z10-REFRACTION OF LIGHT AND LENSES

# z10-REFRACTION OF LIGHT AND LENSES - REFRACTION AND LENSES...

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REFRACTION AND LENSES Purpose: To understand properties of rays when encountering an object and the formation of images by a lens. Apparatus: Optics bench, ray table, slit plate, mask, light source, lenses, cylindrical lens, viewing screen, closed arrow target (object) Background: Light can be described in terms of waves. Light travels from a source in the same way as water waves encircle and propagate in all directions after a stone has been dropped in a pond. If we examine closely the waves traveling in the pond, circles of high and low points can be seen traveling radially outward. A vertical cross section of the water would show patterns similar to those of a wave traveling on a string. The circular rings emanating from where the stone was dropped are called wave fronts. Normally when one wants to represent the direction in which the wave is moving directed lines are drawn perpendicular to the wave fronts. These lines are called rays. Studying the properties of rays when they strike objects begins our study of geometric optics. When a light ray encounters a boundary leading into a second medium, part of the energy passes through to the second medium, and part of the energy is sent back into the first medium. The direction of the ray is changed in both cases. When the ray is sent back into the first medium, it is reflected, and when it moves into the second medium it is refracted. Angles of light rays are measured relative to the perpendicular or normal line at the interface. A perpendicular or normal line can be drawn from all surfaces. The direction, or the angle, of the incident, reflected, and refracted rays are measured with respect to this normal line. We will use this standard in class and for all problems. Refraction changes the direction of light when it passes through the boundary. The change in direction is determined by the indexes of refraction of the materials which make up the boundary. The index of refraction is expressed as the ratio of the speed of light in a vacuum, c, to the speed of light in the medium, v n = c/v Eq. 1). The way of describing the direction change in the ray is by examining Snell's Law. Snell's Law states the relationship of the incident angle and the refracted angle of the of the ray as it passes from one medium to another and is expressed as n 1 Sin( θ 1 ) = n 2 Sin( θ 2 ). Eq. 2).

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z10-REFRACTION OF LIGHT AND LENSES - REFRACTION AND LENSES...

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