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| Light as material In this section I want to look into the nature of light as a material along with some of the history of the discoveries in light and optics. The reason for this is to gain a scientific knowledge which we can then go back and apply to our experience of light. The following was part of Goethe's philosophy which, Arthur Zajonc, author of "Catching the Light, The Entwined History of Light and Mind" explains: "In Goethe’s scientific approach, one sets aside models and systematically investigates the phenomena themselves through three stages—what he called the first stage of empirical phenomena, the second stage of scientific phenomena and the third stage of pure, archetypal phenomena. Throughout these three stages, one moves from working with first observations, and empirical phenomena to a systematic exploration of changing the conditions of appearance, so that you can distinguish the essential from the unessential factors. That’s the scientific domain, the scientific phenomena. Then you come, after having made that whole journey, to a point when you stand before the archetypal phenomena itself—where only the essential conditions of appearance are present in the most simple and eloquent instance of the law, one you see. That is, you’re not writing the law down mathematically but actually perceiving it." http://www.collectivewisdominitiative.org/papers/zajonc_interv.htm#three
(accessed 12-04-06) It is the actual perception that Zajonc separates from 'writing the law' which is so important. However, in order to get to that perception an understanding of the laws need to be understood. The First Theories on Light In 1666 at Trinity College, Cambridge, Isaac Newton (1642-1727) blacked out his window leaving only a small hole. In the path of the light, Newton placed a glass prism and to his amazement, on a screen 18.5 feet away, a broad band of colours appeared. Next, Newton made a hole in the screen, allowing one colour through. In the path of the coloured light beam he placed another prism. This time the light stayed the same colour but was still bent by the prism. Newton deduced that light was made up of constituent colours which he defined as seven colours which he called the Spectrum and that light could not be broken down any further than these colours. Newton would already have known about the bending of light through media from his forerunners Snell and Descartes. Snell died at the age of 35 leaving his ideas unpublished, René Descartes picked up the baton and published the Law of Refraction or Snell's Law in 1637:
This bending of light is caused by the speed of light being slowed down by the medium it passes into. The greater the refractive index the more the light is slowed. This bending is also determined by the wavelength of the electro-magnetic wave, the greater the wavelength the less it is bent. This explains the spectrum as seen through a prism, the blue is refracted more than red light so they split, one bending to a greater extent than the other. At this point I'd like to refer to Johann Wolfgang von Goethe (1749– 1832) with a proper introduction. Goethe was more a student of the arts, a writer and theatrical director than a scientist, but his explorations into colour and light are inspirational and insightful. His work draws direct parallels between art and science, making it of particular interest to architects. Goethe writes with eloquence and astuteness whilst he stares into the mysteries of colour and light. He doesn't have the scientific knowledge to produce any great discoveries, in fact the theory of colours doesn't stand up to scientific scrutiny in many senses, but this doesn't matter. It is his practical experimentation and documentation of his observations that is so interesting. The Theory of Colours was first published in 1810, the book is broken down in to numbered sections, only a paragraph or so long, each one bringing an insight into Goethe's interpretation of colour and perception. Despite the wide acceptance of Newton's wavelength theory of light, Goethe doesn't mention it once, preferring to describe his observations of light through a prism without it. It is amazing the extra insight that can be absorbed from this simple exposition that a hundred readings of a physics text book on light couldn't give. He assesses that most important thing is the displacement of the image rather than the dispersion of the light: "Yet we will not, in employing them (prisms), suffer ourselves to be dazzled by the splendid appearances they exhibit, but keep the above well- established, simple principles calmly in view." (pg 87 Goethe, 1810) (Those of light displaced by parallel surfaced or angled surface medium) His observations lead him to see the dispersion, not as a strange occurrence of it's own, but as part of an image making process. This can be experienced best if you hold a prism to your eye and look around. The first thing you notice is that the image is not the scene in front of your eye but to the left or right of where your eye is looking, this is the bending of the rays. Second, you are likely to notice the bands of colour which appear