Quantum Mechanics

Microwave Tunneling analog

3 cm microwaves and prisms made of plastic beads demonstrate total internal reflection in one prism, and coupling of the evanescent wave to a second prism. An audio signal corresponds to the one kiloHertz modulation of the microwaves.

The prisms are made of foam core board, cut and hot glued, then filled with small pony beads.


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Tunneling Analog

What it shows:

In quantum mechanics, it is possible for a particle to tunnel through a potential barrier because its wave function has a small but finite value in the classically forbidden region. Here we use FTIR as an optical analog of this quantum mechanical phenomenon.

How it works:

A 45°-90° prism will deflect a beam of light by total internal reflection. When two such prisms are sandwiched back-to-back and pressed together, the air-glass interface can be made to vanish and the beam then propagates onward undisturbed. This...

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Optical Analog of Uncertainty Principle

What it shows:

In the Heisenberg uncertainty relation, the momentum of a particle cannot be known with any greater accuracy than h/∆x where h is Planck's constant and ∆x is the uncertainty in spatial position. The more you localize its spatial position, the less certain you become about its momentum. An optical illustration for this is the diffraction of light though a slit.

How it works:

For a laser beam, the transverse momentum is pretty well known (i.e. it's zero) but you have no localization of its spatial x coordinate. You...

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Uncertainty Principle

What it shows:

A pulse-modulated electromagnetic signal is simultaneously displayed in the time domain (on an oscilloscope) and in the frequency domain (on a spectrum analyzer). Using ∆n for the frequency spread (uncertainty in frequency) and ∆t for the duration of the pulse (uncertainty in the time domain), the frequency-time uncertainty relation is given by 1

∆n ∆t ≥ 1/

By progressively shortening the length of time that the carrier signal is on, the inverse relation between pulse length and spectral-energy...

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Electron Diffraction

What it shows:

Louis de Broglie predicted that matter under certain circumstances would exhibit wave-like properties. A proof of this is the repeat of X-ray diffraction experiments using electrons, whose de Broglie wavelengths at high accelerating potentials are similar to X-ray wavelengths. Here we accelerate electrons into crystal targets and get diffraction patterns identical to those from X-ray diffraction.


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Planck's Constant Determination

What it shows:

The photoemission of electrons from a metal surface depends on the energy of the incident radiation and not on its intensity. Knowing the energy of the emitted photoelectrons and the frequency of the incident light, you can calculate a value for Planck's constant h.

How it works:

Using a mercury source, we have at our disposal three very bright visible lines, in the blue, green and yellow (doublet), and a rich selection of ultra-violet. Our main source is a Phillips Lifeguard 1000W street lamp with its outer (uv...

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Photoelectric Effect

What it shows:

A direct observation that the photoelectric effect is color (i.e. frequency) dependent and not intensity dependent. We discharge an electroscope using UV radiation after all attempts to discharge it with light of a longer wavelength has failed.

How it works:

An ebonite rod and fur is used to place a negative charge onto a Braun electroscope (figure 1) fitted with a thick zinc plate. Deviation of the electroscope arm from the vertical indicates a net negative charge. Next we hit it with light from a 1000W...

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Tether-ball Catastrophe

What it shows:

An accelerated electric charge radiates energy. So according to classical physics, an electron in orbit about an atomic nucleus should emit electromagnetic radiation by virtue of its orbital motion. As it radiates energy, the radius of its orbit decreases. The electron should spiral into the nucleus amidst a burst of radiation in about 10-16 seconds.


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Wien's Displacement Law

Changes in the spectral intensity distribution of a hot tungsten filament are observed as the temperature is varied.


How It Works

A slide projector (Beseler Slide King II) with a 1kW lamp and adjustable lens is plugged into a Variac. The light from the projector passes through an inline...

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Single Photon Interference

Wave/particle duality observed in Young's double slit experiment with camera sensitive to individual photons.

What it shows:
In this demonstration we perform the double-slit interference experiment with extremely dim light and show that even when the light intensity is reduced down to several photons/sec, the audience can see the familiar Young's double-slit interference pattern build up over a period of time. This addresses the question of how can single photons interfere with photons that have already gone through the apparatus in the past, or with those that...

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