Sound Waves Sound pressure wave that causes vibrations

Sound Waves • Sound = pressure wave that causes vibrations – Sound “Circus” to show sound phenomena • Waves = periodic motion – Types = longitudinal (sound) & transverse (light) • Properties of Sound Wave – Amplitude (loudness) and Wavelength (pitch) – Loudness Scale (Decibel scale) – Speed in air p. 1

Recap Loncapa conversion problem! p. 2

Activity: Dancing Salt • Hold a vibrating tuning fork near a plastic membrane that is stretched across a cup and is covered with salt. What happens and why? p. 3

Activity: Singing Wine Glass • Fill a wine glass half full and rub your slightly wet finger around its edges. What happens and why? http: //www. youtube. com/watch? v=17 tq. Xgv. CN 0 E&playnext=1&list=PL 8590 BD 456 DED 1994&index=51 p. 4

Activity: Amazing Fork • Tie a string midway around a fork and then wrap the ends of the string around your index fingers and put them into your ears. Bang the fork and against the table and “listen”. Does the fork sound different with and without the string in your ears? Why? p. 5

Activity: Longitudinal and Transverse Waves Longitudinal Transverse Sound Wave Light Wave • For a wave traveling to the right: – Coils move left/right for longitudinal wave – Coils move up/down for transverse wave Figure from Tipler p. 6

ACTIVITY: Waves (wrap-up) • What was the point of this activity? • Explain how you would introduce this to your class. • Where do you see a student having difficulty with this? • What changes or extensions could you do to this activity? p. 7

Animation: Longitudinal and Transverse Waves “Snapshots” from a Movie Showing Particle Motion Longitudinal Sound Wave Particles vibrate Left/Right Transverse Light Wave Particles vibrate Up/Down For a real-time demonstration of a moving wave, see this website: http: //www. gmi. edu/~drussell/Demos/wavemotion. html p. 8

Sound Wave = Pressure Wave Vibration of speaker produces pressure waves Low Pressure High Pressure (changes with time) • Vibrations of an object cause the surrounding air molecules to start vibrating. • These vibrating molecules create pressure waves that travel in the direction of the sound. Figure from Cutnell & Johnson p. 9

Question: Sound Waves > Can you hear sound in the vacuum of outer space? Why? (a) Yes (b) No > If a sound wave travels from left to right through the air, the air molecules move as follows: (a) Travel to the right (b) Travel to the left (c) Vibrate left and right (d) Vibrate up and down p. 10

Amplitude of Wave IMPORTANT: For simplicity, we draw both transverse and longitudinal waves as shown below. Amplitude Distance Amplitude = Height of wave from baseline to peak – Larger amplitude = Louder sound – Smaller amplitude = Softer sound p. 11

Wavelength of Wavelength Distance Wavelength = Distance of wave from peak to peak – Longer wavelength = Lower “pitch” (or frequency) – Shorter wavelength = Sharper or Higher “pitch” p. 12

Question: Loudness & Pitch of Sound > What happens to the loudness of the sound wave from left to right? (a) Becoming softer (b) Becoming louder (c) Staying same volume > What happens to the pitch (or frequency) of the sound wave from left to right? (a) Becoming lower (b) Becoming higher (c) Staying same pitch p. 13

Question: Loudness & Pitch of Sound, cont. > What is happening to the sound wave from left to right? (a) Becoming softer and lower pitch (b) Becoming softer and higher pitch (c) Becoming louder and lower pitch (d) Becoming louder and higher pitch p. 14

Sound: Loudness Scale = Decibels Figure from Reese • Our ears hear a VERY LARGE range of volumes. • To measure volume, we use a logarithmic “decibel (db)” scale. – Increase of 10 d. B (decibels) = 10 times louder (“ 1” + one zero) – Increase of 20 d. B = 100 times louder (“ 1” + two zeros) p. 15

Question: How Much Louder is 60 decibels? > If a sound level increases from 60 decibels (conversation level) to 120 decibels (rock concert), how many times louder is it? p. 16