wherever there is a contrast of light and dark. Goethe assessed that the dispersion was due to this contrast. Depending on which way you hold the prism the bands of colour are red/yellow or blue/violet from light to dark or dark to light. It is only when the colours blend that the full spectrum, as Newton saw it, is produced. Image Making As Goethe saw it is the refractive index that causes lenses to form images. In order to understand how a lense forms an image it helps to use the example of a pin hole camera. A pinhole camera is a light-tight box with a small hole at one end and a piece of tracing paper or photographic paper at the opposite end to record the image. The tiny hole created at the front of the box is very selective about the light it lets in, only letting a very small amount in from every point in the scene the box is directed at. As the pinhole only lets in such a selective amount of light from the mass of light rays being reflected off every part of the scene, it produces an image by projecting these points onto a screen at the back of the box. The size of the hole or aperture is very important, a larger hole is less selective than a small one so the image becomes blurred, a tiny hole produces a sharp image. This disparity can be rectified by using a converging lense over the larger aperture to focus the blurred light to a sharp point. Light as Wave and Particle During the 1600s a debate was started between Christiaan Huygens and Newton as to whether light was a wave or a particle. Huygens developed his theory of light as a wave and published it in his Treatise on Light in 1690. However Newton's theory of light as particles or corpuscles as he called them was more pervasive. That was until Thomas Young did a series of experiments in 1801, the double-slit experiments, in which he passed a beam of light through two parallel slits in an opaque screen, forming a pattern of alternating light and dark bands on a white surface beyond. This demonstrated diffraction of light through a narrow slit providing evidence that light acted like a wave. (A common illustration is that of ocean waves passing through a gap in a harbour wall). In 1905 Einstein came up with the photoelectric effect which swung popular belief back to light as particle. Current understanding is of wave-particle duality in which light has both wave like and particle like attributes. In a piece of work I did in December 2005 I tried to apply some of the things I'd read about light to an installation called 'Coma Maker'. This was a lense of designed focal length which focused light through three grids that acted as diffraction gratings. Commonly, optics are used for imaging purposes to produce the best quality image, camera lenses become increasingly expensive as the lense elements produce a cleaner picture. In order to produce a high quality image, effects like diffraction and dispersion, or aberrations, are designed out. Coma maker is designed to include as many of these aberrations as possible, the name is taken from the effect of light passing through a lens at an angle, stretching the image and producing a tail on it like a comet (images of this project are in the portfolio pages). The lense was made by vacuum forming 1mm polypropylene over a dome shaped mould, this was done twice to make the two sides of the lense, they were bolted together with a silicone gasket. This left a hollow form which could then be filled with water, the refracting medium of the lense. The apparatus was designed around the mould because it determined the focal length of the lense, this can be worked out by putting the relevant numbers into the lensemaker's equation which is a function of the curvature of the lense and refractive index. The positions of the grids, light and screen were then all related to the focal length of the lense. The apparatus was designed so that light from a bright source (a 'red-head' theatre light) passed through three grids in turn, which diffracted the light. The light then passed through the lense which could be twisted and turned via a gyroscope mechanism to produce coma aberrations which where reflected off a white wall (the screen). As it turned out, the projection on the wall was not that interesting in itself. The real interest was in playing with the gyroscope lense and watching the effect change with your movements. the light effect was best when you substituted the screen for your eye and looked back through the lense to the light. In looking directly through the lense the chromatic aberration is more obvious (the dispersion of light through the lense into the colours of the spectrum). The installation served to display simple light phenomena within the structure of the apparatus and allow some interaction in moving parts of the apparatus to produce a changing effect. Being able to change the setup of the apparatus might take the spectator to a deeper understanding of the phenomena and give them the pleasure of creating their own light effects. In the next section I want to look at the way we perceive light through the eye and the brain. The
eye and the brain - How
do we perceive light?
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