Sound and LightUnit 9

Chapter 12

Good vibrations

All sounds are caused by something

that vibrates.

1. When these vibrations collide with air

molecules (or another medium) –

sound waves are formed

2. Sound waves are compressional waves

- they have two regions called

compressions and rarefactions.

Compressional Waves

Rarefaction –air molecules pulled apart

Compression –air molecules pushed together

Medium

The type of matter that the sound waves travel through

1. A sound wave’s speed depends on the substance – solid liquid or gas.

Sound need a medium ––it cannot travel in a vacuum!

2. Sound travels more quickly through solids and liquids because their particles are closer together than in a gas

How Much is faster?

AIR 347 m/s

CORK 500 m/s

WATER 1,498 m/s

BRICK 3,650 m/s

ALUMINUM 4,877 m/s

Turn on the Heat!

3. As a medium’s temperature increases, the molecules move faster and bump into each other more often so it conducts sound faster! –

Properties of Sounds

Speed of sound

• The speed of sound depends on the medium.

• Sound waves travel faster through liquids and solids than through gases.– The particles are much closer in liquids and

solids so the vibrations are transferred much faster from one particle to the next.

– EXCEPTS – Solids such as rubber dampen vibrations so that sound travels very slowly. Materials like this can be used for soundproofing!

How Loud is It?

A. The amount of energy a wave carries

corresponds to its amplitude, which is

related to the density of the particles in the

compressions and rarefactions

1. Intensity – The amount of energy that flows through a certain area in a specific amount of time

2. Loudness – human perception of sound

intensity

Intensity

Intensity of a sound describes the loudness at a particular distance from the source of the sound.

Measure It!

3. Sound intensity is measured in decibels a) Decibels are measured in a

logarithmic scale and shown by the symbol db

b) Increasing intensity by 3 db is 2 times as loud. 63 db is 2 X 60 db

c) Increasing intensity by 10 db is 10 times as loud. 70 db is 10 X 60 db

Common Noises

1. weakest sound heard - 0 dB

2. normal conversation at 3-5 ft - 60-70 dB

3. dial tone of telephone - 80 dB

4. city traffic inside car - 85 dB

5. regular sustained exposure may cause

permanent damage - 90-95 dB

6. power mower - 107 dB

7. power saw - 110 dB

Getting Really Loud

1. regular sustained exposure may causepermanent damage - 90-95 dB2. average Ipod on 5/10 setting - 94 dB3. bass drum rolls - 106 dB4. amplified rock music at 4-6 ft. - 120 dB5. Pain begins 125 dB6. pneumatic riveter at 4 ft. - 125 dB7. jet engine at 100 ft. - 140 dB8. rock music peak - 150 dB9. loudest sound that can occur - 194 dB

Pitch

B. Pitch – how low or high a sound seems to be

1. Frequency is the number of compressions or rarefactions of a sound wave that pass per second – humans hear about 20 – 20,000 Hz

2. Ultrasonic – over 20,000. Is outside the range of human hearing.

3. Infrasonic or subsonic – below 20 Hz may be felt like a rumble but not heard. It is any frequency above human hearing range.

Doppler Effect

C. Doppler effect – Change in pitch or

frequency due to a moving listener or

source

Music

Music

• Sounds that are deliberately used in a regular pattern

• Natural Frequency ––the frequency at which the material vibrates

• Resonance ––The ability of a medium to vibrate by absorbing energy at its own natural frequency

Sound Quality

• The difference between sounds of the same pitch and loudness is sound quality

• Overtone ––vibration with a frequency that is a multiple of the fundamental frequency

Musical Instruments

• Devices used to make musical sounds– Strings– Sound produced by plucking, striking, or

drawing a bow across tightly stressed strings bow strings

– Brass and woodwinds ––air vibrations in a resonator or hollow chamber that amplifies sound – pitch determined by length of air tube

– Percussion ––struck shaken rubbed or brushed struck brushed

• Beats ––pulsing vibration in loudness pulsing loudness

Hearing and the Ear

Mechanics of the ear

• The ear is divided into 3 parts or regions:– Outer– Middle– Inner

Mechanics of the ear cont.

• Sound enters through the outer ear and down the ear canal. The ear canal ends with the eardrum (thin flat piece of tissue).

