Chapter 29: Reflection and Refraction

Reflection Occurs when a wave hits a boundary and bounces back into the first medium Metal surfaces are rigid to light waves that shine upon them Light does not pass into the metal/it bounces off (smooth surfaces/mirrors)

The Law of Reflection The angle of incidence is equal to the angle of reflection

Mirrors Plane (flat mirror) Image is virtual, upright, reversed, the same distance

Plane (flat) Mirror (cont’d)

Convex Mirror (bulges outward/back of a spoon) Image is upright, smaller and virtual (light rays do not meet) Side view mirrors on cars, security mirrors

Convex Mirror

Concave Mirror (bent inward/inside of a spoon) Between focal point and mirror: image is enlarged and upright Make-up and shaving mirrors Beyond focal point: image is upside down and real (light rays meet) Also used in headlights and flashlights

Concave Mirror cont’d

Diffuse Reflection When light strikes a rough surface, it is reflected in many directions Single rays still observe the law of reflection A rough surface for some waves may be polished for others Allows us to read!!

Smooth (specular) vs. Diffuse Reflection

Reflection of Sound Echoes Reverberations Acoustics Grooved walls to diffuse sound waves

Refraction Occurs when waves bend due to a change in their speed Occurs when waves pass into a new medium (temperature difference)

Refraction of Sound Sound refracted toward ground at night because it’s at a lower temperature

Refraction of Light Slows as it enters a denser medium Bends toward the normal when it slows down

Refraction: Bending towards the normal because wave speed is decreasing

Refraction: Seeding Up Bends away from the normal

Total Internal Reflection Occurs when a wave reflects totally back into the same medium Critical angle: wave will skim across surface and reflect back into a substance –Water= 48 degrees –Glass= 43 degrees –Diamonds= 24.6 degrees

Total Internal Reflection

The cut of the diamond favors total internal reflection. Most rays entering the top of the diamond will internally reflect until they reach the top face of the diamond where they exit. This gives diamonds their bright sparkle. Notice in this animation that a ray entering the top doesn't exit the diamond until it reaches the top surface.

A fiber optic is a glass "hair" which is so thin that once light enters one end, it can never strike the inside walls at less than the critical angle. The light undergoes total internal reflection each time it strikes the wall. Only when it reaches the other end is it allowed to exit the fiber. Fiber optic cables are used to carry telephone and computer communications. Advantages over electrical wired include: Fiber optics can carry much more information in a much smaller cable. No interference from electromagnet fields result in "clearer" connections. No electrical resistance. No hazard of electrocution if cable breaks.

Lenses Piece of glass that bends parallel rays of light so that they cross and form an image.

Two types of lenses: Converging (convex) and Diverging (concave)

Converging/Convex Lens Thick in the middle Causes parallel rays of light to converge

Converging Lens (cont’d)

Converging/Convex Lens (cont’d.) When the object is between the focal point and the lens, the image is enlarged and virtual.

Converging/Convex Lens (cont’d.) When the object is beyond the focal point, the image formed is upside down and real. The image can be projected onto a screen.

Converging Lens Ray Diagram

The Eye

Farsighted Vision Image forms behind the retina Person can see objects at a distance Corrected with converging lens(es)

Diverging Lens Thin in the middle, thick at edges Causes parallel rays of light to diverge

Diverging Lens (cont’d) Produce images that are upright, smaller and virtual (always) Used to correct nearsighted vision where the person can see well “close up.”

Nearsighted Vision

Astigmatism-Corrected with cylindrical lenses

Topics for Teachers How do mirages and rainbows form? What causes the Aurora Borealis (Northern Lights)? What causes thunder and lightning? Why does wet blacktop turn into “spectacular” colors when gasoline drips on it (iridescence)? Nuclear Fission