Category: 09 Waves

9.1 What are Waves?

Water wave is an example of a 2D wave that propagates along a surface.

Notice the bob does not travel with the wave. The motion of the wave particles are different from the “motion” of the wave.

Each medium particle oscillate about its own equilibrium position. Their oscillatory motion result in the propagation of the energy/momentum.

You have been taught that sound is a wave since primary school. But how do we know that sound is a wave and not a stream of particles?

915 Polarisation by Reflection

When light passes from air to glass, some of it is refracted into the glass, and some of it is reflected. The reflected light is partially polarised because light polarised horizontally is reflected more strongly than light polarised vertically. In fact, at one particular angle of incidence called the Brewster’s angle, light polarised horizontally is 100% refracted, resulting in a 100% vertically polarised reflected light.

This phenomenon occurs for any transparent material, including glass, paint and water.

Further reading:

914 Two Polarised Giants

The picture of Newton was in green only, while the picture of Einstein was in red color only. Green and red light from the LCD projector are polarised perpendicularly with each other. As polarizer was rotated, it cut off either the green or the red so that only either Newton or Einstein comes into view.

Some 3D movies are made this way. The image meant for the left and right eyes are projected using two mutually perpendicular polarised light. The viewers are then given glasses which are fited with polarizers.

913 LCD Projector

Clearly, the light from the LCD projector is polarised.

Not only that, we can tell that of the RGB components, Red and Blue are polarized perpendicuarly to Green.

912 Polarized Light Around Us

If light passing through ONE single polarizer shows variation in brightness as the polarizer is rotated, it must mean that the light is polarized.

As shown in the video, filament lamps, fluorescent lamps and the Sun emit light that is unpolarised.

The display screens of calculators, handphones and LED TV are all based on LCD (Liquid Crystal Display) technology, which produces polarized light.