Sound & Light

Chapter 16, Section 1 - Sound                                                                                                                                                  

Sound Wave - A longitudinal wave that is caused by vibrations and travels through a medium.

The speed of sound depends on the medium.
  • Material - More dense materials carry sound faster than less dense materials.  (Sound travels faster in water than air.)
    • Some solids dampen vibrations. (ex - rubber)
  • Temperature - Particles move faster at higher temps - so sound travels faster at higher temps.
Loudness is determined by intensity.
Intensity - Rate at which a energy moves through a given area.
Greater Intensity => Louder Sound.
10x intensity = 2x sound
Intensity is measured in decibels (dB).
0 dB = Threshold of Hearing (quietest sound humans can hear)
120 dB = Threshold of Pain (pain, headaches; permanent damage with extensive exposure)

Pitch - The highness or lowness of a sound.
Determined by frequency.
Higher frequency = Higher Pitch
Range of Human Hearing: 20 Hz to 20,000 Hz
Ultrasound - above 20,000 Hz
Infrasound - below 20 Hz

Musical Instruments produce sound by vibration of...
  • strings
  • air columns
  • membranes
Strings (like on a guitar) make standing waves.
Nodes - ends of the string
Antinode - center of the string
Primary Standing Wave (one standing wave, whole length of string)
Wavelength = 2x length of string
Fundamental Frequency = the frequency of this wave
Resonance - When two objects naturally vibrate at the same rate.
Forced Vibration - When a vibrating object causes another (adjoining or connected) object to vibrate.
Natural Frequency - The frequency an object is most likely to vibrate at.
Depends on an instrument's shape, size, mass, and material(s) it's made from.
A guitar works because the strings create a forced vibration on the body of the guitar, causing it to vibrate at its natural frequency.  The vibrations are passed on to the air, thus transmitting the sound.

The human ear senses vibrations in the air, amplifies them, and transmits signals to the brain.
  • Outer Ear - Collects sound waves and directs them to the ear drum.
  • Middle Ear - Pass the vibrations down the ear bones to the inner ear (cochlea).
  • Inner Ear - Converts vibrations to nerve signals; sends signals to the brain.
    • Different parts of the cochlea vibrate at different natural frequencies.
      • Let's you hear different frequencies.
Reflected sound waves can be used to determine distances and create images.
High frequency sound waves can pass through most materials.
Some waves are reflected when passing from one material to another.
Reflected waves can be made into a computer image - sonogram.
Sound waves must be smaller than objects being viewed.
ex - 15,000,000 Hz waves have a wavelength less than 1mm - so can be used to see things 1mm across.

Sonar - Sound Navigation and Ranging.  A system that uses reflected sound to measure distances.
Ship send out a sound pulse, and measures the time it takes to reflect off the ocean floor and return.
d = vt    distance = speed x time    (Time must be divided by two - sound goes down and comes back!)

Homework:
    Pg. 551, #1-5.

Chapter 16, Section 2 - The Nature of Light                                                                                                                               

Light can be described as a wave or a stream of particles.

Wave Model of Light
  • Electromagnetic waves - transverse waves that do not need a medium.
    • Made of changing electric and magnetic fields.
  • Light waves create interference patterns.
    • Passing a beam through two slits, then projecting them on a screen gives this pattern (diagram) =>
  • Light waves reflect in a mirror, refract in a lens, diffract when going through a narrow opening.
    Problem:
Dim, blue light can knock electrons off of a metal plate.
Bright, red light cannot.
According to wave model, the bright red light should have more energy!
Particle Model of Light
  • Photon - A "packet" (unit, quantum) of light.
    • Photons have no mass - they are just little packets of energy.
  • A photon of blue light has enough energy to knock electrons out of a metal plate.  A photon of red light doesn't.
  • Particle model explains how light can travel through space (without a medium).
Light has a dual nature - it can behave like a wave OR a particle.

Energy of light is proportional to frequency.
More Energy = Higher Frequency
Speed of light depends on the medium.
In empty space = 3x108 m/s.
In transparent materials, it slows down.

Brightness depends on intensity.
Intensity depends on the number of photons that pass through a given area per second.
Light spreads out as it moves away from the source - same amount of energy appears weaker because it it spread out.

Electromagnetic Spectrum - includes waves at all possible energies, frequencies and wavelengths.

Radio Waves - longest (1/10th of a meter to thousands of meters)
    Communication
    Radar - Radio Detection and Ranging
Microwaves (wavelengths in cm's)
    Communication
    Cooking - Absorbed by water, fat and sugar molecules.  Converted to KE.
Infrared (Just outside red end of visible light)
    Can be felt as warmth.
Ultraviolet (Just outside violet end of visible light)
    Absorbed by ozone layer - can cause sunburn.
X-Rays & Gamma Rays - shortest wavelength, highest energy (Gamma)
    X-Rays - Used to see inside things.
    Gamma Rays - Used to kill cancer cells.
    Both - very high energy, can damage/kill living cells.

Homework:
    Pg. 558 #1-4.


Chapter 16, Section 3 - Reflection and Color                                                                                                                             

Every object reflects some light and absorbs some light.

Light Ray - An imaginary line that goes in the same direction as light travels.  (Represents a light wave or stream of photon particles.)
Ray Diagram - A drawing that shows the path of light rays.

Rough surfaces reflect light in many directions => diffuse reflection.

Smooth surfaces reflect light in one direction.
Law of Reflection - The angle of incidence = The angle of reflection.

Mirrors reflect light by the Law of Reflection.
  • The reflected light reaches your eyes to make an image.
  • The type of image depends on the type of mirror.
Virtual Image - An image formed from the apparent path of light waves (not the actual path).
- appears behind the mirror
- cannot be projected on a screen
- flat mirrors form virtual images

Curved Mirrors distort images (make them appear bigger or smaller).
Convex Mirror - A mirror that is curved outwards.  (Makes a smaller image.)
Concave Mirror - A mirror that is curved inwards.  (Can make a larger or smaller image.)

Real Image - An image formed from the intersection of light rays.
- appears in front of the mirror.
- can be projected on a screen.
- concave mirrors form real images.

The colors that you see depend on the wavelengths of light that reach your eyes.
400 nm - Blue Light            700 nm - Red Light
  • Objects have the color of the wavelength that they reflect.
  • White light has all colors.  Objects absorb some colors, reflect others.
Mixing colors produces other colors.
Additive Primary Colors (light)
Red - Blue - Green
Adding them together makes white light.
Subtractive Primary Colors (paint/filter)
Yellow - Cyan - Magenta
Adding them together makes black (absence of light).

Homework:
    Pg. 565 #1-6.

Chapter 16, Section 4 - Refraction, Lenses, and Prisms                                                                                                             




Homework:
    Pg. 571 #1,4,7,8.


Resources
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