 # Reflection And Refraction In Media

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Refraction:
Refraction occurs when light enters a more or less optically dense medium, which therefore has a different refractive index (measure of the velocity light can travel at in the medium compared to in a vacuum in which it can travel at 2.9 x 108ms-1). This causes the light’s speed to increase or decrease, which results in the rays bending towards or away from the normal, so the position of the image formed is dependent on the refractive indices of the two media. For refraction to occur, the light rays have to hit the boundary between media at an angle to the normal (which is 90 degrees to the boundary), otherwise no change in direction will occur, only a change in velocity. Therefore, if the light rays hit the boundary between the different media at a perpendicular (90 degree) angle, they will continue to go straight. This occurs because the angle at which the rays hit the boundary (called the angle of incidence) determines the angle at which the rays will refract (called the angle of refraction). Light rays are measured from the normal, not from the medium boundary.
Snell’s law shows a mathematical relationship between the light’s angles of incidence and refraction, and the refractive indices of the media it travels through: n1sinθ1=n2sinθ2 Where: θ1=angle of incidence θ2=angle of refraction n1=index of refraction of first medium
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The objective lens has a larger focal length than the eyepiece lens because the objective lens must collect light from a distant object and then refract and ‘channel’ it to the eyepiece end of the telescope. The eyepiece lens, on the other hand, doesn’t need to have a large diameter or focal length because its job isn’t to collect light, but to magnify the image formed by the objective lens. This means the length of a telescope from one lens to the other is approximately the sum of the focal lengths of the two