Eureka!

Simplicity lies concealed in chaos: it is only for us to discover it.

During the past month, I disappeared once again into the optical laboratory of the University of Leiden. This is where, next to Treepol¹ and a cosy plastic Christmas tree, you will find the instrument I am currently working on. This instrument is called the Life Signature Detection Polarimeter, also known as LSDpol. It is a spectropolarimeter which measures the polarisation of light over a spectrum (i.e. the polarisation of the specific colours of the incident light).

The special feature of LSDPol is that it will be the first spectropolarimeter to measure at one moment what percentage of the incoming light is linear and circularly polarised, without any moving part. This is quite important if we want to design an instrument that could potentially be launched into and used in space. However, the principle involves many things. Given that I have not told so much about the instrumentation side of my research up to now, I think it would be nice to change this in the coming posts.

Before I can explain more about the design of and the measurements done with LSDPol, I have to start off with a little theoretical explanation concerning light. Therefore, today I will introduce you to an important physical effect that is important to take into account when designing and building many optical instruments.

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Fresnel diffraction • The propagation of lightwaves behind an aperture

In 1801, Young's two-slit experiment showed that light sometimes behaves like a wave, which was completely contrary to Newton's particle theory. To this day, the two-slit experiment is a classic hands-on experiment for physics students, including myself. In the experiment you shine a light source through a single slit. The light from the light source must be spatially coherent. With coherent light one can think of:

1. laser light: light that consists of one wavelength, i.e. has "one colour", or
2. sunlight: light that consists of several wavelengths in which the so-called phase of the light for all the different wavelengths is similar.

Young used sunlight in his experiment. The sunlight was shining through the single slit on to two slits that together broke up the sunlight. This caused interference bands to appear at the places where the light beams overlapped. Unfortunately, as Newton's influence was so strong at the time, it took some time before the scientific community accepted the theory of "light waves".

Based on a theory developed by Christiaan Huygens, Augustin Fresnel proposed a mathematical model that could describe the diffraction of light rays. In 1808, Fresnel was one of the participants in a scientific competition that sought to find a mathematical description for a diffraction phenomena. The phenomena was observed in the shadow of a circular screen when it is placed in a beam of light. One of the judges, Siméon Poisson, predicted that, according to Fresnel's theory, at the centre of the circular shadow a bright spot would appear with the same luminosity as the light rays hitting the screen. Shortly afterwards, François Arago, one of the other judges, carried out this experiment which showed that a bright spot appeared on the screen. This confirmed Fresnel's theory. This bright dot is now called the Poisson, Arago or Fresnel spot.

Well, why am I telling this story about Fresnel and Fresnel's theory? Apart from the fact that I admire Fresnel for the theoretical and experimental study of the behaviour of light (basically what I also do with my instruments), I recently found out that in order to understand my instrument, I also need to learn much more about the Fresnel theory. The reason why this theory is so important will be explained in the coming posts. Stay tuned!

¹ TreePol is one of the instruments that I use in my search for signs of life on Earth. The design of TreePol is made with the aim to measure the percentage of a light beam that is circularly polarised as accurately as possible. The molecules in vegetation produce this circular polarisation signal when sunlight reflects off the surface of the leafs.