• When sound hits the eardrum, it vibrates.• These vibrations pass through the small

bones of the middle ear (Hammer, anvil, and stirrup)

• When vibrations reach the stirrup, the stirrup strikes a membrane at the opening of the inner ear.

Mechanics of the ear cont.

• The waves in the inner ear go through the spiral-shaped cochlea (also called the basilar membrane).– Different parts of the basilar membrane vibrate at

different natural frequencies.

• As the waves pass through the cochlea, they resonate with specific parts of the basilar membrane.

• Hairs near this area stimulate nerve fibers which send an impulse to the brain.

• The brain interprets this impulse as a sound with a specific frequency.

Using Sound

Sound

• Used for entertainment, warning signals, information

• Acoustics ––study of sound to create a good listening environment

• Echolocation ––locating objects by sending out a signal and interpreting the waves reflected back

Sound

• Sonar ––a system that uses the reflection of underwater sounds waves to locate objects underwater

• Ultrasound ––used in medicine to diagnose, monitor, and treat many conditions– Can produce images of internal

structures– Can treat certain medical problems

such as kidney stones

The Nature of Light

Thomas Young

• In 1801, Thomas Young devised an experiment to test the nature of light. He realized that the pattern created is similar to the pattern caused by water waves interfering such as the ripple tank.

Light can be modeled as a wave

• We have learned light waves can be described as transverse wave which do not require a medium.

• They are also called electromagnetic waves because they consist of changing electric and magnetic fields.

• Light waves can:– Reflect in a mirror– Refract through a lens– Diffract passing through a narrow opening

Wave model does not explain all observations

• When light strikes a piece of metal, electrons get excited and may fly off the metal’s surface.

• Experiments show that not all colors of light can knock the electrons off the metal.– Dim blue light can knock some electrons off– Bright red light cannot knock any electrons

off

• How can we explain this observation?

Light can modeled as a stream of particles

• One explanation to the effects of light striking a metal plate is so assume that the energy of light is contained in small packets.

• These packets are called photons• Photons are particles of light

– They do not have mass – They are more like little bundles of energy– Unlike energy in a wave, the energy in a

photon is located in a particular place

The light model used depends on the situation

• Light can be modeled as either waves or particles.

• Some effects such as:– Interference of light are explained as waves– Light exciting electrons off a metal plate are

explained as particles

• The particle model can also explain how light can travel across an empty space without a medium

• Light can be considered to have a “dual nature.”

Energy of light is proportional to frequency

• Remember light is a form of energy!• Each photon of light carries a small

amount of energy.• The amount of this energy is proportional

to the frequency of the corresponding wavelength.– Photon of red light carries an amount of

energy that corresponds to the frequency of waves in red light (4.5 x 1014 Hz)

Speed of light depends on the medium

• In a vacuum, all light travels at the same speed “c”

• Speed of light is very large– 3 x 108 m/s (~186,000 mi/s)

– It is the fastest signal in the universe

– Nothing can travel faster than the speed of light

Speed of light depends on the medium

• Light also travels through transparent mediums, such as air, water, and glass

• When passing through a medium, it travels slower than it does in a vacuum.

Brightness of light depends on intensity

• Intensity is the rate at which light or any other form of energy flows through a given area of space.– It depends on the amount of light or

the number of photons or waves

– Intensity decreases as the light spreads out in spherical wave fronts.

Electromagnetic Spectrum

Sunlight contains UV light

• The invisible light just beyond violet light falls into the UV portion of the spectrum.

• It has higher energy and shorter wavelengths than visible light.

• 9% of energy emitted by the sun is UV• Due to the high energy, it can pass

through thin layers of clouds causing you to get a sunburn on overcast days.

X ray and gamma rays used in medicine

• X rays have wavelengths less than UV with higher energy

• Gamma rays have the highest electromagnetic energy waves with the shortest wavelength

• X rays are helpful in diagnostic in medicine but can be dangerous to the body.

• Both of these waves can kill living cells or turn them into cancerous cells– Gamma rays can also be used to treat cancer by

killing the diseased cells.

Infrared light can be felt as warmth

• Infrared (IR) light has wavelengths slightly longer than red light

• IR light from the sun or heat lamp warms you

• Used to keep food warm in restaurants without continuing to cook it.

