Lecture 3: Waves, the Physics of Light & Sound Flashcards
What is a wave?
Lights and sounds are composed of waves.
A wave is a type of internal motion of a medium, in which the displaced portion returns to equilibrium.
This disturbance propagates in space as well.
Wave
disturbance propagates in x…
Transverse Waves
This is what occurs in the vibrations of stringed instruments
Longitudinal Waves
This is what occurs in the vibrations of wind instruments
Types of Waves
Transverse Waves – This is what occurs in the vibrations of stringed instruments.
Longitudinal Waves – This is what occurs in the vibrations of wind instruments.
Sound Waves
Sound waves involve the longitudinal oscillations of air molecules.
The sound vibrations in a long, narrow tube, such as a trombone, flute or trumpet, propagates in one direction.
In open air, a sound wave propagates radially in all directions
Light Waves
Light waves involve transverse oscillations in electric and magnetic fields
Light
A wave; a stream of photons, tiny particles that each consist of one quantum of energy
Diagram of a wave
All waves are caused by vibrations
Velocity, Frequency and Wavelength
Within a given medium, light & sound waves travel at a constant velocity.
Thus, long wavelengths oscillate at low frequencies, and short wavelengths oscillate at high frequencies.
Long wavelengths oscillate at _____ frequencies.
low frequencies
short wavelengths oscillate at ____ frequencies.
high frequencies
Two waves with the same velocity and different wavelengths will have different frequencies.
Two waves with the same frequency and different wavelengths will have different velocities
Two waves with the same frequency and different wavelengths will have different velocities
Two waves with the same velocity and different wavelengths will have different frequencies
Interference
The addition and subtraction of waves
When 2 or more waves come together, the individual displacements at each point in space are added together to produce a composite wave.
Absorption
The conversion of energy to a different form, such as heat, when a wave hits an obstacle
Reflection
The reversal of direction when a wave hits an obstacle.
When a wave hits a boundary between two media, some of its energy rebounds in a different direction.
Refraction
The bending of a wave as it crosses the boundary between two media
Refraction is what causes a pencil to appear bent when it is partially submerged in water.
Diffraction
The bending of a wave around an obstacle
Waves can sometimes bend around obstacles.
Long wavelengths diffract more than short wavelengths.
Wave Interactions
Interference – The addition and subtraction of waves.
Absorption – The conversion of energy to a different form, such as heat, when a wave hits an obstacle.
Reflection – The reversal of direction when a wave hits an obstacle.
Refraction – The bending of a wave as it crosses the boundary between two media.
Diffraction – The bending of a wave around an obstacle.
When two or more waves come together, the individual displacements at each point in space are added together to produce a composite wave. This is referred to as ______.
interference
When a wave hits a boundary between two media, some of its energy may be converted to heat.
This is an exmple of….
Absorption
Scattered
Energy that is dispersed in an irregular fashion.
When light enters the atmosphere, much of it is absorbed or scattered and never reaches the perceiver.
When a wave hits a boundary between two media, some of its energy rebounds in a different direction
Reflection
Some animals, such as bats, are able to use reflections of sounds to determine ______ .
the distance of objects
Energy that is passed on through a surface (when it is neither reflected nor absorbed by the surface) is called ……
Transmitted energy
When a wave hits a boundary between two media, some of its energy may be transmitted across the boundary
Transmitted
Energy that is passed on through a surface (when it is neither reflected nor absorbed by the surface)
When a wave hits a boundary between two media, some of its energy may be transmitted across the boundary
What happens to a wave when it changes speed
Refraction
When a wave travels from a less dense to a more dense medium at an angle off the line of normal, it _____ and bends ______ the line of normal.
When a wave travels from a less dense to a more dense medium at an angle off the line of normal, it *slows down and bends *towards the line of normal.
When a wave travels from a more dense to a less dense medium at an angle off the line of normal, it ____ and bends *away from the line of normal.
When a wave travels from a more dense to a less dense medium at an angle off the line of normal, it *speeds up and bends *away from the line of normal.
____ wave lengths refract MORE than _____ wavelengths.
Short wave lengths refract MORE than long wavelengths.
Rainbows are caused by light that is _____ through particles of water.
Rainbows are caused by light that is refracted through particles of water.
