Properties of sound

Properties of sound• Longitudinal waves travel at different

speeds depending on the medium (air @ 25oC 346m/s, water 1490 m/s)

• Denser the medium, the faster it travels, energy can transfer faster through molecules closer together.

• Loudness of a sound– depends on intensity (the amplitude of the

wave. Higher amplitude … louder sound) – Measured in decibels (dB), talking is about 50

dB over 120 dB is painful.

Loudness/intensity

Properties of sound• Pitch (high or low) is related to

frequency. Higher frequency = higher pitch

• (Also related to wavelength, indirectly. Bigger wavelength = lower pitch)

• Movie: pitch• Humans hear between 20 Hz and

20,000 Hz. Below that range is infrasound, above is ultrasound.

Musical Instruments• Rely on standing waves to make sound …

only certain wavelengths• Fundamental frequency: the lowest freq.

standing wave made by the instrument. It can be determined by the size of the instrument itself.

• String instr.: the wavelength of the fundamental frequency is twice the length of string.

• Changing the size of the string changes the wavelength and the frequency.

• Movie: fundamental freq.

Musical Instruments• Wind/brass instr.: change size of wave by

opening closing holes, changes size of tube that standing wave is made in.

• Wavelength is related to size of the tube. • In an open tube, you get ½ the

wavelength, so multiply by 2 to find the fundamental frequency

• In a closed end tube, you get ¼ the wavelength, so multiply by 4 to find the fundamental frequency

Musical Instruments• Strings make ½

wave, so mult. by 2.

• Open tubes make ½ wave so

mult. by 2• Closed tubes

make ¼ wave so mult. by 4.

Musical Instruments• Most instruments vibrate at the

fundamental frequency plus other standing waves…called harmonics.

• That’s why everyone’s voice sounds different and why instruments sound different when they play the same note.

• Vibrations can cause other things to vibrate at same frequency … called resonance.

• Movie: resonance• Waves interfere, called beats

How we hear• Outer ear focuses

waves, passes them to ear drum

(middle ear) which passes them to three bones (hammer, anvil,

stirrup) to the cochlea (inner ear) which resonates at certain frequencies.

• Hair cells pick that up and transmit to brain.

Ultrasound and Sonar• Use speed of sound in medium and

time to figure out how far away something is (e.g. bottom of

ocean floor)• Movie: sonar• Ultrasound...frequencies beyond our

hearing used in medicine (babies).

Light: Waves and Particles• Like a wave … light shows interference (reflect,

refract, diffract, standing waves) does not require a medium

• Like a particle, light can move electrons• So light is described as having a dual nature• Comes in “packets” called photons (more like

bundles of energy)• Amount of energy directly proportional to

frequency. Higher freq. = more energy.• Indirectly proportional to wavelength. Bigger

wave = less energy• Speed of light 3 x 108 m/s in vacuum• Speed changes for medium, but not significantly

Electromagnetic spectrum• all possible kinds of light, classified

according to frequency and wavelength• Radio/TV Waves …lowest energy,

used in communication• Microwaves…used in cooking and

communication• Infrared Waves… felt as warmth• Visible light…we can see ROYGBIV• Ultraviolet waves…cause sunburn• X Rays… used in medicine• Gamma Rays… used in medicine• Stars, like the sun, give off all of these

forms of energy

Light Rays• use lines called rays to see

where light goes• rough surfaces reflect the rays in

all directions• smooth surfaces reflect the

light in one direction• the angle light rays come in at

equals the angle it’s reflected at.• Angle of incidence equal the

angle of reflection (Law of Reflection)

• Angles are measured between the ray and an imaginary line perpendicular to the surface called the normal.

• movie: Law of Reflection

Mirrors• Plane (flat) mirrors form virtual

images inside of the mirror. Virtual images are from where

light appears to come from, not the actual path.

• Curved mirrors distort the image, but make real images. Real images form when light rays come together at a certain point

• movie: comparing real and virtual images

Color• Light hits objects. • Many wavelengths are absorbed

by the object and not seen by you.

• Wavelengths that are reflected are seen by you as color.

• Technically, you don’t see something, you see light bouncing off of something.

• Colors can add together to make new colors (e.g. light. White light = all colors of light, black = absence of light)

• Colors can subtract to make new colors (e.g. pigments, black paint = all colors of paint, white paint = absence of color)

• movie: additive color mixing

Refraction• Light bends as it goes

from one substance (medium) to another because it changes speed

• Compare the bend to the normal, just like reflection

• Light bends towards the normal when it enters a denser medium and slows down

• Light bends away from the normal when it enters a less dense medium

and speeds up

Lenses• Mirrors and lenses can be curved out

(convex) or can curve in (concave … like a cave).

• movie: converging and diverging lenses

Light Rays• There’s an angle called the critical

angle, where light will not go back out of something and just keeps bouncing back around inside.

• Diamonds are cut like this to keep the light from leaving and fiber optic cables are made so that light does not escape.

• movie: Total Internal Reflection