Polarization by reflection

Experiment number : 4300

  • Goal of experiment

    The goal of the experiment is to demonstrate that reflection pro duces partially or fully polarized light from unpolarized light.

  • Theory

    When a light wave is incident on the interface of two media, the original wave is decomposed into a refracted wave and a reflected wave. Even if the incident wave is unpolarized, the reflected wave is partially linearly polarized, with its plane parallel to the plane of the interface.

    The extent of polarization of the reflected wave depends on the angle of incidence. A reflected wave can even be fully polarized if the light is incident at the so-called Brewster angle Brewster’s angle.

  • Tools

    Linearly polarized light filter, an aquarium with water, a burning candle.

  • Procedure

    1. Place the burning candle above the aquarium of water so that the reflection of its flame from the water surface can be observed.

    2. Look at the reflection of the flame through the linearly polarized light filter and slowly rotate it.

  • Sample result

    The experiment showed that not only the reflection on the water surface, but also the reflection on the painted surface of the metal lift (originally intended only for candle placement) leads to partially polarized light – as the polarizing filter is rotated, the reflection alternately weakens and strengthens, as shown in video 1. The transition from the maximum intensity of the reflected light to the minimum intensity corresponds to a 90° rotation of the filter.

    Video 1: Polarization by reflection

    Video 2 shows the same effect, but this time not only for the painted surface of the lift, but also for the reflection of the candle flame, which changes its intensity according to the rotation of the polarizing filter.

    Video 2: Polarization by reflection (water + candle flame)

  • Methodical notes

    • If you have several polarizing filters, after this experiment, you can hand them out to the students in groups and have them find other surfaces in the room that significantly polarize the light when reflected.

    • Photographers use polarizing filters for their work when they want to get rid of unwanted glares and achieve rich colours in their photos. If you have a large number of polarizing filters, you can hand them out and have students use their own cell phones to take the same picture once with and once without the filter (...and then compare them).

Type of experiment: Qualitative
Difficulty level: From Upper secondary level
Necessary tools: Tools and equipment usually found at schools
Preparation time: Under 3 minutes
Duration of experiment: Under 3 minutes
Experiment is video recorded
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