Modern Physics

Operation of semiconductor diodes

The demonstration is aimed to understand and analyze silicon diode rectification. The apparatus contains full wave and half wave rectification circuitries. A selection switch is also provided to select between the two circuitries.
An alternating voltage is supplied on the input terminals and the rectified output is observed on the output terminals. The LabView VI provided with the demonstration requires a DAQ card with one analog voltage output, one differential voltage input and one absolute voltage input channels. A sinusoid of amplitude up to 10V and frequency up to 400Hz is supplied through the analog output channel Ao0. The output of the rectifier circuit is received through differential channels Ai0 and Ai4. The output signal is also read back as a reference through the Ai1 input channel. The wiring has to be made according to these requirements for the VI to work.

Magic eye

A magic eye is a simplistic triode tube that was used as a tuning indicator in radio receivers around the time of world-war II. It has a bowl shaped anode coated with a phosphorescent material that produces a green glow when electrons strike on it. This makes possible to measure the curvature of electrons’ trajectory when the tube is placed in a magnetic field. By measuring the radius of curvature, the applied voltage between cathode and anode and the magnetic field, charge-to-mass ratio of the electron can be measured.

By Azeem Iqbal | Modern Physics

Electron diffraction

This interesting demonstration utilizes a highly evacuated electron diffraction tube to show the wave behavior of electrons. The electrons are emitted by the thermionic emission and accelerated towards target by applying a very high potential (2000-5000 V). The target is a micro meshed nickel grid on which a thin layer of graphite is deposited. The electrons being diffracted through the graphite satisfy the Bragg’s condition and produce an interference pattern consisting of two rings.

By Azeem Iqbal | Modern Physics . Solid State Physics

Michelson Interfrometer

This demonstration uses the famous Michelson interferometer which is used either for precise distance measurements or the wavelength of the laser. An assembly of optical components including HeNe laser, mirrors and converging lenses is used. The interference pattern is produced by splitting the beam into two paths using a 50:50 beam splitter. The movable mirror is motor controlled and computer interfaced, a source of changing the path length and produce a interference fringes.

By Azeem Iqbal | Modern Physics . Optics

Statistics of natural radioactivity

The classroom demonstrations uses a gamma ray source of Co-60 placed inside a lead container. The radiation is detected with a Geiger muller tube, whose data is brought into the computer. A histogram is built up showing a Poisson distribution. The distribution of decay times is readily observable and the statistical nature of the phenomenon could be explained.

By Azeem Iqbal | Modern Physics

Optical tunneling

Optical tunneling is demonstrated by frustrated total internal reflection.

  • A green laser diode (from a laser pointer) falls on a glass prism. The angle of incidence is greater than a critical angle, resulting in light being totally internal reflection.
  • There is evanescent light leaking from the prism. But this light is difficult to tap off, unless a second prism is placed really close to the first one.
    the second prism is almost conjoined with the first prism using an index matching fluid (Cargille’s BK 7 Matching Liquid).
  • The fluid is able to mate the prisms really well, yet it keeps a small gap between the prisms. Light can tunnel across this classically forbidden region and transmit into the second prism. Total internal reflection has been frustrated.
By Azeem Iqbal | Modern Physics . Quantum Physics

Hall effect in Semiconductor materials

When a magnetic field is applied perpendicular to the direction of flow of charge carriers, in a semiconductor material, charge carriers experience a force in transverse to the direction of applied magnetic field and carriers flow. This effect is known as Hall effect. Being very simple and straight forward phenomena in physics, Hall effect is a fundamental principle in magnetic field sensing and have many practical applications in our daily life. This demonstration shows Hall effect in semiconductor materials and shows how n-type and p-type semiconductors can be identified.

Here is a software required to acquire data using Keithley instruments

By Azeem Iqbal | Modern Physics . Solid State Physics

Fluorescence from quantum dots

Cadmium selenide (CdSe) quantum dots are chemically synthesized in the diameter ranges of 2 to 6 nm. These colloidal solutions are excited with a blue laser of wavelength 405 nm and the fluorescence emission spectrum is observed using a fiber optic spectrometer. The emission wavelength depends on the sizes of the dots, the wavelength is directly proportional to the size (radius) of the nanoparticle. The demonstration is a beautiful, lucid example of the effect of size on the quantization of energy levels and brings home the idea of the potential well quite neatly.

Here is a Presentation on fluorescence from CdSe quantum dots shown in class on 18 April 2013.


By Azeem Iqbal | Modern Physics . Spectroscopy
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