Sound vs. Light • Sound and light both travel as waves, but they are VERY DIFFERENT phenomena. • Sound = longitudinal pressure wave that travels ~300 m/s in air. • Light = transverse electromagnetic wave that travels ~300, 000 km/s. • Why do we see lightning before hearing thunder? Figure from Cutnell & Johnson p. 17

Recap Strong field S N N S N Weak field N Figure from Serway p. 18

Light Waves • Light is an electromagnetic wave – EM spectrum and types of waves • Reflection of light with planar/convex/concave mirrors – Activity with light box and bendable mirrors – Activity with periscope and ladle • Refraction of light with convex/concave lenses – Activity with light box and lenses – Eye and Eyeglasses • Dispersion of light in Prisms/Rainbows – Activity with flashlights p. 19

EM Radiation: Wave Electric Field Magnetic Field Wavelength • Electromagnetic (EM) wave has changing electric & magnetic fields. • Visible light has a wavelength of 400 to 750 nanometers (nm) Paper thickness = 0. 1 mm or 100 microns or 100, 000 nm • Velocity of EM wave = 300, 000 km/s in air. – Light travels to the moon in just over one second! p. 20

EM Radiation: Spectrum Longer Wavelength Lower Energy Shorter Wavelength Higher Energy Micro AM FM wave Radio Waves Infrared UV X-rays Gamma Rays Visible Light Red Violet p. 21

Questions: EM Spectrum > What type of EM radiation has longer wavelength and is used for communication devices? > What type of EM radiation has shorter wavelength and is used for imaging devices (particularly for the human body)? > What type of higher-energy EM radiation comes from the sun and is not sufficiently absorbed by our atmosphere, thereby damaging our skin and causing cancer? Enter a 3 -digit number. (1) Radio (2) Infrared (3) Visible (4) Ultraviolet (5) X-ray p. 22

Activity: Light Box and Mirrors Planar Mirror Concave Mirror Convex Mirror • What happens when the flexible mirror is “bent” more inwards or outwards? p. 23

ACTIVITY: Light Box and Mirrors (wrap-up) • What was the point of this activity? • Explain how you would introduce this to your class. • Where do you see a student having difficulty with this? • What changes or extensions could you do to this activity? p. 24

Reflection: Light Ray Diagrams for Mirrors • When light reflects from a planar (flat) mirror, the incoming incident angle and outgoing reflection angle are equal. • Why does a curved mirror cause light rays to “focus together” or “spread outwards. ” Hint: Think of the curved mirror as made of short mirror segments. Figure from Niculescu p. 25

Activity: Reflections for Periscope • Draw and label the two mirrors in the periscope. • Draw the light pathway that allows you to see the frog. p. 26

Activity: Convex “Spoon” Mirror • A convex mirror is bent outwards and “distorts” the image. • The reflection always appears upright and is smaller. p. 27

Reflection: Convex Mirror Application • Because images always appear smaller in a convex mirror, a larger “field of view” is possible. • Such mirrors are used for car sideview mirrors and surveillance mirrors in stores. • Do you understand why car mirrors have the warning given in the cartoon? p. 28

Activity: Concave “Spoon” Mirror When an object is outside the focus of a concave mirror, the reflection appears inverted. When an object is inside the focus of a concave mirror, the reflection appears upright and larger. p. 29

ACTIVITY: Mirror (wrap-up) • What was the point of this activity? • Explain how you would introduce this to your class. • Where do you see a student having difficulty with this? • What changes or extensions could you do to this activity? p. 30

Question: Reflections from Mirrors > Your reflection from the outside of a ladle at arm’s length appears: > Your reflection from the inside of a ladle at arm’s length appears: > Your reflection from a planar mirror at arm’s length appears: (a) (b) (c) (d) upright and smaller than real size upright and bigger inverted and smaller inverted and bigger p. 31

Fun with Mirrors: Piggy Mirage > What types of mirrors are used to create the mirage? (a) one convex + one concave (b) two convex (c) two concave p. 32

