Coloured Reflections?

John Moore imaged his spectacles. The lenses appear normal but their reflections in the table are strongly coloured.


©John Moore, shown with permission.


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The first key is polarization.

The scene was lit by the light of a white LED monitor screen. These emit polarized light and are an excellent source for polarization experiments.

The light was further polarized when it was reflected from the non-metallic surface of the table. The extent of polarization by reflection depends on the viewing angle.

     Sunglasses are made of polarizing material to suppress reflections. They let through light polarized at right angles to that reflected by horizontal surfaces or water and so block or reduce the polarized light from reflections.

But the dual polarisation will not produce colours unless the spectacle lenses have particular properties.


Some materials - plastic spectacle lenses, calcite and other crystals, ice - are birefringent. They are anisotropic and their optical properties are dependent on direction.

In the simplest type of birefringence light is split into two distinct rays which are polarised and refracted differently.

The two rays also have their colours dispersed differently and this becomes apparent when the incoming light is first polarised and the emerging light is viewed through a second polariser. We then see colour patches.

The colours arise from constructive and destructive interference between the two differently polarised rays. The two rays have slightly different optical path lengths as they traverse the plastic.  On emerging, their wave crests can be in phase and combine to give a bright colour. They could also be out of phase giving less or no light. The phase condition depends on the wavelength (colour) and the viewing angle.

The result is patches of colour. In plastics the colours are irregular because the birefringence was generated when the plastic was extruded and moulded. The extent and direction of the anisotropy causing the birefringence is the result of strains and molecular orientation frozen in when the plastic solidified.

Birefringence is best seen by illuminating the material with plane polarized light and then viewing or photographing it through a second polarizer. The colours can be seen, but more weakly, without these strenuous conditions. The colours seen through aircraft windows are examples.