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Physics Unit Summary - Light and Sight in Focus

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Christiane Soluri Fox

on 15 December 2012

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Transcript of Physics Unit Summary - Light and Sight in Focus

Light: What is it, and how does it function? Light is only possible to describe with the use of a model - in this case, the photon model.
The photon model states that light appears to act as a particle simultaneously functioning in the same way as a wave - this is called wave-particle duality. What is colour? The Human Eye What did we learn in this unit? Light is a tiny part of the electromagnetic spectrum, called the visible spectrum. Electromagnetic waves, as their names suggest, are waves which possess an electric part, and a magnetic part, which travel in a manner perpendicular to each other. How does the electromagnetic spectrum relate to light? Where does light come from? Light comes from two types of sources:
Incandescent sources, which produce light as a result of heating. As the temperature of the source increases, it's electrons move randomly in all directions, occasionally colliding. The random accelerations, and decelerations of the source's electrons cause light radiation to be emitted.
Luminescent sources also produce light - yet their difference to incandescent sources are that they produce light as a result from absorbing forms of energy which are not thermal. The atoms of the luminescent light source move more rapidly as they absorb this energy, and this energy is 'released,' if you will, as they emit light radiation as they move back to their original state. An incandescent light source A luminescent light source How does light travel? Light always travels in straight lines, which traverse time, and space. This prevents us from being able to see around corners, and also creates shadows.
Light interacts differently with different materials - for example, opaque materials reflect light, transparent material allows light to traverse it, and translucent materials only partially allows light through. Opaque Translucent Transparent
Light is reflected in 2 different ways:
regular reflection, and diffused reflection.
Regular reflection occurs upon smooth surfaces, such as mirrors, and causes apparent reflections to appear.
Diffused reflection occurs on non-smooth surfaces. The fact that the light is reflected in an irregular manner is why reflections cannot be made upon non-smooth, or 'shiny' surfaces.
The law of reflection states that the angle of the incidence ray to the normal shall equal the angle of the reflected ray to the normal. This is true for both cases of regular, and diffused reflection. The Refraction of Light Light refracts differently with different prisms. For example, in a triangular prism, the white light shall be split to reveal the full visible spectrum due to the prism's non-parallel sides, whereas it shall exhibit endemic refraction with a semi-circular prism.
http://www.freezeray.com/flashFiles/TotalInternalReflection.htm As light travels through a material, 2 things occur: light refracts, or changes direction, and decelerates. The extremity of the refraction is determined by the material's refractive index. The refractive index demonstrates the speed of light through the material as compared to that of it's speed inside a vacuum. The Snell-Descartes Law of Refraction states that the refractive index (n) = sine of the angle of incidence/sine of the angle of refraction.
The path of light The path of light can be changed using lenses. Diverging (concave) lenses cause rays of light to diverge from a 'virtual' focus point (so called as it is imaginary, and would be the point of intersection of the emergent rays if they were continued back through the lens).
Converging (convex) lens cause rays of light to converge from a 'real' focus point. The usage of diverging, and converging lenses have many uses to us, such as ocular aids, magnifying glasses, and microscopes.
Colour and the electromagnetic spectrum Colour is light, a tiny part of the electromagnetic spectrum, as discussed earlier. White light comprises all of the colours of the visible spectrum, so called as it is the only part of the electromagnetic spectrum which we can see - red, orange, yellow, green, blue, indigo, and violet.
These colours can be 'revealed' using a prism with non-parallel sides, which splits the base of white light.
Specific colours can be shown in a variety of ways, with the use of filters. Additive mixing occurs when a primary colour of light (either red, green, or blue) is separated from white light using a filter of the same colour. This occurs because filters absorb every colour except their own, which they allow to pass through. The primary colours of light combine to form white light. When two primary colours overlap, they produce a secondary colour, which may be magenta, cyan, or yellow, depending upon the primary colours used. Subtractive mixing is a process in which a single colour is removed from white light with the use of 2 filters, one in a secondary colour, and one in a primary colour, or 2 secondary colours. For example, 2 secondary colours, cyan, (consisting of blue, and green) and magenta (consisting of red, and blue) are placed one behind the other, in that order. The cyan filter only allows blue, and green light to pass through it, and all other colours are absorbed. The magenta filter than only allows the blue light to pass through,, absorbing the green. The result is blue light.
http://www.freezeray.com/flashFiles/colouredFilters.htm What makes objects appear coloured? Different objects have different colours because they reflect, and absorb, different colours of light. For example, an object which appears to be a secondary colour under white light, such as cyan 3-D glasses, only appear so as they absorb all colours except blue, and green. An object which appears to be a primary colour, such as a green leaf, will appear black under all lights except green, and white, as green as a primary colour, and only green is reflected. How does the eye work? Light passes through the cornea, and then through the lens of the eye, which causes it to refract, and then converge, causing the image to be upside down. To be able to focus upon objects at different distances, the lens can change shape using the contraction of ciliary muscles attached to it, in a process called accomodation. The light then reaches the back of the eye, called the retina. The retina is covered with millions of photoreceptors, which convert light into electric impulses which are sent from the optic nerve to the brain. There are two types of photoreceptors: rods, and cones. Rods allow us to see in dark light, and 120 million are spread across the back of the retina. Cones allow us to see in colour, and are concentrated upon the fovea, which is located in the centre of the macula pit. The macula pit is located directly across from the lens, and the most light falls there, hence the majority of the cones being situated there. Visual Impairments There are many vision problems related to refraction issues of the lens. In hypermetropia (long-sightedness) the eyeball is either too short, or the lens is unable to change shape. Hypermetropia is corrected by the administration of a converging (convex) lens. In myopia, the eyeball is either too long, or the lens is unable to change shape. Myopia is corrected by the prescription of a diverging (concave) lens. Astigmatism is caused by the irregular shape of the lens - egg-shaped, or oval, as opposed to spherical. This causes the lens to have 2 different focal points, one for the vertical, and one for the horizontal part of the image. This means that some areas of an image appear more out of focus than others. http://www.freezeray.com/flashFiles/eyeDefects.htm Other visual impairments People may gain visual impairments due to a variety of factors, asides from genetic - such as their environment, and diet. For example, river-blindness is contracted by interaction with the environment of the black-fly, which transmits parasitic worms which carry the disease. How is science helping people with visual impairments? Scientists worldwide are helping people with visual impairments, and are trying to find their underlying causes, and eliminate them as far as is possible. For example, scientists have raised awareness about the damaging effects of long-term exposure to sunlight, and it's consequential cataracts, and have been working tirelessly to create new vaccines, and cures to aid those in danger of, and suffering from diseases such as trachoma, and river-blindness. Scientists work with multi-national organizations, such as the WHO to distribute these cures, and to educate others about what can be done to prevent such impairments. Sources Source 1: Champion, Neil, and Rick Armstrong. Physics 4/5: For the International Student. South Melbourne, Vic.: Cengage Learning, 2010. Print.
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