With the kind permission of the Dean & Chapter of York Minster

Did You Know…

…that Light Has a Dual Nature?

Without light, there would presumably be no life on earth. Light is the reason why fields appear green and the sky blue. It is why stars shine in the night sky, and it illuminates processes which would not be visible to the naked eye.

The multifaceted quality of light is presumably what has fascinated people through the ages. And yet right up until modern times, it was largely unknown what light actually is. In the 17th century, Isaac Newton endeavored to explain light propagation as the movement of small particles. Almost 200 years later, the Briton George Bidell Airy found out why stars look blurry when observed through a telescope, confirming the then predominant view that light behaves like a wave. Only a portion of the infinitely long light wave emanating from a star is captured by the diameter – or aperture – of the optical system, which is why the objects in the image plane appear blurred. Since that time, the wave theory of light has been used to explain most phenomena observed. However, not all phenomena can be explained in this way. Thus, in 1905 Albert Einstein once again postulated that light consists of indivisible particles, referred to as photons, which can only be generated and absorbed in their entirety.

Such an apparent contradiction requires two complementary theories to be resolved: light therefore has to be made up of both waves and particles at the same time. Quantum mechanics has a simple explanation for this: light is born as a particle, lives as a wave, and finally dies as a particle again. This means that every source of light emits photons. This corresponds to the “birth” of light. Light then “lives” as a wave and is diffracted at the aperture of the lens. In the detector or eye, light is finally converted into electric signals. Only individual photons are detected, meaning that light “dies” as a particle again.

All optical systems are subject to these laws of physics – although it is not normally very noticeable, since a great many photons are usually being detected all the time. If only a few photons are present, however, individual points of light – arriving at different parts scattered across the detector – are indeed visible. The optical wave diffraction pattern can be recognized only in this scatter pattern of the individual photons.

The same is true in microscopy, where the light of a fluorescence molecule is diffracted at the microscope aperture. It can emit around 50,000 photons before it finally “expires.” We have to make do with only a very few photons to be able to recognize both aspects of light.

16 May 2012

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