Module 4.4 - Waves Flashcards
Describe progressive waves
Waves that transfer energy away from a source. All the particles oscillate vertically but they do not move forwards or backwards.
What are longitudinal waves?
A wave where oscillations are parallel (left and right) to the direction of wave propagation.
What are some examples of longitudinal waves?
> Sound
> Ultrasound
What are transverse waves?
A wave where the oscillations are perpendicular (up and down) to the direction of wave propagation.
What is one example of transverse waves?
Electromagnetic waves
Define wavelength (m)
The distance moved between two successive identical points that have the same pattern of oscillation.
Define period (s)
The time taken for one complete oscillation to take place.
Define frequency (Hz)
The number of oscillations per unit time
Define displacement (m) - waves
Distance any point of the wave has moved from its rest position.
What is amplitude (m)?
The maximum displacement - peak/trough to rest position.
How do you use an oscilloscope to find the frequency of a wave?
- Work out the period - = distance between peaks x time base setting
- Use f = 1 / T (period)
What is the time base setting on an oscilloscope?
Time taken for luminous dot to move 1cm horizontally on the screen
What are the two important wave equations?
> f (Hz) = 1/T (S)
> v (m/s) = f (Hz) x wavelength (m)
What does the intensity of a progressive wave mean?
Rate of energy transfer as the wave travels through space.
How do you work out intensity?
Power ÷ area
What is the relationship between intensity and amplitude?
Intensity is directly proportional to the square of amplitude.
What is the order of the EM spectrum from largest wavelength to smallest?
- Radio
- Microwave
- Infrared
- Visible light
- UV
- X-rays
- Gamma rays
Which EM waves have a wavelength around 10^4 - 10^-1?
Radio
Which EM waves have a wavelength around 10^-7?
Visible light
Which EM waves have a wavelength around 10^-12 - 10^-14?
X-rays
Which EM waves have a wavelength around 10^-3 - 10^-7?
Infrared
Which EM waves have a wavelength around 10^-9 - 10^-16?
Gamma rays
Which EM waves have a wavelength around 10^-7 - 10^-9?
UV
Which EM waves have a wavelength around 10^-1 - 10^-4?
Microwave
Which end of the EM spectrum is the least and most dangerous?
- Radio waves - least
… - Gamma rays - most
Which end of the EM spectrum has the highest/lowest frequency?
- Radio - lowest
… - Gamma rays - highest
Give one use of radio waves
Communications
Give one use of microwaves
Microwave ovens
Give one use of infrared
Remote controls
Give one use of UV
Counterfeit detection
Give one use of visible light
Sight
Give one use of x-rays
CT scans
Give one use of gamma rays
Radiotherapy
Give 6 common properties of all EM waves
> Travel in vacuum > Travel at speed of light, c > Transverse waves > Can be reflected, diffracted and refracted > Can be polarised > Can demonstrate interference
Describe polarisation
It occurs when the oscillation of a wave
is restricted to one plane only.
What is the direction of propagation?
Direction of travel
When is a transverse wave plane polarised?
If the direction of oscillation remains in one plane.
When is a transverse wave unpolarised?
If the direction of oscillation is in multiple planes.
When does wave speed decrease when passing through different mediums?
When the wave travels from a material with a lower refractive index to a material with a higher refractive index.
When does wave speed increase when passing through different mediums?
When the wave travels from a material with a higher refractive index to a material with a lower refractive index.
Where is the direction of oscillation perpendicular to the direction of propagation?
In transverse waves
Describe the refraction of light
> Occurs when passing from a material of lower refractive index to a material of higher refractive index.
Wave speed changes
Wave direction changes
When does the wave direction remain the same in refraction?
When the wave is travelling along the normal.
How do you work out the refractive index of a material?
= Speed of light in a vacuum ÷ speed of light in the material
n = c ÷ v
What is Snell’s law?
Refractive index of material 1 = n1
ø1 = theta 1 = angle to normal of wave in material 1
Refractive index of material 2 = n2
ø2 = theta 2 = angle to normal of wave in material 2
n1 x sin(ø1) = n2 x sin(ø2)
What is the critical angle?
