4.4 Waves Flashcards
- What is a progressive wave?
- Two types and their definitions + examples:
- Oscillation that transfers energy from one place to another.
- Particles vibrate around a fixed place, but don’t move with the wave.
- Transverse - oscillations perpendicular to direction of wave travel, have peaks and troughs. EM Waves, water.
- Longitudinal - oscillations parallel to direction of wave travel, have compression and rarefaction (particles together vs spread out) Sound.
KEY DEFINITIONS
- Displacement, peaks and troughs.
- Amplitude
- Wavelength
- Period
- Frequency
- Wave Speed
- Distance from rest/equilibrium position, trough = minimum, peak = maximum.
- Maximum displacement from rest.
- Distance between same point on two waves oscillating in phase.
- Time taken for one oscillation of a wavelength to pass a point.
- Number of whole oscillations per time set.
- Distance travelled by wave within set time.
What is a mechanical wave? Give examples.
- Wave caused by a physical movement that then creates oscillations.
- If you remove the medium, the wave doesn’t exist.
- E.g. earthquakes, water, sound.
What is an electromagnetic wave? Give examples.
- Atoms provided with energy, electrons go up energy levels, then fall, emitting waves.
- Don’t need particles to travel through, can travel through vacuums.
- E.g. X-ray, infra-red, radio.
Equipment needed for speed of sound practical:
- Oscilloscope, microphone, ruler, loudspeaker, signal generator.
What is phase difference?
- Difference in where in the cycle two waves are.
What is the phase difference of a wave in phase with another? Anti-phase?
- 0 or 2pi radians
- Pi radians difference.
What are the equations for calculating phase difference?
- 0=(x/wavelength) x 360
- 0= (x/wavelength) x 2pi radians
What is the law of reflection?
Draw a diagram to demonstrate this.
- Angle of incidence = angle of refraction.
What is reflection?
What is refraction?
- The change in the way a wave travels off a surface of an object when it hits it.
- The change in speed of a wave as it travels through a different medium.
Where does sound and light travel fastest?
- Sound - solid - liquid - gas.
- Light - gas - liquid - solid.
What happens as water waves refract?
- Refract due to depth.
- As wave gets to shallow water, it slows due to friction.
- Wavelength is smaller.
- Bends towards normal.
What is diffraction?
- When waves spread out as they pass through a gap or around an object.
- Most noticeable when the gap is the same size as the wavelength.
- Can’t see it in light, can in sound.
3 similarities of EM waves.
2 differences.
- Transverse
- Can travel through a vacuum
- 3x10*8 m/s is how fast they travel
- Wavelength
- Frequency
Order of EM waves:
Longest wave lengths and lowest frequency:
Radio
Microwave
Infra-red
Visible light
Ultra-violet
X-rays
Gamma rays
Short wave lengths and highest frequency.
Why are EM waves called electromagnetic?
- Transverse progressive waves.
- Magnetic and electric fields oscillate perpendicular to each other.
Inverse Square Law:
Intensity proportional to 1r^2
Area of a sphere:
4pi r^2
Inverse square law in a sphere:
Intensity:
P=output
P/4pir^2
Intensity - radiant power passing through a surface per unit area.
Intensity=power/area
I = W/m^2
Relationship between intensity and amplitude:
- As waves spread out, intensity drops.
- As a result, amplitude drops.
- Amplitude drops, particles move slower (by same factor as amplitude drops).
- Energy of particles relates to kinetic energy.
KE= 1/2 m v^2 - Half speed = energy decreases by factor 4.
- Intensity proportionate to amplitude^2.
What type of waves can be polarised?
Transverse.
How can transverse waves be made?
Making particles oscillate in multiple directions, as long as the particles are at 90 degrees to the direction of wave travel.
Wave length (m)
Radio 10^3
Micro 10^-2
Infra red 10^-5
Visible 0.5x10^-6
Ultraviolet 10^-8
X-ray 10^-10
Gamma 10^-12
What is a plane polarised wave?
Wave that only exists in a singular plane.
What types of waves are mostly unpolarised?
What type of wave is nearly always polarised?
Light
Microwaves
What is an unpolarised wave?
A wave oscillating in more than one plane.
What is a polaroid?
How does it work?
- Like a series of parallel slits.
- Polarises unpolarised waves.
- Waves vibrating at 90 degrees to the polaroid slits can’t pass through. Only waves that are parallel to the slits can pass through so the wave changes from unpolarised to polarised.
- Reduces intensity by factor of 2.
- Second filter rotated perpendicular to the first reduces it to 0.
3 uses of polarisation:
Identify transverse vs longitudinal waves.
Study earthquakes.
Optimise microscopy.
Use of a metal grille to polarise.
As grill with slits is rotated, intensity of radiation rises and falls from a max to min of 0.
Refractive law.
Snell’s Law
n sin 0 = k
What is the refractive index of air?
1
What is total internal refraction?
When no refraction occurs at the boundary, the wave is completely reflected back into the the original medium.
Equation for TIR:
Sin C = 1/n
Only when from air to another substance.
2 conditions needed for TIR:
Light going from a more to a less refractive index substance.
Angle of incidence must be above 90 degrees.