• Devices and photographic film are sensitive to IR light

• You can detect IR radiation areas of different temperature. Therefore, mapping the area

Microwave for cooking and communication

• Microwaves are centimeters longer than IR waves

• Microwave are reflected by metals but easily transmitted through air, glass, paper, and plastic

• Microwaves are also used to carry telecommunication signals

Radio waves used in communications and radar

• Radio waves are longer than microwaves

• Radio waves range from 1/10th of a meter to millions of meters

• This portion includes TV signals, AM and FM radio signals, and other radio waves

Radio waves used in communications and radar

• Air traffic control towers at airports use radar to determine the locations of aircrafts– Antennas at the control tower emit radio

waves, or sometimes microwaves, out in all directions

– When the signal reaches an airplane, a transmitter on the plane sends another radio signal back to the control tower indicating the planes location and elevation above the ground.

Radio waves used in communications and radar

• Radar is also used by police to monitor the speed of vehicles– The radar gun fires a signal of known

frequency at a moving vehicle then measures the frequency of the reflected waves

– Because the vehicle is moving, the reflected waves have a different frequency and use the Doppler effect to determine the speed.

Reflection and Color

Reflection of Light

• A light ray is a model of light that represents light traveling through space in an imaginary straight line– It is the same as the direction of wave travel

in the wave model or the path of photons in the particle model of light

• Geometrical optics is the study of light in circumstances where it behaves like a ray.– Using the light rays, the path of light can be

traced in ray diagrams

Reflection

• When a light wave hits an object and bounces off it is reflected

• Law of Reflection –angle of incidence = angle of reflection

• Regular reflection –reflection from a smooth surface

• Diffuse reflection –reflection from a rough surface

Law of reflectionnormal

Mirrors

• Flat mirrors form virtual images from reflection

• Virtual image is an image that forms at a point from which light rays appear to come but do not actually come.

Mirrors

• Curved mirrors can distort images• Mirrors that bulge out are called convex

mirrors• Indented mirrors are called concave

mirrors

Concave mirrors create real images

• Concave mirrors are used to focus reflected light

• It can form one of two kinds of images– A virtual image behind the mirror or a

real image in front of the mirror.– Real image is an image of an object

formed by many light rays coming together in a specific location

Telescope use curved surfaces to focus light

Colors

• Determined by wavelengths of light an object reflects

• Objects appear white because they reflect all colors

• Objects appear black because they absorb all colors

Mixing colors

• Pigment –colored material that absorbs some colors and reflects other

• Primary colors of light –red, green, blue

• Primary pigments –magenta, cyan, and yellow

Mixing colors

• When mixing light, colors are additive –they combine to form white

• When mixing pigment they are subtractive –they combine to form black

Refraction, Lenses, and Prisms

Refraction of Light

• Light changes speed when it passes from one material to another -can cause light to bend

• Index of refraction – indicates how much light slows down, the greater the index, the more light slows down –greater the index, the more the bending

Total Internal Reflection

• Total internal reflection is the complete reflection of light at the boundary between two transparent mediums when the angle of incidence exceeds the critical angle

• Really due to refraction –light strikes a surface between two materials and is completely reflected back into the first material

• Used for fiber optics

Fiber Optics

Lenses

• Lenses rely on refraction– Light traveling at an angle through a flat

piece of glass is refracted twice – once when it enters the glass and again when it reenters the air.

– Lens are a transparent object that refracts light rays, causing them to converge or diverge to create an image

• Converging lens – bends light inward• Diverging lens – bends light outward

Lenses can magnify images

• Magnification is a change in the size of an image compared with the size of an object– It usually produces an image

larger than the object - - but not always!

Eye depends on refraction and lenses

• Light enters the eye and is focused on the retina

• Retina is made of types of cells that absorb light

• Cones –distinguish color and detailed shape

• Rods –Good in dim light• Color Blindness occurs when one or

more sets of cones don’t work properly

Dispersion and Prisms

• Prisms –separate white light into visible spectrum based on λ– A prism is a transparent block

with a triangular cross section

• Refraction of light through air of different densities can cause a mirage

Dispersion and Prisms

• Dispersion is an effect in which white light separates into its component colors– The light separates into different colors

because of differences in the wave speed

Rainbows

• Rainbows –caused by water droplets refracting white light– They are caused by the

dispersion of the sun and the reflection of water drops