DEMONSTRATION TOPICS

A hologram contains information about the 3-dimnensional shape of an object in the pattern of fine lines contained on the holgram film. The hologram can be projected using a laser pointer and a ground glass plate to diffuse the laser light. This hologram is set on a rotating table so that it can be seen from all directions as the table turns.

This is an image of the hologram produced by the simple setup to the right. The information about the shape of the toy car is contained in the pattern of lines on the film which is attached to the surface of the plastic cylinder.

A prism or a diffraction grating can be used to separate the colours in light emitted from electrically excited atoms. The frequencies of light is proportional to the energy of the photons emitted by the atoms. The spectrum of light emitted by specific atoms (like hydrogen and neon in this case) can be related to the separation between energy levels in the atoms.

In a laser, electrons in some of the atoms can be excited to energy levels where they get stuck for a short while. Photons of the same frequency as the light that is emitted when the electrons drop from such energy levels can stimulate the emission of such photons. This picture shows the tube from a helium-neon laser, some laser pointers, some diffraction gratings, a lucite rod that can be used to demonstrate total internal reflection, the principle behind fibre optics. The pattern of bight and dark regions (interference) resulting when light passes through an array of slits or a grating demonstrates the wave nature of light and can be used to measure wavelength.

Light waves involve electric and magnetic fields oscillating perpendicular to the direction in which the light wave is travelling. Materials with long conducting molecules aligned along a specific direction (polarizers) can be used to filter out all of the light waves except for those with electric fields perpendicular to the long molecules. Polarizing material can be used to observe the polarization of light due to reflection and the separation of light into beams of different polarization by birefringent crystals like calcite.

Light is also polarized by scattering from small particles (in this case, coffee creamer suspended in water). This is the reason that light scattered from molecules in the sky is polarized.

The light from a toy pig at the focal point of one (the top) parabolic mirror converges to form a real image of the pig at the focal point of the bottom parabolic mirror.

In a light emitting diode, one photon is produced for each electron that crosses the "bandgap" of the semiconductor. Almost all of the energy lost by the electron goes into the photon. The energy lost by each electron is just the voltage acros the leads of the light emitting diode. It is thus possible to measure the energy of photons with different frequencies (colours) by measuring the voltage across the diodes that produce them. The voltage drop across a red diode is lower than that across a yellow or blue diode.