MOD 5  LECTURE 4                                         REFLECTION

In this lecture you will learn about:

SUMMARY OF WAVE - MATTER INTERACTIONS
Type of waves Main interaction  SOUND LIGHT (EMS)
INTERFERENCE Run into each other acoustics (loud/soft) holograms (bright/dark)
REFLECTION Hit and bounce off surfaces acoustics: echoes and reverberations mirrors, colors, shiny and matte
REFRACTION Enter clear materials and bends rumble of thunderstorms lenses, diamonds, rainbows
DIFFRACTION Bend around solid or little slit how we hear around a post slits, rainbow effect on CD or DVD

 
SOUND REFLECTION
WAVES HIT AND BOUNCE OFF OF SOLID, NON TRANSPARENT SURFACES
"Reflection of sound waves off of surfaces can lead to one of two phenomenon - an echo  or a reverberation. A reverberation often occurs in a small room with height, width, and length dimensions of approximately 17 meters or less. Why the magical 17 meters? The effect of a particular sound wave upon the brain endures for more than a tiny fraction of a second; the human brain keeps a sound in memory for up to 0.1 seconds. If a reflected sound wave reaches the ear within 0.1 seconds of the initial sound, then it seems to the person that the sound is prolonged. The reception of multiple reflections off of walls and ceilings within 0.1 seconds of each other causes reverberations - the prolonging of a sound.

Perhaps you have observed reverberations when talking in an empty room, when honking the horn while driving through a highway tunnel or underpass, or when singing in the shower. In auditoriums and concert halls, reverberations occasionally occur and lead to the displeasing garbling of a sound.

But reflection of sound waves in auditoriums and concert halls do not always lead to displeasing results, especially if the reflections are designed right. Smooth walls have a tendency to direct sound waves in a specific direction. Subsequently the use of smooth walls in an auditorium will cause spectators to receive a large amount of sound from one location along the wall; there would be only one possible path by which sound waves could travel from the speakers to the listener. The auditorium would not seem to be as lively and full of sound. Rough walls tend to diffuse sound, reflecting it in a variety of directions. This allows a spectator to perceive sounds from every part of the room, making it seem lively and full. For this reason, auditorium and concert hall designers prefer construction materials which are rough rather than smooth.

Reflection of sound waves also lead to echoes. Echoes are different than reverberations. Echoes occur when a reflected sound wave reaches the ear more than 0.1 seconds after the original sound wave was heard. If the elapsed time between the arrival of the two sound waves is more than 0.1 seconds, then the sensation of the first sound will have died out . In this case, the arrival of the second sound wave will be perceived as a second sound rather than the prolonging of the first sound. There will be an echo instead of a reverberation." CITE


 
LIGHT REFLECTION
"Light incident upon a surface will in general be partially reflected and partially transmitted as a refracted ray. The angle relationships for both reflection and refraction can be derived from Fermat's principle. The fact that the angle of incidence is equal to the angle of reflection is sometimes called the "law of reflection".
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
"A mirror is an object with at least one reflective surface. The most familiar type of mirror is the plane mirror, which has a flat surface. Curved mirrors are also used, to produce magnified or diminished images or focus light or simply distort the reflected image.

Mirrors are manufactured by applying a reflective coating to a suitable substrate. The most common such substrate is glass, due to its transparency, ease of fabrication, rigidity, hardness, and ability to take a smooth finish. The reflective coating is typically applied to the back surface of the glass, so that the reflecting side of the coating is protected from corrosion and accidental damage by the glass on one side and the coating itself and optional paint for further protection on the other.

The method of making mirrors out of plate glass was invented by 16th century Venetian glass makers on the island of Murano, who covered the back of the glass with mercury, obtaining near perfect and undistorted reflection. 

In modern times the mirror substrate is shaped, polished and cleaned, and is then coated. Glass mirrors are most often coated with non-toxic silver [18] or aluminum, implemented by a series of coatings.  1. Tin (II) chloride, 2. Silver, 3. Chemical activator, 4. Copper,  5. Paint"
http://en.wikipedia.org/wiki/Mirror


 
REFLECTION AND ABSORPTION

When a ray of light hits a solid, non transparent surface of material it can be absorbed and/or reflected

Most matter reflects light.  "COLOR" is what happens when waves are reflected.  The color of an item is the wavelength that is reflected back to the eye. So black is the absorption of all colors, white the reflection of all colors. 

The energy of the absorbed wavelengths are converted to kinetic energy of the absorbing atoms. So black garments in winter heat up faster than white ones because black absorbs all light wavelengths (and IR). In summer heat white garments reflect all waves and so dont absorb much and dont heat up. 


 
REFLECTION AND INTERFERENCE
Objects are "shiny" or "matte" depending on how smooth or rough the surface of the item is.  A  smooth surface reflects the light in nice parallel lines (in phase) while the rough surface reflects at odd angles which interfere and CANCEL the light waves (out of phase). 
Thin film interference is a combination of reflection and interference.  It is responsible for the rainbow effect of soap bubbles and oil spills
"Constructive and destructive interference of light waves is also the reason why thin films, such as soap bubbles, show colorful patterns. This is known as thin film interference, because it is the interference of light waves reflecting off the top surface of a film with the waves reflecting from the bottom surface. To obtain a nice colored pattern, the thickness of the film has to be on the order of the wavelength of light.
Consider the case of a thin film of oil floating on water. Thin film interference can take place if these two light waves interfere constructively:
   1. the light from the air reflecting off the top surface
   2. the light traveling from the air, through the oil, reflecting off the bottom surface, traveling back through the oil and out into the air again. "

"Iridescence is an optical phenomenon of surfaces in which hue changes in correspondence with the angle from which a surface is viewed.

Iridescence is caused by multiple reflections from multi-layered, semi-transparent surfaces in which phase shift and interference of the reflections modulates the incident light (by amplifying or attenuating some frequencies more than others). This process is the functional analog of selective wavelength attenuation as seen with the Fabry-Pérot interferometer."



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SUMMARY OF EFFECTS
loud and soft areas of sound in concerts sound interference
bright or dim areas lined up in a row on a surface light interference
holograms  light interference using lasers
reverberation sound reflection
echoes sound reflection
mirrors light reflection
colors  light reflection and absorption
shiny or matte light reflection and interference
soap bubbles and oil spill rainbows light reflection and interference
iridescence  reflection and interference