Evans SPH 4U1

Physics Grade 12

Unit 5: Waves

Note 9: Polarization of Light


Reference: Chapter 10.1


Young's double slit experiment its demonstration that light does produce an interference pattern led to the support of the wave theory of light.  However, he had not proven whether light was a transverse wave or a longitudinal wave.



If transverse waves move up and down and then horizontally back and forth, they are said to be unpolarized.

If this transverse wave was to pass through a filter that only allowed the vibrations to occur in one plane (i.e. horizontally or vertically) then the wave would be plane polarized.  Example: If we pass an unpolarized transverse wave through a vertical slit, the transmitted waves now only vibrate up and down.  If we then pass these vertically polarized waves through a second horizontal slit, then the wave will basically be stopped. This also occurs for light.


A longitudinal wave cannot be polarized (see figure 5 in text). Its rarefractions and compressions would not be affected even if the wave travelled through a vertical slit or a horizontal slit. THEREFORE LIGHT BEHAVES LIKE A TRAVERSE WAVE NOT A LONGITUDINAL WAVE.


Whenever the axes of the polarizing filters are at right angles to each other, light energy is almost completely absorbed (i.e. analogous to first passing through a horizontal slit and then passing through a vertical slit)

When the axes of the two filters are parallel, the light polarized by the first filter passes through the second without any further absorption.