Unit 7 – Waves – Sound & Light
What are waves?
waves – repeating patterns that carry only energy not matter
= ripples in water after throwing a rock in the water
= throwing a ball is not a wave – the ball is made of matter
2 types of waves
mechanical waves – use matter to transfer energy  the matter transferred through mechanical waves
= medium
-- ripples on the pond  energy
-- water in the pond is the medium for transferring energy
2 types of mechanical waves
transverse – energy from the wave causes the medium to move up and down or back and forth
- crest – high points on the wave
- trough – low points on the wave
longitudinal or compressional – energy from the wave causes the medium to move forward and
backward in the same direction in which the wave travels
sound waves – a type of compressional wave produced from vibrating energy. the vibrations push
molecules in the air that were close to the thing producing the vibration forcing all the molecules close
together = compression
electromagnetic waves – don't use matter to transfer energy – radio waves, x-ray waves & microwaves
- transverse waves produced by the movement of electrically charged particles  they can travel
through liquids, solids and gases – can also travel in outer space = no matter
- a wave's amplitude is related to the energy that the wave carries = waves of greater amplitude (higher
rises & falls) carry more energy and waves of less amplitude carry less energy. Ex. Tsunamis – high
amplitude waves – beach waves – low amplitude waves
frequency – the number of wavelengths that pass a given point in 1 second. Measured in units called
hertz (Hz) => long wavelengths = less frequency
wave speed – depends upon the medium through which it is traveling
light waves travel much faster than sound waves
light waves are electromagnetic waves (which don't) use matter to move energy – travel 300 million m/s
sound waves – mechanical waves which use air as the medium to move energy and travel much slower
through air – 340 m/s. Sound waves also travel faster in solid mediums than liquid but move faster in
liquid than gas
Speed = frequency x wavelength
Interactions of Waves
- waves traveling in the same medium move at a constant speed and in a constant direction
reflection – bouncing back of a wave after it strikes a boundary that doesn't absorb all the wave's energy
(smooth surfaces reflect better than rough surfaces). Ex. reflected light in a mirror to see yourself or an
echo
refraction – when waves pass from one medium to another they bend – speed changes as the waves
travel from one medium to another (ex. straw in a glass of water)
diffraction – bending of waves around the edges of an obstacle – a new series of waves form when the
original waves strike an obstacle
interference – when 2 or more waves arrive at the same place at the same time to interact or combine
to produce a single wave (ex. raindrops on water)
What is Sound?
- anything that vibrates produces sound
- travels as a longitudinal wave
- speed of sound is determined by the temperature, elasticity and density of the medium through which
the sound travels.
- lower temps – sound moves more slowly
- sound travels faster through solids than gases (ex. Native American) because solids are more elastic
(particles move back and forth quickly)
- slower in the denser materials because it has more inertia to overcome
- properties of sound
pitch – depends on how fast the particles of a medium vibrate – the higher the frequency, the higher the
pitch. Sounds with frequencies higher than 20,000 vibrations (hertz) per second are ultrasonic and are
above the range of human hearing (dogs, cats & porpoises). Sounds with frequencies below 20 hertz –
infrasonic (elephants)
Doppler Effect - pitch is higher as an object moves closer to you and then the pitch is lower as it moves
away
intensity – the amount of energy carried by a wave in a certain amount of time.
- determines the loudness of a sound
- scale to measure the relative intensity of sounds. Based on the unit – decibel
0 decibels can barely be heard. 120 decibels = thunder. 170 decibels = jet engine
Applications of Sound
sonar – high frequency ultrasonic waves – Sound Navigation And Ranging used to find oil and minerals
or ultrasound imaging
How do you hear?