The angle of incidence that causes an angle of refraction to be 90º
How do you calculate the critical angle, C?
n1 x sin(ø1) = n2 x sin(ø2)
n1 x sin(C) = n2 x sin(90)
n1 x sin(C) = n2
sin(C) = n2 ÷ n1
If n2 is air: sin(C) = 1 / n1
Describe total internal reflection
> Occurs when light travels from a material of higher refractive index to a material of lower refractive index
Angle of incidence will be BIGGER than the critical angle.
What is the principle of superposition?
When two or more waves meet, the resultant wave can be found by adding the displacement of the individual waves.
Describe constructive interference
> Meet in phase.
Resultant wave will have an amplitude of individual waves added together.
The path difference is a multiples of whole wavelengths (n wavelengths).
What is phase difference?
> How much a wave is in front/behind another in terms of π radians/degrees
Both have the same frequency and referenced to the same point in time.
What is path difference (m)?
> Difference in terms of wavelength between the distances travelled by two waves arriving at the same point
Both have the same frequency and travel at the same velocity.
What is the path difference of two waves with a phase difference of π/2 radians?
π/2 radians = 90º
90º = 1/4 wave cycle
Therefore
Wavelength/4
Describe destructive interference
> Meet in anti-phase.
Resultant wave will have an amplitude of zero as they cancel each other out.
Path difference is an odd multiple of half wavelengths (n/2 wavelengths).
What is one wave cycle equal to?
2π radians or 360º
What is the phase difference from peak to rest position on the same wave?
1/4 wave cycle/wavelength
Therefore
π/2 radians or 90º
What is the phase difference of waves that have a path difference of 3/2(wavelength)m ?
3/2(wavelength) = 3/2 x 360
= 540º
= 3π radians
What is the path difference of waves with a phase difference of 5π radians?
5π = 900º
900 ÷ 360 = 5/2
Therefore
5/2(wavelengths)m
What is coherence?
Two waves with a constant phase difference (same frequency)
Describe the loud and soft spots from two-source sound waves.
The loud sounds are where waves constructively interfere and the soft sounds are where waves destructively interfere.
How do you work out the wavelength of the light source that has been diffracted through slits?
Wavelength = ax ÷ D
a - slit separation
x - fringe (between maxima/minima) separation - add up then divide by n
D - distance between screen and slit
where a «_space;D
Why did Young use two slits?
To achieve coherent waves due to lack of technology.
What is a diffraction grating?
A piece of optical equipment (glass) which has thin, equally spaced parallel lines that cause light to be diffracted at different angles.
What is the diffraction grating equation?
n x wavelength = d x sin(ø)
n - order of maxima
d - slit separation
(Ø) - the angle that the beam makes with the grating
Describe stationary waves
> Produced by interference
> Two waves that overlap must be:
- travelling in opposite directions
- have the same frequency
- have similar amplitudeWhat is the difference between stationary and progressive waves?
Energy is stored in stationary waves and energy is transferred in progressive wave.
What are antinodes?
Points of maximum positive or negative displacement
What are nodes?
Points where displacement is always zero
What is the frequency of stationary waves on strings affected by?
> Mass per length
Tension
Length of string (Adjusted when playing)
Describe the fundamental mode of vibration
In the fundamental mode of vibration, the length of string is half the wavelength. This produces the lowest possible frequency called the first harmonic. Other harmonics are whole number multiples of this.
How does length of the string compare to wavelength in the third harmonic?
The length of the string is one and a half wavelengths
Describe stationary waves in a pipe
> Air particles oscillate to form a longitudinal wave
Wave travels up and down the pipe, reflecting of both ends (closed/open)
Reflected and initial waves superpose to form areas of no displacement (node) and larger displacement (Anti-node)
Where in a pipe is there always a node?
At the closed end of a pipe
Where in a pipe is there always an antinode?
At the open end of a pipe
What happens to all waves?
> refraction
reflection
diffraction
Describe reflection
> waves are reflected into lines of constant phase - wavefronts
wavelength does not change after being reflected
Describe diffraction
> the spreading out of a wave after passing around an obstacle or through a gap
waves are most diffracted when the wavelength of the wave is the same as the gap it is passing through
What is the difference between two nodes/antinodes in terms of wavelength and phase
Wavelength/2 or 180º or π radians
