphthalein indicator bottle.

Using the long forceps, pick out a pea size lump of sodium metal from the mineral oil in the small beaker. Wipe off the lump on the dry paper towels. With the vent fan running, lift the edge of the screen and drop in the sodium metal. Replace the screen and get back.

The sodium will from a hissing ball of molten metal, which bounces...

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Reaction of Hydrogen and Oxygen

Blue balloon with hydrogen, green balloon with helium, red balloon in back with hydrogen and oxygen mixture, and, on a cart, a red water balloon on large watch glass. Candle on a stick with matches, and a needle in the end to prick the water balloon.

Safety glasses and hearing protection is required for the demonstrator and anyone else who can't cover their ears for the red balloon.

Reactions of Li, Na, and K with Water

Lithium, sodium, and potassium undergo reactions with water.

Two liters of warm water in large pyrex vessel, covered with fine mesh stainless steel screen, is on a stool close by in-floor vent hood.  Add a few drops from the phenolphthalein indicator bottle, and a few drops of 1M hydrochloric acid if the warm tap water turns pinkish.

Video camera is clamped to the stool leg, and pointed at the bottom of the beaker. Before class, frame the shot and focus on the center of the beaker.

Using the long forceps, pick out the coil of lithium wire from the mineral oil in...

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Hydrogen Peroxide Decomposition by Iodide

Hydrogen peroxide 30% in a large round flask decomposes to boiling water and oxygen when postassium iodide is added.

The 12L Round Bottom Flask is set on white C-Fold towels covering a large cork ring on the lab bench.  100-150 ml of 30% hydrogen peroxide is carefully poured in. The liquid should be visible against the white towels from the perspective of the class, and any camera, if used.

The catalyst is 5 g of potassium iodide in a small plastic weighing boat labeled KI.

Safety goggles and gloves. Raise the projection screen and make sure the flask 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.

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