_____ wavelengths diffract more than _____ wavelengths
Long wavelengths diffract more than short wavelengths
This animation shows diffraction through a slit.
It also shows how interference patterns can occur when a wave interacts with its own reflection.
Natural Frequencies
Most objects have a specific frequency at which they vibrate most readily. This is called the natural or fundamental frequency.
For a vibrating string, the natural frequency increases with the tension of a string, and decreases with increasing length or mass.
For a vibrating tube, the natural frequency decreases with increasing tube length, and is higher for open tubes than for closed tubes.
The manipulation of these parameters is what allows musicians to play musical instruments.
Rule to remember: Short things have higher natural frequencies than long things
Rule to remember:
Short things have higher natural frequencies than long things
Methods of representing waves
Waveform plots
Spectral plots
Spectrograms
Spectrum plots
Amplitude against frequency
The amplitudes of the sine waves give the amplitude spectrum of the sound.
The amplitude spectrum of a sine wave is a single point at the frequency of the sine wave.
Waveform
Amplitude against time
Sound is a change in the pressure of the air.
The waveform of any sound shows how the pressure changes over time.
The eardrum moves in response to changes in pressure.
Any waveform shape can be produced by adding together sine waves of appropriate frequencies, amplitudes and phases.
Sound is ….
….a change in the pressure of the air.
The amplitudes of the sine waves give the amplitude spectrum of the sound.
The amplitude spectrum of a sine wave is a single point at the frequency of the sine wave.
Amplitude is a measure of the pressure change of a sound and is related to how loud the sound is.
Amplitude squared is proportional to the energy or intensity (I) of a sound.
complex sound
A sound which has more than one (sine-wave) frequency component
periodic sound
which repeats itself at regular intervals
A sine wave is a _____ sound
simple periodic
_______ produce complex periodic sounds.
They have a spectrum consisting of a series of _____.
Musical instruments or the voice produce complex periodic sounds.
They have a spectrum consisting of a series of harmonics.
fundamental frequency
The lowest frequency (of which all the others a re multiples)
harmonics
Each harmonic is a sine wave that has a frequency that is an integer multiple of the fundamental frequency.
Musical instruments or the voice produce complex periodic sounds. They have a spectrum consisting of a series of harmonics.
Here is 1/20th of a second of the waveform and also the spectrum of a complex periodic sound consisting of the first 4 harmonics of a fundamental of 100 Hz.
All the frequency components are integer multiples of 100 Hz.
A periodic sound consists of a section of waveform that repeats itself.
The period of the complex wave is the duration of this section.
In this case, it is 1/100s or 0.01s, or 10 ms.
The period is the reciprocal of the fundamental frequency (in this case 100 Hz).
If you change the period of a complex sound, you change its pitch.
Shorter periods - higher fundamental frequency - higher pitch.
If you change the period of a complex sound, you change its _____.
If you change the period of a complex sound, you change its pitch.
Integer multiples of the fundamental frequency
Harmonics
Spectrograms
A spectrogram is a 3-dimensional plot of frequency and amplitude as a function of time. Amplitude is represented in a spectrogram by shades of gray
Spectrograms are especially useful for representing complex waveforms like speech that change over time.
Properties of Waves
Amplitude: Controlled by the magnitude of the forces that started the wave
Frequency: f of oscillations: controlled by forces starting the wave and by the nature of the material or object that is oscillating
Spectral Composition: Refers to the mixture of different frequencies in a wave
Amplitude
Controlled by the magnitude of the forces that started the wave
Frequency f of oscillations
controlled by forces starting the wave and by the nature of the material or object that is oscillating
Loudness
most related to intensity
Timbre
most related to spectral composition
The 3 Main Perceptual Attributes of Sound
The terms pitch, loudness, and timbre refer not to the physical characteristics of sound, but to the mental experiences that occur in the minds of listeners.
The 3 Main Perceptual Attributes of Sound
The terms pitch, loudness, and timbre refer not to the physical characteristics of sound, but to the mental experiences that occur in the minds of listeners.
The 3 Main Perceptual Attributes of Light
The terms Brightness, Hue, and Saturation refer not to the physical characteristics of light, but to the mental experiences that occur in the minds of observers.
Brightness
most related to amplitude
Hue
most related to wave length or frequency
Saturation
most related to spectral composition
