Wave Interactions

WAVES Waves can interact with other waves, other objects, or change medium (travel through different substances) When a wave meets a new surface or boundery, the wave reflects When a wave passes the edge of an object or passes through an opening, the wave diffracts When a wave passes from one medium to another at an angle, it will refract

REFLECTION of LIGHT Reflection is the bouncing back of a wave when it hits a surface or boundary that is hard enough that it can’t travel through. Examples: water waves bouncing off of a boat, images in a mirror (and glass or very still water), echos. Light obeys the “Law of Reflection” The angle of incidence will equal the angle of reflection

REFLECTION (Law of Reflection) Flat Surfaces (like mirrors) Every single incident ray will hit at the same angle, and bounce off at the same angle Rough Surfaces (like water or frosted glass) Every single incident ray will hit at a different angle, and scatter when they reflect

Diffraction (Light, Sound and Water) Diffraction: When waves pass the edge of an object, or reach an opening, they will bend or spread: spread out as if they were created there (for small openings) bend around the object (when waves pass an edge)

Diffraction (Light, Sound and Water) Diffraction: When waves pass the edge of an object, or reach an opening, they will bend or spread: Wind “bends” around this bottle to blow out the candle

Diffraction (Light, Sound and Water) http://phet.colorado.edu/en/simulation/wave-interference http://phet.colorado.edu/en/simulation/sound

REFRACTION Refraction: The bending of waves when they pass from one medium to the next Occurs when waves reach a new medium (that it can travel through) at an angle. Medium = the material the wave is traveling through Each time a wave reaches a new medium, they are deflected (change direction) as they change speed The pencil looks broken because of refraction: The light traveling to/from the part of the pencil out of the water does not ever change speeds. The light that travels to/from the part of the pencil in the water has to change speeds (and changes angles) as it enters the water. This causes light from each end to reach your eye at different angles.

INDEX OF REFRACTION--Examples Different substances have different angles of refraction ie: Waves bend/change directions at different angles when they enter that specific substance This is because waves travel at different speeds in different substances The slower light travels through an object, the more light will “bend” and change direction when it enters it

INDEX OF REFRACTION--Examples A laser and a glass cube Only wavelength, so it can’t separate into different wavelengths - That single wavelength slows down in the glass cube to the left White Light and a PRISM White light is made of ROYGBIV, many different wavelengths. It can separate into different wavelengths - Each wavelength slows down at a different rate, separating in the prism to the right

REFRACTION OF LIGHT—PRISMS Light slows down when it enters a prism and “bends down” when it strikes at this angle. Each wavelength of light slows down and bends differently. Violet end slows down more than the red end, so it bends more. When light exits the prism, it speeds back up and bends down again.

REFRACTION OF LIGHT Water will do the same thing… it will refract light. When light enters water, it will slow down (or bend) This causes objects underwater to appear as if they are somewhere else. The light is tricking your eye.

REFRACTION OF LIGHT Water will do the same thing… it will refract light.

REFRACTION OF LIGHT So this is a polymer ball. Why does it disappear in water? It has the same angle of refraction as water, so the light never changes direction…

WAVE INTERACTIONS Sometimes waves will interact with other waves… Interference: When several waves are in the same location, they combine to produce a single, new wave that is different than the original wave Once the waves pass through each other, they will return to their original shape and continue on their path. 2 types Constructive (add to each other) Destructive (cancel each other out)

Constructive Interference When the CREST of one wave overlaps the CREST of another wave the waves reinforce each other These wave are also called “in phase” When this happens, the amplitude INCREASES

Constructive Interference http://www.acs.psu.edu/drussell/demos/superposition/superposition.html

Constructive Interference

Destructive Interference When the CREST of one wave overlaps the TROUGH of another wave, the resulting wave has a SMALLER amplitude than the larger of the two original waves When this happens, the waves can even cancel out These wave are also called “out of phase”

Destructive Interference

Wave Interference: Both Types http://www.kettering.edu/physics/drussell/Demos/superposition/superposition.html

LENSES A lens is an optical device with symmetry that transmits AND refracts light We have 2 types of lenses: Convex (converging) and concave (diverging) We can either use: A Simple Lens: one lens Examples: reading glasses, magnifying glass, your retina A Compound Lens: uses more than one lens Examples: telescope, microscope, cameras, etc

LENSES Lenses can either be: Convex  (converging) OR Concave  (diverging)

LENSES An eye that can see at 20/20 vision will allow light to focus directly at the back of the retina. If you don’t see 20/20, your focal point is not directly on the retina, and you get a blurry image

LENSES To correct nearsighted eyes (myopia), you use concave lenses They move the focal point backward To correct farsighted eyes (hyperopia), glasses will be made with convex lenses They move the focal point forward.