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Chapter 8 - Chapter 8 Visual System Light Optics and the...

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Chapter 8 Visual System: Light, Optics, and the Eye A. The Nature of Light History - Ether: an idea that dates back to Aristotle; believed to be the medium of light and filler of space. - Mid 17 th c: particle theory of light began to be questioned + a wave theory emerged. - 1800s: became clear that phenomena such as diffraction could not be explained by particle theory and thus light must have a wave-like motion. - Early 20 th c: swing back in favour of particle theory when it was found that a light beam can produce an ejection of electrons from an object under appropriate conditions (Einstein). - Idea that light travels in packets- or photons- that have a wave-like quality began to take hold. o This dual-theory of light is the cornerstone of current understanding of its nature. 1. General properties - 1873, James Clerk Maxwell: showed that an oscillatory charged particle produces electromagnetic radiation - a type of moving energy field found everywhere in nature. - The electric+magnetic fields are linked to each other perpendicularly +vary in intensity over time. - If magnetic field intensity oscillates on vertical axis, electric field does so on the horizontal. - Electric/magnetic waveform= sinusoidal function. - Wavelength (one full cycle’s distance) = lambda. Spectrum - From longer to shorter wavelengths: ELF (extremely low frequency) waves, AM radio, TV, FM radio, microwaves, radar, infrared rays, visible light (400-700 nm), UV rays, X-rays, gamma rays. - Why is visible light the only electromagnetic radiation we can see? o The emission spectrum of the sun is narrow, ~100-4000 nm. o This region contains the infrared, UV, and visible light bands. o Must also consider transmission spectrum of the atmosphere. o Transmission + emission spectra overlap most significantly around the wavelengths of visible light; thus it would appear humans/life on Earth evolved to take advantage of this. Speed of light and light propagation - =, in vacuum, 299,792 km/second. - Light can travel in absence of medium (unlike sound), as the nature of electromagnetic radiation is such that the fields can propagate in total vacuum. - Point source : very small light source form which light radiates outwards in all directions. - Wave fronts : representation of light propagation in space at a particular point in time through a series of expanding concentric circles. - Optical infinity : a distance of 6m or more from the source at which wave fronts flatten out and light rays become parallel. - What emerges from a point source can be depicted as a radiating pattern of straight line. - The term “parallel incident light” is used to describe the nature of light rays at optical infinity. Effects of distance on light intensity - Divergence of light rays w/ increasing distance is responsible for a reduction in intensity, a reduction that is not linear but rather is related to the square of the distance.
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