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Do Now Draw a reflected sound wave, labeling the angle of incidence and the angle of reflection. How do these angles compare? Draw the wave fronts of a light wave passing through three different media. The first medium be medium speed The second should be faster speed The should be slow speed Your picture should show realistic angles of refraction and changes in wave length.

Index of Refraction (n) The index of refraction (n) is a value that describes the relative speed of light in a particular medium n = index of refraction, has no units c = speed of light in a vacuum, 3.00 X 108 m/s v = speed of light in a particular medium Since c is the fastest speed of light, n is always greater than 1. The larger the index the _______________ the speed in that medium . The greater the difference in n, the ______________________ the amount of refraction. MEDIUM n v (m/s) Vacuum air Exactly 1 1.000293 300,000,000 water 1.33 225,564,000 glass 1.52 197,368,000 diamond 2.42 123,967,000 slower greater

Index of Refraction (n) Draw a light ray passing from air to water (n=1.3) , and another passing from air to glass (n=1.5). In each case, the incident ray approaches the new medium from a 45o angle. Use Snell’s law to determine the refracted angle. 45o 45o 32o 28o glass Water

Total Internal Reflection(n) q2=900 qc n1 Normally, when a wave reaches a new medium, some of the energy is reflected and some is refracted. However, when passing from a slow medium to a fast medium, the angle of refraction is greater than the angle of incidence. At a certain point, the angle of refraction becomes 90o and the light no longer is transmitted to the other medium; instead it is completely reflected back. Critical angle: qc - angle of incidence for which angle of refraction is 900

Application: Fiber Optics A fiber optic cable is a bunch (thousandths) of very fine (less than the diameter of a hair) glass strands clad together made of material with high index of refraction.  Critical angle is very small  almost everything is totally internally reflected. Click me  The light is guided through the cable by successive internal reflections with almost no loss (a little escapes).  Even if the light pipe is bent into a complicated shape (tied into knots), light is transmitted practically undiminished to the other end.

Application: Fiber Optics Why use fiber optics? can carry more info with less distortion over long distances One single optical fiber can transmit several TV programs and tens of thousands of telephone conversations, all at the same time. takes 300 lbs of copper to carry same info as 1 lb of fiber optics Downside: expensive Click me

JFF: Optical Illusions Caused by Refraction Is the straw really broken? refracted ray perceived straw incident ray real straw mirage, not a puddle! Mirage – hot air by road surface less optically dense JFF: Optical Illusions Caused by Refraction

Where is the fish? Deeper than you think! Apparent location of the fish JFF: Optical Illusions Caused by Refraction

Dispersion Even though all colors of the visible spectrum travel with the same speed in vacuum, the speed of the colors of the visible spectrum varies when they pass through a transparent medium like glass and water. That is, the refractive index of glass is different for different colors. Different colors (frequencies) are refracted by different amounts! True for different pitches, as well! True for sound as well!

A smile in the sky Rainbows are caused by dispersion of sunlight by water droplets 1. When white sunlight enters droplet its component colors are refracted at different angles (dispersion) 2. These colored lights then undergo total internal reflection. observer is between the Sun and a rain shower. 3. Second refraction from droplet into air – more dispersion If you view a rainbow from an airplane, you can see a complete circular, below and above you! 4. Each droplet produces a complete spectrum, but only one color from each drop is seen by observer – you have your own personal rainbow and I have MINE!

Diffraction When waves pass through a small opening, or pass the edge of a barrier, they always spread out to some extent into the region that is not directly in the path of the waves. The spreading of a wave into a region behind an obstruction is called diffraction.

Diffraction Diffraction effects are greater when the wavelength is big relative to the size of the obstruction. Small obstruction, large wavelength = noticeable diffraction Water waves diffracting through two different sized openings.. diffraction effects are small when slit is much larger than the incident λ. The waves are diffracted more through the narrower opening, when wavelength is larger than the opening.

Diffraction Diffraction explains why we can hear but not see around corners and other obstructions... Wavelengths of audible sound: 16.7 mm – 16.7m Wavelengths of visible light: 400 nm – 700 nm Audible sound wavelengths are around the same size, or even bigger, than most obstructions (open doorway, side of a building, etc.), so they diffract around the obstructions. Visible EM wavelengths are much much smaller than these obstructions, and so they reflect backwards, rather than diffracting around the obstruction.

Diffraction Turn and Talk: Why do dolphins, bats, SONAR, and doctors use ultrasound for localizing objects, instead of lower frequency sounds? High frequency = short wavelength  Little diffraction! Instead of diffracting around obstructions, ultrasound will reflect back towards the source, allowing the animal / instrument to identify the location of objects in the environment.