Chapter 28 Chapter 28 Reflection and Refraction
1. REFLECTION Most objects we see reflect light.
2. PRINCIPLE OF LEAST TIME Fermat's principle: Light travels in straight lines and will take the path of least time.
5 4 1 2 3 3. LAW OF REFLECTION • Using Fermat's principle one can show the law of reflection. A B Mirror
The law of reflection states that the angle of incidence (qi) equals the angle of reflection (qr). • This is true for specular reflection. (Specular reflection is “mirror-like” reflection.)
Included in the law of reflection is the fact that the incident ray, the normal, and the reflected ray all lie in the same plane. Normal Reflected Ray Incident Ray qr qi Specular Reflecting Surface
Demo - Candle Burning Under Water Video - Candle Burning Under Water Video - Water Wave Reflections Demo - Mirror Track Tracing – Take home exercise for PQ Points
Plane Mirror Ray Diagramming Plane Mirror
Plane Mirror Ray Diagramming Plane Mirror
Plane Mirrors Using ray diagramming one finds that the image is 1. Upright 2. Same size as the object 3. Virtual
Diffuse Reflection • Video - Diffuse Reflection • If successive elevations of a surface are no more than l/8 apart, then the surface is said to be polished at that wavelength. • Most objects are seen by diffuse reflection.
Magnification of a surface Diffuse Reflection Colored lines are for the purpose of distinguishing reflected rays from incident rays.
Something that is barely polished for red light would not be polished for blue light. • Consider microwave satellite dishes. They are polished for microwaves but not for visible. • The metal screen in a microwave oven serves what purpose?
f Reflection from Curved Surfaces(Concave shown here) Ray Diagramming The law of specular reflection is still obeyed. Principal axis Reflecting Surface
f Reflection from Curved Surfaces(Concave shown here) Ray Diagramming Principal axis Reflecting Surface The law of specular reflection is still obeyed.
Demo - Coin Mirage • Demo – Deep Concave Mirror • Demo - Make-up Mirror Images • Concave mirrors can produce real inverted images that are magnified, diminished, or the same size as the object. • Concave mirrors can produce virtual upright images that are magnified.
More on Mirrors • Convex mirrors always produce a diminished, upright, virtual image. • Demo - Convex Mirror • Demo - Two Perpendicular Mirrors • Demo - Three Perpendicular Mirrors • Demo – Road Reflectors • Question - Why is it hard to see at night after or during a rain?
What type of mirror would you use to produce a magnified image of your face? (a) flat (b) concave (c) convex (d) you could use a concave or a convex mirror
What is (are) the purpose(s) of the wire screen in the door of a microwave oven? (a) to absorb microwaves (b) to allow you to see what's cooking (c) to reflect microwaves (d) all of the above (e) only (b) and (c)
4. REFRACTION • Fermat's principle can also explain refraction (remember the beach). • Refraction of light is the bending of light as it passes obliquely from one medium to another. • It is due to the different speeds of light in the two different media.
Index of Refraction Index of refraction of a material equals the speed of light in a vacuum divided by the speed of light in the material.
Demo – Green Laser Refraction in an Aquarium Demo - Twinkling with Laser Slide - Twinkling Cartoon
Video - Water Wave Refractions • Video - "Broken" Pencil Refraction
Because of atmospheric refraction, we have lingering, elliptical sunsets. Sun Earth Sun
Mirage Cool air Warm air Surface of water? Highway Mirage
Looming Warm air Cool air
An oar partially immersed in water appears "broken" because of (a) refraction (b) diffraction (c) polarization (d) interference (e) absorption
5. CAUSE OF REFRACTION • When light passes from one medium to another, its speed changes which in turn causes a bending of the light. • Examples: Car running onto shoulder of road Light passing from air into water
Wave fronts of light qi Air Water qR
This bending produces illusions. • Example: Objects in water appear closer and nearer to the surface. Eye Air Water
Fish can see everything above the surface of water within a 960 cone.
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. • Demo - Aquarium Prism
Dispersion in aPrism Slit White Light Source Prism
Picture - Rainbow • Individual drops act as dispersers. • The 42o cone • Demo– Rainbow Model • A single eye can only see a small range of colors from a single raindrop.
SecondaryRainbow Double rainbows are dimmer, higher, and have colors reversed. Link to picture.
Green Flash Earth Sun Dispersion occurs causing multiple images of the sun. The last to set would be blue, but most of the blue has been scattered which leaves green.
6. TOTAL INTERNAL REFLECTION • Video - Laser Under Water • 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. • Binoculars make use of TIR.
Two Prisms in a Monocular Flex Cam – Monocular
Demo - Laser and Light Pipe • Video - Woman at Edge of Pool • Flex Cam – Aquarium and Finger Dip in Water • Fiber optic devices make use of TIR.
A lingering sunset is (a) a looming effect (b) caused by an elliptical (oval) sun (c) due to atmospheric refraction (d) caused by reflections from the upper atmosphere
When a beam of light emerges at a nonzero angle from water to air, the beam (a) bends away from the normal (b) continues in the same direction (c) bends toward the normal (d) changes frequency (e) slows down