Reflection and Refraction California Content Standards: 4f Students know how to identify the phenomena of refraction and reflection and know that these are characteristic wave properties. Discuss how slides becoming more of outlines of what is said and that will need to listen more carefully because of this. We are moving to being able to take good notes even when speaker doesn’t give you any.

Reflection in General Definition: bouncing of wave off an interface. Velocity, frequency and wavelength remain constant. Law of reflection Define reflection. How do the frequency, wavelength and velocity of the incident wave compare to the reflected wave? State the law of reflection. Distinguish between a specular and diffuse reflection and give examples of each. Show transparency and define reflection as bouncing of wave. Discuss how velocity, frequency and wavelength all will remain the same for the incident and reflected wave since they are in the same medium. Write equation for law of reflection and define the incident and reflected angle from the normal to the interface Draw two pictures showing parallel waves bouncing off a rough and smooth surface. Define specular and diffuse reflection using these pictures. Give examples of each including waxed vs unwaxed floor, shiny vs rough paper and wet vs dry road. Discuss breifly how the relative size of the wave (wavelength) determines the smoothness of a surface. 1/8 th wavelength = smooth All waves can be refracted

Specular and Diffuse Reflections All rays reflected parallel to one another off smooth surface Examples include plane mirror, gleam off polished floor and glare off wet asphalt road Rays reflected at different angles relative to one another off rough surface Examples include unwaxed floor, dry asphalt and newsprint

Plane Mirrors Plane Mirror Virtual Image Image and Object distance same Image and Object sizes same Reverse front-back

Curved Mirrors Converging Mirror Diverging Mirror Thinnest in middle Reduced or magnified image Real or virtual image Inverted or erect image The FAT mirror Mirror Diverging Mirror Thickest in middle Reduced image Virtual Image Erect image The THIN mirror

Reflection of Sound Echo Amphitheatre Sound Balls and Half Shells Krikorian Theatre sound baffles eliminate reflections Upward sloping ceiling directs reflections to back where absorbed

Refraction in General Dfn: Change in direction of wave due to a change in speed Velocity and wavelength changes, frequency remains constant Fast to Slow/Slow to Fast Snell’s Law Define refraction. How do frequency, velocity and wavelength vary for the incident and refracted wave? Which direction will the wave be bent when moving from a fast to a slow medium? How about a slow to fast medium? Show Physical Science laser disk Chp. 28 refraction and wheel analogy Use above anology to derive bending of wave traveling from fast to slow. Generalize result by saying when going from fast to slow the ray will bend towards the normal. Do the same for slow to fast Describe how frequency is same because peaks will arrive at interface at same time interval. However wavelength will change because different velocity means wave travels longer or shorter distance in same period of time.

Refraction Examples For Sound Bending of sound over lake where warm air overlays cool air For Light Bending of spoon in water Mirages Pumpkin Sun Length of day Star Twinkling (scintillation) List some examples of refraction for light. What conditions will produce a mirage on the road? Does daylight last longer or shorter due to refraction of light from our atmosphere? Give an example of sound refraction. Under what conditions does this occur? Show spoon in water Discuss coke bottle illusion Discuss mirages (cold over hot) Discuss lengthening of day due to light refraction through atmosphere Show half of slide Talk about camping days and listening to hear what girls said Sound mirage is hot over cold air Show second half of slide

Snell’s Law Sin q I / sin q r = vr/vi N = index of refraction = c/v = speed light in vacuum / speed of light in substance N>1

Light Mirage Cold air over hot air Eye assumes straight line path for rays See a piece of sky on ground Brain interprets waving as water

Light after Sunset and Pumpkin Sun Light after sunset: Earth’s atmosphere refracts sun’s rays so they bend over horizon Pumpkin sun: rays lower on horizon pass through more atmosphere and are bent more Lunar Eclipse

Sound Mirage Warm air over cool air Usually occurs at night during summer Sound waves bent over lake

Check Question 4 If the speed of light where the same in a fluid as it is in glass, what would glass look like when placed in the fluid?

