Waves Flashcards
What are waves ?
The transmission of energy without the transmission of matter or movement of energy from one place to another without moving a medium from one place to another .
How do mechanical waves work?
They move through the particles in a medium by either moving them backwards and forwards( longitudinal) or side to side (transverse)
After a wave passes, where do the particles go?
The particles return to its original place but the energy is transferred to the wave’s destination.
Longitudinal Waves
The particles are oscillated in a back and forth motion that is parallel to the motion that the energy travels.
Examples
- Sound waves
- P waves from earthquakes
-tuning fork ( compression and rarefactions in the air particles)
How longitudinal waves transfers?
By compressions and rarefactions
Compressions
The regions where the particles are pushed closest together.
Wavelength: from one compression to the next successive one
Rarefactions
Regions where the particles are pulled apart.
Wavelength: from one rarefaction to the next successive one
Transverse waves
Particles move at right angles (side to side or up or down) to the direction that the wave travels ( perpendicular to the wave direction). The particles return to equilibrium after the wave passed. However, they do not require a medium to travel.
- the majority of waves
Example: Electromagnetic waves
Electromagnetic Waves
They are transverse waves and need no medium to travel through.
Speed of light
c=3.00x10^8
Electromagnetic waves
Increase in frequency
Radio & microwaves –> Infrared –> visible light(ROYGBIV) –> Ultraviolet –> X-rays –> Gamma Rays
Amplitude
Measured from the peak or maximum displacement to the equilibrium point ( how much energy a wave has)
Wavelength , λ
Distance traveled in a complete oscillation or the distance between two successive troughs or crests.
Frequency
The number of cycles in one second and is measured in hertz, Hz.
f= cycles/ seconds
Period, T
Time taken to complete one cycle
T= 1/f
Velocity of a wave
v= Frequency x wavelength v = fλ
Velocity at maximum displacement/ extremes
Equal to zero
Speed of a wave of a string
This is dependent on the properties of the medium through which the wave travels
Velocity of a wave of a string
The velocity that the wave moves with to the specific direction is dependent on the acceleration at which each consecutive particle moves with. ( upwards or side to side) due to the net pulling that each particle has on the next.
Speed of a wave of a string formula deriving
From newton’s 2nd Law
- As F increases, acc increases and Velocity increase
- for one particle to exert a force on the other, tension needed
as tension increases, velocity increases
-smaller mass, velocity increases
mass per unit length/ linear density
m/L
Therefore v=√F applied / m/L
Force applied= tension
m/L = μ
Therefore v=√ T/μ
1st Law of Reflection
Angle of incidence = angle of reflection
- Measured from a normal perpendicular to the surface at point of incidence.
2nd Law of reflection
Incident ray, reflected ray and normal all lie in the same plane
Types of reflection ( determined by smoothness)
Regular reflection
for very smooth surfaces (mirrors) all incident rays are reflected parallel to each other
Diffused reflection
on rough surfaces, the rays are reflected at many different angles
Reflection of waves
All waves can be reflected
At any point where there is a change in wave velocity or where the wave meets upon a boundary and is reflected
Reflection of waves ( Phase)
When reflected, waves are out of phase or opposite to the original wave.
Reflection (phase change)
At a fixed end, there is a change of phase but at a free end there is no phase change
A change in medium brings about a ……?
Change in speed of wave and some energy of incident wave is reflected(phase change, v1=constant) and some is transmitted(no phase change,v2
Ray
Arrow drawn on diagram to show direction of propagation of a set of waves and is at 90 degrees to the wavelength.
Huygen’s Principle
When predicting the future position of a wavelength ,each point on the wave is considered a source of waves, called “secondary wavelets”
Application of Huygen’s Principle
- Choose a point on the wavefront
- Draw an arc to the point chosen and call the radius ‘vt’, distance moved in a particular time
- Choose another point and repeat first steps
- Draw a tangent to the two arc and this is the new wavefront
Refraction has:
- change in direction of wave
- When waves move across a boundary between two different mediums with change in speed
- Change in speed results in change in the propagation of a wave.`
Huygen’s Principle predictions
Huygen’s principles predicts where a wavefront will appear due to the difference in speed when a wave passes through a different medium.
- using the wavelets principle, each wavelet meets the boundary at different times and passes through the medium with distance ‘vt ‘
Huygen’s Principle predictions part 2
Since each wavelet of a wavefront moves a specific distance across the boundary at different distances in the same time, then this translates into a change in wavelength.
Mathematical Application of Huygen
When a wave front and its refraction are taken into consideration, it makes a quadrilateral with corners = 90 degrees.
- Therefore
sin∅i÷sin∅r=vi ÷vr
Snell’s Law
n= index of refraction
i= angle of incidence
r= angle of refraction
* Only applies to rays travelling from air into another medium
