Sound and light
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Sound and Light. Physical Science Chapter 16. 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. Sound waves. Longitudinal waves . Need a medium to travel through .

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Sound and Light

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Sound and Light

Physical Science

Chapter 16


  • 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.

Sound waves

  • Longitudinal waves.

  • Need a medium to travel through.

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


  • Depends on the amount of energy in each wave.

  • Depends on the amplitude of the wave


  • 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


  • 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


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

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

What are two factors that affect the speed of sound?

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.


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

  • This amplifies the sound.


Sound waves make the eardrum vibrate.

Vibrations pass through three small bones – hammer, anvil, and stirrup.

Stirrup sends waves to cochlea and basilar membrane. Hairs near the membrane send an impulse through nerves to the brain.

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.


  • 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.


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

Is light a wave or a particle?

  • Newton -- particles.

  • In the early 19th century, Young, Fresnel, and others -- wave.

  • In 1860 Maxwell -- electromagnetic wave.

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


  • 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.

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.

Speed of light

  • In a vacuum (or in air) 3 x 108 m/s

    • Nothing in the universe goes faster than this

  • Slower in media where the particles are closer together

Intensity of light

  • Determines brightness

    • Brighter light has higher intensity

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.

Radio waves

  • Long wavelength and low frequency

  • Low energy


  • The highest energy (highest frequency) radio waves.

Infrared radiation

  • Wavelength slightly longer than visible light.

    • Just outside the red of visible light.

  • Felt as heat.

Visible radiation

  • The light we see.

  • White light is a combination of all the colors

Ultraviolet radiation

  • Higher frequency than visible light

    • More energy

    • More penetrating

  • Just outside the violet of the visible spectrum

X Rays

  • Higher frequency than UV rays

  • Greater penetrating power

    • Can go through skin and muscles, but not bone

  • Harmful in large doses

Gamma rays

  • Emitted from radioactive atoms

  • Come to Earth from space

  • Highest frequency (and energy)

    • Most penetrating


  • 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

Incident and reflected waves

  • Incident wave – before it hits the surface

  • Reflected wave – after it reflects off the surface

Law of Reflection

  • The angle of reflection equals the angle of incidence.

  • Angles are measured from the normal, not the surface.

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

Concave mirrors

  • Surface is curved inward

  • Form images differently

  • Depends on how far in front of the mirror the object is

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.

Concave mirrors

  • If the object is farther from the mirror than the focal point, the image is

    • larger

    • Inverted

    • Farther in front of the mirror than the object

Concave mirrors

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

    • larger

    • virtual

    • upright

    • Behind the mirror

Concave mirror

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

Convex mirrors

  • Surface curves outward

  • Light rays parallel to axis always reflect as if they came from the focal point

  • Image is always

    • Virtual

    • Upright

    • Smaller than object

    • Behind mirror


  • 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


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


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

  • Entering a slower medium

    • bends towards the normal.

  • Entering a faster medium

    • bends away from the normal




Convex lenses

  • Thicker in the middle

  • Parallel light rays are refracted towards the center

  • Images can be

    • Real or virtual

    • Upright or inverted

    • Larger or smaller

Concave lenses

  • Thicker at the edges

  • Bend light towards edges

  • Image is

    • Virtual

    • Smaller

    • Upright

    • In front of lens


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


  • Rainbows are reflected light from water droplets in the air.

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


Describe how a mirage is formed.

If light traveled at the same speed in raindrops as it does in air, could rainbows exist? Explain your reasoning.

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