Critical Angle and Dispersion Questions: 1) Under what conditions will a wave be absorbed? Under what conditions will it produce a reflected and refracted wave? Under what conditions will it produce only a reflection? What does the critical angle depend upon? 2) A Native American stands on a river bank with a spear. A fish appears in the water. Should the Native American aim in front, behind or right at the image of the fish? Does the fish appear shallower or deeper than it actually is in the water? Describe what the fish sees when it looks at the surface of the water. 3) Under what conditions does total internal reflection occur? Does fiber designed to carry light using TIR have a large or small difference between the speed of light in it and the material that surrounds it? Give some examples of both natural and manmade materials that experience TIR. 4) Define dispersion. What color is bent best by most dispersive mediums? What color bends the least? List some dispersive mediums. 5) Draw a picture of a rain drop and explain how it separates colors. Which color will appear highest in the sky in a primary bow? Which color appears highest in the sky for a secondary bow? What accounts for this difference? Where do you have to be relative to the sun and rain in order to see a rainbow?

Critical Angle Dfn: Largest angle for which both a reflected and refracted wave are produced. Above critical angle you get only reflections At critical angle ray is refracted at 90 degrees

The Fish Example Where should native american aim arrow? Nearer and deeper than actually appears What does fish see? Inside critical angle sees refracted waves from above water Outside 48 degrees sees reflection of bottom

Speed of Sound vs Light Speed increases w/ density Speed increases with temperature Fastest in solid Speed decreases w/ density Speed increases w/ temperature Fastest in vacuum

Refraction Lecture

Check Question 1 What is the ultimate fate of a wave striking an interface when the following are true: A) The wave is the same frequency as the natural frequency of the solid it hits. B) The wave is not at the solids natural frequency and is incident at an angle less than the critical angle of the interface. C) The wave is not at the solids natural frequency and is incident at an angle more than the critical angle of the interface.

Mirrors (Reflection of Light) Work due to reflection of light Law of reflection holds for all mirrors Plane Mirror Virtual, erect, magnification=1 Reverses direction perpendicular to surface (front and back) Image distance = Object distance Curved Mirrors Convex=>virtual, erect and reduced image=>skinny mirror Concave=> real/reduced/inverted, virtual/erect/magnified big time=>fat mirror 1/f= 1/p + 1/q; M=-q/p; positive on object side of mirror

Check Question 2 What type of reflection do the rocks in asphalt allow the road to give? If water covers the road, what type of reflection can be expected? Why will turning your brights on not help you to see on a rainy night in which the road is covered with water?

Reflection Examples For Sound Echos Amphitheatre Hemispheres at Davies Hall Krikorian Theatre ceiling and sound baffles For Light Plane mirror Virtual erect image; same size as object; object distance = image distance; reverses F and R Curved Mirrors Concave: virtual, magnified and erect image (< f.l.); or real, reduced and inverted (>2f.l.) Convex: virtual, reduced and erect Give several examples of reflection of sound. Explain two ways in which the design of the Krikorian theatre in Redlands controls sound. Do all mirrors follow the law of reflection? Which type of mirror magnifies? Which is used to look around corners? Do plane mirrors switch left and right or front and back? 1) Mention that echo is obvious example of sound being reflected. 2) Discuss how amphitheatre, sound balls at Hollywood bowl and plastic hemispheres at Davies hall in SF all have shapes that reflect sound into the audience. The later case, what you see is what you hear. 3) Discuss how the upward slanted ceiling of the Krickorian reflects unwanted sound from the speakers behind the screen to the back wall where it is absorbed while the sound baffles on the sides reflect sound throughout the theatre to make sure there are no dead areas 4) Show transparencies and point out all mirrors follow law of reflection 5) Treat mirrors lightly discussing different images you can get and defining terms erect and inverted, real and virtual and magnified and reduced. Do point out front-back reversal and image d = object d for plane mirror. 

Check Question 3 Feedback occurs when reflected sound constructively interferes with the sound a microphone picks up from the speaker’s voice. Why is the gym so susceptible to feedback? What measures could be taken to reduce feedback in the gym?

Fate of a Wave Less than or equal to critical angle Greater than critical angle Angle of Incidence Not equal to natural frequency Equal to natural frequency frequency Reflected and Refracted Reflected only Absorbed Make a chart showing the three fates of a wave once it hits an interface. What two quantities determine the fate of a wave? Draw picture and label incident, refracted and reflected wave. Define interface as place where wave speed changes. Define ray as representation of wave that shows only wave direction. Draw wave fronts and discuss how rays are perpendicular to fronts. Two variables determine the fate of a wav: frequency and angle of incidence Discuss how if frequency = natural frequency of material wave is incident on then the wave will be absorbed and become heat if material is solid or be scattered (re-emitted in all directions) if liquid or gas. Show slide