Recap Straight Wire Looped Wire p. 33

Activity: Light Box and Lenses Convex Lens Light rays initially focus together Concave Lens Light rays spread out p. 34

ACTIVITY: Light Box and Lenses (wrap-up) • What was the point of this activity? • Explain how you would introduce this to your class. • Where do you see a student having difficulty with this? • What changes or extensions could you do to this activity? p. 35

Refraction: Air/Glass Boundary AIR GLASS Denser Material • Light “bends” or refracts between different types of material (due to slower velocity in denser materials). • Light is bent closer to the perpendicular in denser materials (water, glass). Figure from Niculescu p. 36

Refraction: Light Ray Diagrams for Lenses Convex Lens Concave Lens Focal Point • A convex glass lens bends light inwards, and a concave lens bends light outwards. • For a convex lens, parallel light rays are bent to a focal point. Figure from Niculescu p. 37

Question: Convex Lens > Which convex lens is “stronger” and bends light rays more? (a) Flatter lens (b) Rounder lens p. 38

Question: Multiple Lenses > If light travels through a convex plus concave combination of lenses, how would the rays be bent as compared to one convex lens? > If light travels through two convex lenses, how would the rays be bent as compared to one convex lens? Enter a 2 -digit number. (1) Rays would be bent less. (2) Rays would be bent more. (3) Rays would be bent the same. p. 39

Lenses: Eye • Your eye uses a convex lens to focus light images onto the retina. • Notice that the retinal image is inverted!! Your brain processes this information so that objects “appear” upright to you. Figure from Niculescu p. 40

Lenses: Eyeglasses for Near- and Farsightedness Normal Eye Nearby object Far-away object Nearsighted Eye far-away objects out of focus eye lens too strong Concave Lens “weakens” eye lens Farsighted Eye nearby objects out of focus eye lens too weak Convex Lens “strengthens” eye lens Figure from Niculescu p. 41

Questions: Eyeglasses > If someone is near-sighted (i. e. , eye lenses are too “strong”), then their eyeglasses must be: (a) thinner in the middle and thicker on the edges (b) thicker in the middle and thinner on the edges > When you need to focus on an object that is very close, your convex eye lens becomes: (a) flatter (or weaker) (b) rounder (or stronger) > Without glasses, a near-sighted person can only clearly see objects that are: (a) farther away (b) closer p. 42

Refraction of light We see the sun even after it has set http: //sol. sci. uop. edu/~jfalward/refraction. html p. 43

Refraction of light Mirage Light is refracted by layers of air at different temperatures near the surface of the road. http: //sol. sci. uop. edu/~jfalward/refraction. html p. 44

Refraction of light What happens when the angle of incidence is changed? Air Glass What will happen next? p. 45

Refraction of light If the incident angle is greater than the critical angle, the ray is no longer refracted. It is reflected back into the more dense media. This is known as “ Total Internal Reflection” Incident light http: //sol. sci. uop. edu/~jfalward/refraction. html Incident light p. 46

Total Internal Reflection http: //sol. sci. uop. edu/~jfalward/refraction. html p. 47

Dispersion: “Spreading” of Visible Light Figure from Cutnell & Johnson Figure from Halliday • When light is refracted, blue light bends more than red. • Refraction therefore causes light to “spread” or disperse into its component colors, just as you see for sunlight hitting a prism. p. 48

Activity: Additive Colors (mixing light!) Green Blue Red • Red light + Green light = ________ • Green light + Blue light = ________ • Blue light + Red light = ________ • Red light + Green light + Blue light = ________ p. 49

Dispersion: Rainbow • Refraction also causes rainbows to form, where the sun’s light is refracted inside raindrops and spreads to form the rainbow colors: red, orange, yellow, green, blue, indigo, violet. Figure from Halliday p. 50

Questions: Rainbow > When looking at a rainbow, the sun is located: (a) directly in front of you (b) to your side (c) behind you > What color of light is bent (or refracted) more by the raindrops? (a) red (b) yellow (c) blue p. 51

Recap (a) (b) p. 52