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Sound and Light

Physical ScienceChapter 16

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Sound

Produced by the vibration of objects. The energy from the vibrations is carried

through a medium. Sound wave spread out in all directions

from the source of the sound.

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Sound waves

Longitudinal waves. Need a medium to travel through.

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Sound waves

Travel faster through media where the particles are closer together.Particles can transmit the motion faster when

they are closer together Travel faster when the temperature is

higher.Particles move faster when they are warmer

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Intensity

Depends on the amount of energy in each wave.

Depends on the amplitude of the wave

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Loudness

The human perception of the intensity of sound waves

Measured in decibels (dB) 0 dB is the softest sound most people can

hear. 120 dB is the pain threshold – can quickly

damage your hearing

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Pitch

How high or low a sound sounds Depends on frequency

Higher pitches have higher frequencies Most people can hear sounds from 20 Hz

to 20 000 Hz.Ultrasound – above 20 000 Hz Infrasound – below 20 Hz

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Discuss

1. Which two properties of a sound wave change when the pitch gets higher?

2. Which two properties of a sound wave change when the sound gets louder

3. What are two factors that affect the speed of sound?

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Natural frequency

Most objects have one. The frequency it naturally vibrates at. When you pluck a guitar string, it produces

a standing wave at its natural frequency.

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Resonance

When an object is subjected to a sound vibrating at its natural frequency it begins to vibrate.

This amplifies the sound.

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Hearing

1. Sound waves make the eardrum vibrate.2. Vibrations pass through three small

bones – hammer, anvil, and stirrup.3. Stirrup sends waves to cochlea and

basilar membrane. Hairs near the membrane send an impulse through nerves to the brain.

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Ultrasound imaging

Ultrasound waves can travel through most materials, but some are reflected at a boundary between different materials.

Reflected waves (echoes) can be made into a computer image called a sonogram.

In order to see details, the wavelength must be smaller than the smallest parts of the object being viewed.

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Sonar

Uses reflected ultrasound waves to determine distance.Measures the time it takes for the waves to

come back. Used to calculate ocean depth and detect

fish or submarines Used by bats to “see” their surroundings

and find food.

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Discuss

To create sonograms, why are ultrasound waves used instead of audible sound waves?

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Is light a wave or a particle?

Newton -- particles. In the early 19th century, Young, Fresnel,

and others -- wave. In 1860 Maxwell -- electromagnetic wave.

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Photoelectric effect

19th century -- Hertz -- shining light on a metal plate would make it emit electrons – producing an electric current.

Kinetic energy of the emitted electrons was independent of the intensity of the light.

Didn’t fit with wave theory

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Photons

In 1905 Einstein proposed that light comes in small bundles called photons.

The energy of a photon depends on the frequency of the light, not the intensity.

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Wave Particle Duality

In some situations, light behaves like a wave.

In other situations, it behaves like a particle.

Bottom line – there probably is a single model that works all the time, but no one has figured it out yet.

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Speed of light

In a vacuum (or in air) 3 x 108 m/sNothing in the universe goes faster than this

Slower in media where the particles are closer together

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Intensity of light

Determines brightnessBrighter light has higher intensity

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Electromagnetic waves

All types travel the same speed in a given medium.

Wavelengths and frequencies vary. Short wavelengths have high frequencies. Long wavelengths have low frequencies.

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Radio waves

Long wavelength and low frequency Low energy

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Microwaves

The highest energy (highest frequency) radio waves.

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Infrared radiation

Wavelength slightly longer than visible light.Just outside the red of visible light.

Felt as heat.

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Visible radiation

The light we see. White light is a combination of all the

colors

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Ultraviolet radiation Higher frequency than visible light

More energyMore penetrating

Just outside the violet of the visible spectrum

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X Rays

Higher frequency than UV rays Greater penetrating power

Can go through skin and muscles, but not bone

Harmful in large doses

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Gamma rays

Emitted from radioactive atoms Come to Earth from space Highest frequency (and energy)

Most penetrating

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Reflection

When a wave strikes an object and bounces off.

See diagram on page 561 To study reflection, we draw a normal to

the surface.Normal means perpendicular

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Incident and reflected waves

Incident wave – before it hits the surface Reflected wave – after it reflects off the

surface

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Law of Reflection

The angle of reflection equals the angle of incidence.

Angles are measured from the normal, not the surface.

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Plane mirrors

One flat surface Image is behind the mirror and upright Image is same size as object Image is same distance from mirror as

object Image is virtual

No light rays pass through it

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Concave mirrors

Surface is curved inward Form images differently Depends on how far in front of the mirror

the object is

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Focal point

Light rays parallel to the optical axis are all reflected through the focal point.

The distance from the center of the mirror to the focal point is the focal length.

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Concave mirrors

If the object is farther from the mirror than the focal point, the image is larger InvertedFarther in front of the mirror than the object

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Concave mirrors

If the object is closer to the mirror than the focal point, the image is largervirtualuprightBehind the mirror

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Concave mirror

If the object is at the focal point, no image is formed.

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Convex mirrors

Surface curves outward Light rays parallel to axis always reflect as

if they came from the focal point Image is always

VirtualUprightSmaller than objectBehind mirror

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Colors

Objects that appear blue reflect blue light and absorb all other colors

Objects that appear white reflect all colors Objects that appear black absorb all colors

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Discuss

How is reflection from objects that appear blue different than reflection from objects that appear yellow?

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Refraction

The bending of light waves caused by a change in their speed.

Entering a slower mediumbends towards the normal.

Entering a faster mediumbends away from the normal

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Refraction

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Mirages

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Mirages

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Convex lenses

Thicker in the middle Parallel light rays are refracted towards

the center Images can be

Real or virtualUpright or invertedLarger or smaller

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Concave lenses

Thicker at the edges Bend light towards edges Image is

VirtualSmallerUpright In front of lens

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Color

Red (longer wavelength) is refracted less than violet (shorter wavelength).

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Rainbows

Rainbows are reflected light from water droplets in the air.

Different colors are refracted at different angles by the water, so they are separated.

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Discuss

1. Describe how a mirage is formed.2. If light traveled at the same speed in

raindrops as it does in air, could rainbows exist? Explain your reasoning.