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physics

# physics - Pre-AP Physics Notes Ch 15 Refraction You should...

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Pre-AP Physics Notes: Ch. 15 – Refraction You should remember from the previous chapter that whenever light encounters a different medium one of three things can happen. The light can be absorbed by the new medium and turned into internal energy and/or heat, the light can be transmitted through the new medium, or the light can be reflected back into the original medium. If the ray is transmitted into the new medium the ray may be refracted (bent) at the boundary. We will focus on refraction this chapter. Refraction of light When light travels from one transparent medium into another transparent medium with different properties at any angle other than straight on , the light ray changes direction (bends) at the boundary; that is, the ray is refracted. Refraction occurs because the light’s velocity changes. If the velocity decreases when the light enters the new medium, then the angle of refraction < angle of incidence and the ray is bent towards the normal. If the velocity increases when the light enters the new medium, then the angle of refraction > angle of incidence and the ray is bent away from the normal. The path of the light ray across a boundary is reversible. The amount that the ray bends depends upon the ratio of the speeds in the different mediums. To relate the speeds it is useful to define a quantity known as the index of refraction . The index of refraction (n) of a material is the ratio of the speed of light in a vacuum to the speed of light in the material ( n=c/v ). The larger the index of refraction the slower the speed of light. . Frequency does not change when light passes from one medium into another since frequency depends upon the source that created the light. If frequency does not change but speed does change, then wavelength must change proportionally ( if speed decreases then wavelength must decrease ). One interesting consequence of refraction is that an object lying under water appears to be closer to the surface than it actually is. This is because light speeds up when it moves from water into air, bending away from the normal. Your brain perceives the light to travel in a straight line. Another interesting consequence of refraction can be observed by putting a pencil in a glass of water. It appears as if the pencil is broken. The light from the pencil is refracted as it passes from the water to the glass to air, causing it to be displaced. Since the surface of the glass is curved, the water in the glass also acts as a magnifying glass, slightly enlarging the pencil. Mirages are also a result of light refraction. A mirage can be observed when the ground is so hot that the air directly above it is warmer than the air at higher elevations. These layers of air at different heights above the Earth have different densities and different refractive indices, causing the light to bend as shown below. Because both an upright and an inverted image are seen

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