outer ear acts as a funnel for sound waves to strike the eardrum causing it to vibrate which enter the
middle ear – contains the smallest bones in the body – the hammer picks up the vibrations from the
eardrum and passes the vibrations to the anvil which transmits the vibrations to the stirrup which sets
another membrane vibrating that transmits vibrations to the liquid filled inner ear which channels
vibrations to the cochlea – contains the nerve fibers that goes to the brain where the electrical impulses
are interpreted as sound
Electromagnetic Waves
- transverse waves that consist of an electric-magnetic field
- charges particles = electron – create electric or magnetic fields that vibrate. Ex. electrons moving back
and forth in an antenna create radio waves
- speed of all electromagnetic waves = 300 million meters/second in a vacuum. Ex. light from the Sun
travels 150 kilometers to Earth in 8.3 minutes
Electromagnetic Spectrum
- electromagnetic waves are arranged in order of wavelength and frequency
Radio waves
AM – amplitude modulation
FM – frequency modulation
also used in medicine – MRIs
microwaves – the highest frequency radio waves. used for cell phones, weather forecasting to locate
storms. Not as easily blocked by structures as radio waves
radar – radio detecting and ranging. used to locate objects and monitor speed. Weather forecasts use
them to locate and track storms by recording the speed of reflected radar waves.
-- Doppler effect
Infrared Rays – cannot be seen but are felt as heat given off by all objects
Visible Spectrum – almost ½ of Sun's energy is given off as visible light. Essential for photosynthesis
Ultraviolet Rays – cause your cells to produce Vitamin D. Tanning protects body from ultraviolet's
harmful rays. Ozone protects by absorbing most of Sun's ultraviolet rays
X-Rays – energy of x-rays is great enough to pass easily through many materials (ex. skin) but denser
materials absorb (ex. bone). Exploding stars give off most of their energy as X-rays
Gamma Rays – EM waves with the highest frequencies and shortest wavelength. Have the highest
energy of the EM spectrum – can penetrate up to 3 meters of concrete
Light
Reflection  bouncing of light waves off a surface = mirror. Smooth surfaces reflect better than rough
surfaces  rough surfaces deflect waves in many ways instead of straight back to you (lake and
throwing stone in lake)
Mirrors  most are flat (like cosmetic mirrors) but some are curved (circus mirrors, spoon) – the wave
hit different places on the curve and bounced off in different directions = distorts the image – flat mirror
= waves bounce back straight toward you – doesn't distort image
concave mirror  curves like the inside of a spoon. Can be useful – headlights, flashlights
convex mirror  curves like the outside of a spoon. Can also be useful – automobile rearview mirror or
in department stores or Circle K.
Refraction – the mending of a wave as it moves from one material to another  p. 232  light waves
pass from air to water which slow down once they hit the water. When the light waves hit the water
they hit at an angle and change speed = distorts image = light waves are refracted. Light waves also
refract when they pass from air  glass or plastic  lens
convex lenses  magnifying lens – make objects look bigger (microscopes, cameras, telescopes, far
sighted glasses)
concave lenses  helps you see far off objects – glasses for nearsightedness
Color and Reflected Light – most objects don't give off light – most are opaque = object that doesn't
allow light to pass through. Like the shoes p. 237 – light doesn't pass through. The shoes reflect light to
your eyes. They absorb some of the light and reflect what you see. Takes the color of the light it reflects
(red shoe absorbs all color except red = red shoe). Snow reflects all the light that hits it. White =
presence of all colors. Black = absence of all colors (space). Black absorbs
Color and Transmitted Light
transparent objects (windows) transmit = allow light to pass through
translucent objects (wax paper, frosted glass) – transmitted light is scattered producing a fuzzy image
How you see – light enters the eye through an opening = pupil – the iris (colored area surrounding the
pupil) controls the amount of light that enters the pupil – the light entering the eye is refracted by the
cornea and focused on the retina (the image is upside down and smaller than the actual object) – the
image is transferred to and interpreted by the nervous system. The retina contains nerve cells called
rods and cones
rods – sensitive to light and dark
cones – sensitive to a particular color
The lens – makes adjustments for focusing on objects at different distances