Doppler Effect• The change in frequency due to the

motion of the source (or receiver) is called the Doppler effect. The greater the speed of the source, the greater the Doppler effect will be.

The Doppler Effect for Sound• The pitch of sound is higher when the source

moves toward you, and lower when the source moves away from.

• The police calculate a car’s speed by measuring the effect of radar waves. The reflected radar waves of a speeding car are higher than the emitted waves from the police.

Bow Waves• Supersonic means faster than

sound. A supersonic airplane flies into smooth undisturbed air because no sound wave can propagate out in front of it.

• A bow wave is a V-shaped wave produced by an object moving on a liquid surface faster than the wave speed

Shock Wave• A shock wave is a cone

shaped wave produced by an object moving at supersonic speed through a fluid.

• When the conical shell of the compressed air that sweeps behind a supersonic aircraft reaches the listener on the ground below, the sharp crack is heard as a sonic boom`

• Aircrafts break the “sound barrier” long before you hear the sonic boom

What is Sound?• As the tines of the tuning fork vibrate back and

forth, they disturb the surrounding air.• The disturbance is passed along to other air

molecules.• The motion of the disturbance is called a sound

wave.

The Origin of Sound• All sounds are produced by the vibrations of the

material objects.• The frequency of the vibrating source equals the

frequency of the sound wave produced.• Pitch refers to how high or how low sound

frequencies appear to be.

1) high frequency = high pitch (siren)

2) low frequency = low pitch (fog horn)

Sound waves are longitudinal waves.

Sound travels much slower than light.

Infrasonic vs. Ultrasonic

• People can usually hear pitches with frequencies from about 20 to 20,000 Hz

• Infrasonic = sound waves whose frequencies are below 20 Hertz.

1) humans cannot hear

• Ultrasonic = sound waves whose frequencies are above 20,000 Hertz

1) humans cannot hear

Media that Transmit Sound• Most sounds we hear are transmitted through air.

However, solids and liquids are generally good conductors of sound – much better than air.

• The speed of sound is different in different materials. In general, sound is transmitted faster in liquids than in gases, and still faster in solids.

• Sound DOES NOT travel through a vacuum (empty space where there is no air)

Loudness• Intensity = the amount of energy

transported through a given area per unit of time.

• The greater the amplitude of a sound wave, the larger the intensity.

• Loudness = the physiological sensation sensed in the brain.

• Loudness is subjective but does relate to intensity.

• The unit for sound intensity is the decibel.

Examples of Natural Frequency• Wind instruments

• Bottle pop instruments Fill soda bottles with different amount of water to get different tones

• Water goblet

Resonance• Resonance occurs when the frequency of

forced vibrations on an object matches the natural frequency of a nearby object and causes a dramatic increase in amplitude.

• Resonance means to resound or sound again.

• Only elastic material resonate because it needs a force to pull it back to its resting position and enough energy to keep it vibrating

Resonance Demo 1• One tuning fork forces another to resonate

- Once the first tuning fork is struck, it is forced to vibrate at its natural frequency. The energy carried by the sound wave is tuned to the natural frequency of the second tuning fork. The second tuning fork then begins to vibrate at its natural frequency. Realize the both tuning forks have the same natural frequency.

Resonance Demo 2

• When the natural frequency of the air column is tuned to the frequency of the vibrating tuning fork, resonance occurs and a loud sound results.

Beats

• Beats are produced when sounds of two slightly different frequencies are sounded together.

Light

The evidence that supported the wave theory was that light will spread out (diffract).

Currently, light is believed to have a dual nature - part particle, part wave

The speed of light is 3 x 108 m/s

The speed of light is constant when it travels through one type of medium. Light travels faster in air than in water.

Light takes 8 minutes to travel from the sun to the earth.

27.3 Electromagnetic Waves• Light is energy that is emitted by

accelerating charges – often electrons in atoms.

• The energy travels in a wave that is partly electric and partly magnetic. These waves are called electromagnetic waves.

• The range of electromagnetic waves is called the electromagnetic spectrum.

• Red light is the lowest frequency of light we can see.

• Violet light is the highest frequency light we can see.

• Infrared waves are lower in frequency than red light. Heat lamps give off infrared radiation.

• Ultraviolet waves are higher in frequency than violet light. UV rays from the sun cause sunburn

Transparent, Opaque and Shiny• Transparent materials allow light to pass

through

• Opaque materials do not allow light to pass through. When light hits opaque materials, the light energy is converted into heat.

• The electrons of shiny materials, like metals, are too far apart to transfer energy so the energy is re-emitted as light.

Shadows

• Shadows are places where light rays do not reach.

• Two types of shadows:

1) Umbra total shadow where all light is blocked

2) Penumbra partial shadow

• Solar eclipses do not occur as often as lunar eclipses because the sun is much larger than the moon.

Polarization• Polarized light travels in only one direction.

• Unpolarized light travels in many directions.

• A polarizing filter polarizes light.

• Light passes through a pair of polarizing filters when their axes are aligned, but when the filters are crossed at right angles, no light will pass through.

28.1 The Color Spectrum• Sunlight is a mixture of all the colors of the

rainbow.

• Sunlight is an example of white light. White is not a color, but a combination of all colors.

• Black is also not a color, but is the absence of light. An object appears black when all the frequencies of light are absorbed.

Color by Reflection• Most material absorb light of some

frequencies and reflect the rest. We see the reflected light.