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Lecture #11 Reflection, Refraction, Difraction, and Total Internal Reflection

Lecture #11 Reflection, Refraction, Difraction, and Total Internal Reflection. February, 24 th. Agenda Review Reflection 2. Refraction (…the bending of light…). Refraction occurs when light is traveling in one material and then enters a new material. 3. Dispersion

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Lecture #11 Reflection, Refraction, Difraction, and Total Internal Reflection

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  1. Lecture #11Reflection, Refraction, Difraction, and Total Internal Reflection February, 24th

  2. Agenda • Review Reflection • 2. Refraction (…the bending of light…). Refraction occurs when light is traveling in one material and then enters a new material. • 3. Dispersion • 4. Total Internal Reflection

  3. Geometric Optics (tracing rays) We are interested about the direction of travel of the electromagnetic wave (propagation direction).Light does cool things1. Reflection2. Refraction 3. Dispersion

  4. Reflection One obvious property of light is that it reflects off of surfaces. Among other things, this gives rise to the images we see in mirrors.

  5. Reflection light waves can be reflected at a surface between two media refl inc Law of Reflection inc = refl

  6. 5 4 1 2 3 LAW OF REFLECTION • Using Fermat's principle one can show the law of reflection. A B Fermat's principle: Light travels in straight lines and will take the path of least time.

  7. Plane Mirrors Using ray diagramming one finds that the image is upright, the same size as the object, and is virtual.

  8. Ray Diagramming Plane Mirror

  9. Refraction Light refracts, which means that it bends when passing from one medium to another. When light enters a more dense medium from one that is less dense, it bends towards a line normal to the boundary between the two media. Example:

  10. 5. CAUSE OF REFRACTION • When light passes from one medium to another, its speed changes which in turn causes a bending of the light. • Example: car running onto shoulder • This bending produces illusions. • Example: objects in water appear closer and nearer the surface

  11. Refraction:1. At surface, the direction of the ray changes.2. When speed of ray decreases, the ray is bent towards the normal.3. Usually, a more dense medium…decreases the light speed.

  12. Refraction Snell found (in 1621) a law called the Law of Refraction (Bending) Its qualitative form: If a ray of light enters a more dense medium, then the ray is bent towards the normal. If it enters a less dense medium, the ray is bent away from the normal. This explains the direction of bending.

  13. Refraction Snell found (in 1621) a law called the Law of Refraction (Bending) Its qualitative form: If a ray of light enters a more dense medium, then the ray is bent towards the normal. If it enters a less dense medium, the ray is bent away from the normal. This explains the direction of bending.

  14. Question: Why does the wave-speed of light get slower, in a material medium? Model: The atoms of the material absorb (temporarily) the light. Then they re-emit it. On its way from one atom to another, light travels with speed c. But, there is a time-delay between the absorption of light by an atom, and its re-emission. Problem: Suppose white light is sent through a prism. It then is dispersed into a rainbow of colors. Which takes longer to get through the prism – red light or green light?

  15. The Speed of Light: In empty space, there is just one value for the speed of light. It is 300,000 km per second. This value is denoted as c. This value of c does not change, no matter what the frequency or wavelength. For red light, f = 400 trillion cycles per second. For blue light, f = 700 trillion cycles per second. In a material, the speed of light is less than c. Examples: In water, v=0.75c; In glass, v=0.67c; In diamond, v=0.41c, perhaps the slowest speed of light in any simple material.

  16. Index of Refraction (Refractive index) Index of refraction of a material equals the speed of light in a vacuum divided by the speed of light in the material. Note that v < c always. So n is always less than 1. From experiment, n is also a measure of the strength of light-bending of the material, in going from air to the material. n1sin 1=n2sin 2

  17. Taking Advantage of Refraction The greater the density difference between the two materials, the more the light bends. One place where this is used is in lenses for a variety of optical devices, such as microscopes, magnifying glasses, and glasses for correcting vision. An example of an image formed from a lens is shown below.

  18. Dispersion Another aspect of light that is quite familiar is dispersion. If a beam of white light enters a glass prism, what emerges from the other side is a spread out beam of many colored light. The various colors are refracted through different angles by the glass, and are ``dispersed'', or spread out.

  19. Dispersion • Different frequencies are bent different amounts which causes a separation of white light into its constituent colors. • This is the basic principle behind the operation of a prism. We say that a prism disperses the light. • The higher frequencies interact most (slow down the most) and thus are bent the most.

  20. Internal Reflection An effect that combines both refraction and reflection is total internal reflection. Consider light coming from a dense medium like water into a less dense medium like air.

  21. Rainbows Rainbows are phenomena that involve refraction, dispersion, and internal reflection. In order to see a rainbow, it is necessary to look at a portion of the sky containing raindrops with the Sun directly behind you. White light from the Sun enters the raindrops, and gets refracted and dispersed inside the raindrop.

  22. Rainbows • Individual drops act as dispersers. • The 42o cone • A single eye can only see a small range of colors from a single raindrop

  23. TOTAL INTERNAL REFLECTION (TIR) • Critical angle is the angle where total internal reflection (TIR) begins. • TIR is possible only when light is entering a medium of lesser index of refraction. • Suppose that the incidence angle is gradually increased. Then, the law of refraction requires that the refraction angle also increases. So, eventually, the refraction angle will reach 90 degrees. • Result: the light does not go into the new material. It just skims the surface. • Binoculars make use of TIR. Fiber optics uses this property of light to keep light beams focused without significant loss.

  24. This happens at a certain incidence angle called the critical angle. At the critical angle, there is no transmission of light. Conclude: When the incidence angle is equal to or larger than the critical angle, there is no transmission. All the light is reflected. This is called total internal reflection (TIR).

  25. Summary on TIR Light goes from a dense medium to a less-dense medium. Then, the refraction angle 2 exceeds the incidence angle 1. Increase 1, then 2 will reach 90 degrees. The value of 1 (when 2 is 90 degrees) is the critical angle, c. The the refracted beam skims the surface. If 1 > c no light is transmitted. We have TIR.

  26. Supplementary Materials: Chapter 23rd, pp. 633-634, 642-644, 645-646 Look at examples Homework assignment: Chapter 23, Practice problems!!! #1, #5,

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