Month: May 2015

104 High Pressure Sodium Lamp

There are many interesting features in the spectrum of light emitted by a high pressure sodium (HPS) lamp.

Most of the blue lines are emission lines from mercury.

The dominant light of the HPS lamp comes from the yellowish sodium D-line. In low pressure sodium lamp, the D-line would have been a narrow discrete line (a doublet actually). But in HPS lamps, this line is broadened so much by high pressure that it appears like a continuous spectrum. This broadening is caused by the emitting atoms suffering collisions during the emitting process. Because the collision time is much shorter than the lifetime of the emission process, the uncertainty in the energy emitted is increased (Heisenberg Uncertainty Principle, ΔE Δt > h/4π).

Notice also the dark absorption line in the pressure broadened sodium D-line. This is due to absorption by the cooler sodium at the outer layers of the lamp. Because the probability of absorption is much higher near the line centre than at the wings, the sodium D-line undergoes a self-reversal near the line centre.


103 Electron Diffraction

The electron diffraction experiment demonstrates the wave nature of electrons. An electron with momentum p has a de Broglie’s wavelength of λ=h/p. When a beam of electrons is passed through a crystalline structure (such as graphite), an interference pattern of bright and dark rings is formed. If the electrons are accelerated to higher momentum (by turning up the accelerating voltage), the electrons’ wavelength will decrease. The separation between the bright and dark rings will thus decreases. (similar to fringe separation in the double slit experiment being dependent on wavelength Δy=Lλ/d)

102: Solar Spectrum

The sun’s core emits a continuous spectrum of light. However, the gas atoms in the sun’s “atmosphere” are capable of absorbing some of this light. Thanks to the discrete energy levels in gas atoms, these gas atoms can only absorb photons of certain energies (and since E=hc/λ, photons of certain wavelengths) that match the energy gaps in the energy levels of the gas atoms (|E2-E1|= hc/λ), resulting in dark lines in the solar spectrum at those wavelengths.


Out of curiosity, I tried to match the absorption lines in my video to those published on the internet (see With a little confidence, I think four of the more prominent dark lines (labelled C, D, E and F) in the spectrum are due to absorption by Sodium, Iron, Hydrogen and Iron atoms in the Sun’s atmosphere.


It came as a surprise to me that the absorption lines of the solar spectrum can be viewed directly with the naked eyes using a grating. Although the video camera tends to over exposure the image (thus resulting in the absorption lines being washed out), the dark lines are clearly visible momentarily when the camera was in the midst of “correcting” the exposure.

101 Van de Graaff and Cupcake Cups


For the sake of discussion, let’s assume that the globe is positively charged.

The electrons in the cupcake cups feel the electrical attraction by the globe. Since the cupcake cups are in electrical contact with the globe, these electrons move into the globe, leaving the cupcake cups positively charged as well.

Like charges repel. Thus fly the cups.

100 Van der Graaff and Foil


For the sake of discussion, let’s assume that the globe is positively charged.

When the foil is near the globe, the globe’s electric field causes the electrons in the foil to shift, inducing negative and positive charges on opposite ends of the foil.

Thanks to the inverse-square nature of Coulomb’s forces, the electrical attraction acting on the negatively charged near end of the foil is stronger than the electrical repulsion acting on the positively charged far end of the foil. The foil thus accelerates towards the globe.

When the foil touches the globe, electrons are attracted into the globe, leaving the foil positively charged. The foil is thus quickly repelled away from the globe.

As the foil slowly loses its positive charges to the surrounding air molecules, the cycle repeats again.

099 Simple Motor



First note that an electric current is flowing from the external wire, through the magnet, into the screw.

This current is sitting in the magnetic field of the magnet. (In fact, it is the field inside the magnet)

There is thus a F=BIL force acting on that part of the magnet which carries the current. This results in a net torque about the centre of mass of the magnet, which causes the magnet to rotate.

Alternatively, you can think of it as charges in the magnet moving in the magnet’s magnetic field. You will then be thinking of F=Bqv forces acting on the moving charges, which will result in the same spinning motion.