Activity 2.6 - How do sunglasses work?

Learning Aims:
  • Understanding that light can be polarized
  • Understanding that the intensity through two polarizers may be described by Malus’s Law
Materials:
  • Two polarizers, two neutral density filters
Suggestions for use:

The students should be asked to discuss how sunglasses work. Typical responses will involve ‘coloured glass/plastic’ that ‘only allows some of the light through’ which could be used as a starting point for a discussion on absorption, filters, and energy.

Next, the students are invited to examine the properties of ‘neutral density’ filters of the kind found on most sunglasses and asked to consider the following questions:

If each filter only allows 50% of the light through it, what percentage is transmitted through both filters when placed on top of one another?

This question establishes that the total transmission is found by multiplying (50% of 50% is 25% or 0.50 x 0.50 = 0.25). The students are then posed the question as to whether the order or orientation of the filters makes a difference.

The students are then given Polaroid filters and asked to examine their properties:

Does the relative orientation of the filters change the light intensity transmitted? If zero degrees is defined as the relative orientation providing maximum throughput, what relationship in terms of angle gives minimum throughput?

Does the order of the filters make a difference?

The Worksheet includes an optional experiment utilising a data logger to examine Malus’s Law.
Possible questions:
  • How can you use a polarizer to remove reflections from a surface?
  • How do 3D movies work and why do you have to wear glasses?