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Without light there would be no life on earth. Light is the reason why fields appear green and the sky blue. It makes the stars shine in the sky at night and illuminates processes which cannot be seen by the naked eye.

It may well be the many different forms that explain why it has been a source of great fascination to people down through the ages. Right up until modern times, it was largely unclear what exactly light consisted of. In the 17th century, Isaac Newton endeavored to attribute light propagation to the movement of small particles. Almost 200 years later, Briton George Bidell Airy discovered why stars look blurred when observed through a telescope – confirming the then commonly-held view that light behaves like a wave. Only a part of the infinitely long light wave which a star emanates is captured by the diameter or aperture of the optical system, which is why the objects in the image plane appear blurred. Since then, most phenomena observed have been accounted for using the wave nature of light. However, not all of them can be explained in this way. Therefore, Albert Einstein again postulated in 1905 that light consists of particles called photons. They are indivisible and may only be generated and absorbed in their entirety.

Only two complementary theories could resolve such an obvious contradiction: Light must be both wave and particle in one. Quantum mechanics offers a ready-made, simple concept to address this conundrum. Light comes to earth as a particle, lives as a wave and finally dies as a particle. This means that light is “born” when light sources emit photons. It then “lives” as a wave and is diffracted at the aperture of the lens. The light is finally converted into electric signals on the detector or in the eye. Only individual photons are detected. Light then “dies” in particle form.

All optical systems are subject to these laws of physics – but normally it is not very noticeable because in most cases, many photons are detected. However, if there are few photons, individual light points – which arrive scattered in different places on the detector – are indeed seen. Only in this scattered pattern of the individual photons can the wave-optical diffraction image be recognized.

Similarly, in microscopy, the light of a fluorescence molecule is diffracted at the microscope aperture. It can emit around 50,000 photons before it finally “expires.” We must therefore make do with very few photons and we can recognize both aspects of light.


May 16, 2012