Refraction And Total Internal Reflection P
Visão Geral
Somewhere on the floor of the Atlantic there is a cable about the thickness of a garden hose, and inside it are strands of glass finer than a human hair. The words on this screen may well have crossed an ocean inside one of them, as flashes of light bouncing off the inside of the glass tens of thousands of times a second without a single flash escaping. Nothing holds that light in. There is no mirror, no coating, no clever machinery. The light simply cannot get out, and the reason is one of the neatest arguments in the whole of school physics.
It begins somewhere much more ordinary: with the fact that light travels at different speeds through different materials. Slow a wave down as it crosses a surface at an angle and it has no choice but to swing round, which is why a straw in a glass of water looks broken and why a swimming pool is always deeper than it looks. In this lesson you will learn how to put a number on that bending, how to calculate exactly how far a ray turns, why a glass prism splits sunlight into colours, and where the angle lies beyond which light stops escaping altogether and gets trapped. By the end you will be able to work out that angle for any material from a single measurement, and explain how surgeons see inside a living body through a bundle of glass threads.
Objetivos
- The velocity of waves is affected by the medium they are travelling through. The speed changes. Unless the wave enters at 90° to the surface (along the normal) the direction also changes. This is called refraction.
- Light waves are refracted at an interface: when light enters a denser medium it is refracted towards the normal; when light enters a less dense medium it is refracted away from the normal. Students should have the opportunity to use wave front diagrams to explain refraction in terms of the change in speed that happens when a wave travels from one medium to another.
- Refraction by a prism can lead to dispersion of light waves and the formation of a spectrum.
- Refractive index can be defined in terms of wave speed. The refractive index of a medium is defined as: speed of light in vacuum (air) speed of light in the medium
- The relationship between refractive index, n, angle of incidence, i , and angle of refraction, r, is: n = sin i / sin r Required practical: Investigate the refraction of light by different substances.
- The relationship between refractive index, n, and critical angle, c, is: n = 1 / sin c Recall of the values of critical angles is not required.
- Total internal reflection is a special case of refraction, which occurs if the angle of incidence within the denser medium is greater than the critical angle.
- Visible light and infra red can be transmitted through optical fibres by total internal reflection. Students should be able to describe the application and benefits of optical fibres in medicine and communication technology.
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- A ray of light passes from air into a glass block. What happens to the light? A. It speeds up and bends away from the normal B. It slows down and bends towards the normal C. It slows down and bends away from the normal D. It keeps the same speed and bends towards the normal Answer: B
- What is the unit of refractive index? A. m/s B. degrees C. hertz D. it has no unit Answer: D
- The speed of light in air is 3.0 x 10^8 m/s. What is the speed of light in a material of refractive index 1.5? A. 1.5 x 10^8 m/s B. 2.0 x 10^8 m/s C. 4.5 x 10^8 m/s D. 3.0 x 10^8 m/s Answer: B
- A transparent material has a refractive index of 2.00. What is its critical angle for a boundary with air? A. 26.6 degrees B. 30.0 degrees C. 45.0 degrees D. 60.0 degrees Answer: B
- Total internal reflection of a ray at a glass to air boundary happens when: A. the ray is inside the glass and the angle of incidence is greater than the critical angle B. the ray is inside the glass and the angle of incidence is less than the critical angle C. the ray is in the air and the angle of incidence is greater than the critical angle D. the ray travels along the normal Answer: A
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