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11.1 – Refraction of Light

11.1 – Refraction of Light. What is Refraction?. Refraction is a property of light in which the speed of light and its direction of travel change as it passes from one medium to another. Air. Water. How does refraction occur?.

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11.1 – Refraction of Light

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  1. 11.1 – Refraction of Light

  2. What is Refraction? • Refraction is a property of light in which the speed of light and its direction of travel change as it passes from one medium to another Air Water

  3. How does refraction occur? • Light travels in a straight line and at a constant speed as long as the medium it is travelling in remains the same • When light travels from one medium to another, however (e.g., from air to water), the light rays bend, or refract • This means that both its direction and speed changes

  4. How does refraction occur? • Light travels as a wave • Crests and troughs of waves can be thought of as wave fronts, with rays perpendicular to them showing the direction the wave is travelling in

  5. How does refraction occur? • Think of a marching band: • The mud is sticky, so students cannot march as fast in it • As each student reaches the mud, he or she slows down • The slower students “pull” the line back and cause a bend in the line, representing the wave front • As a result, the direction in which the entire row is marching changes – light behaves the same way

  6. Fermat’s Principle • The exact path of light as it travels from one medium to another can be found by applying Fermat’s Principle: When light travels from one point to another, it follows the path that will take the least time. • In a single medium, the path that takes the least time is a straight line • When travelling from one medium to another, the path that takes the least time is not a straight line

  7. Fermat’s Principle • Compare the dashed line with the solid, bent line going from point A in the air to point B in the water • In air, where light travels faster, the solid line is longer than the dashed line • In water, where light travels slower, the solid line is shorter than the dashed line

  8. Fermat’s Principle • Light travels a longer distance in air and a shorter distance in water than it would if it followed a straight line • Following the bent path (solid line) takes less time than following the straight path (dashed line).

  9. Describing Refraction Using Rays • Mostly similar terminology to reflection • The refracted ray is the ray that is bent upon entering a second medium • The angle of refraction, R, is the angle between the normal and a refracted ray

  10. The Direction of the Refracted Ray • If a light ray travels from a medium in which its speed is faster (such as air) to a medium in which its speed is slower (such as water), the refracted ray bends towards the normal

  11. The Direction of the Refracted Ray • If a light ray travels from a medium in which its speed is slower to a medium in which its speed is faster, the refracted ray bends away from the normal

  12. The Direction of the Refracted Ray • In summary: • Faster medium  Slower medium = Bends towards the normal • Slower medium  Faster medium = Bends away from the normal

  13. The Index of Refraction • How much a light ray refracts is determined by how much the speed of light changes as it travels from one medium to another • In a vacuum (such as space), the speed of light is 3.00 x 108 m/s • The speed of light is less in any other medium • e.g., the speed of light in water is 2.26 x 108 m/s • It is inconvenient to use these large numbers to describe relative speeds, so we use the index of refraction instead

  14. The Index of Refraction • The index of refraction is the ratio of the speed of light in a vacuum to the speed of light in a given medium

  15. The Index of Refraction • Example: calculate the index of refraction of water if light travels through it at 2.26 x 108 m/s

  16. Using the Index of Refraction (Found on pg. 454)

  17. Dispersion • Dispersion is the process of separating colours by refraction • Each colour of light travels at a slightly different speed in any medium except a vacuum (where they all travel the same speed) • Can see this by shining white light into a prism

  18. Dispersion • White light contains all the wavelengths (i.e., colours) of visible light • When white light is shined into a prism, it refracts twice – once when it enters, and once when it leaves • As it exits, the light is separated into a spectrum of colours • Blue light bends more than red light, so it must be travelling at a slower speed

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