Waves Flashcards
Define Superposition
- When two waves of the same type meet and overlap the resultant displacement is the vector sum of individual displacements
What is constructive interference/superposition?
- In phase
- Amplitude doubles if same amplitude
- n wavelength, only whole numbers
What is destructive interference?
- In antiphase
- Amplitude zero
What is constructive path difference?
- 2 sound sources
- Path difference is 0
- Loud because arrive in phase
- Path difference is a whole wavelength
- n λ is path difference for constructive
What is destructive path difference?
- Leave in phase
- Arrive out of phase
- Path difference is 1/2 λ, 1 1/2 λ ex
- The sound is quiet
What is a coherent source?
- Have a constant phase difference
- Same type of wave of same frequency
What is the radiation flux equation?
- Radiation flux = P/A
What is Radiation Flux the same as?
- Intensity
Define Amplitude
- Maximum displacment from the midpoint of the oscillation (m)
Define period?
- Time for one complete oscillation
Define frequency?
- Number of oscillations per second
Define wave speed
- Distance travelled by wave each second
Define Wavelength
- Distance between consecutive points which are in phase
What are the two key wave equations?
- v=fλ
- f=1/T
Define a Transverse wave
- Oscillate perpendicular to there direction of travel
Define Longditudinal wave
- Osillate parrael to there direction of travel
What is a Standing Wave?
- Set up when two waves of equal frequency and amplitude that are travelling at the same speed in opposite direction superimpose
What are the differences between Standing and Progressive waves
- Energy: Energy transffered in direction of wave for progressive waves. Energy stored within each particle for stationary wave
- Amplitude: All points along wave have same amplitude for prog wave. Amplitude varies for stationary wave (min node to max antinode)
- Phase: Adjacent points have different phase relationships for prog waves. All points between each pair of consecutive nodes have constant phase relationship for standing waves
What is the equation for standing wave on a string?
- v=(T/μ)1/2
- v is velocity
- T is tension
- Mew is Mass per unit length
What is fundamental frequency?
- Standing waves with shorter wavelengths are called overtones or harmonies
- This is only the case if frequency is an integer of the fundamental frequency
What is the equation for refraction?
- n1 * sin(x)1 = n2 * sin(x)2
What does refactive index =.
- n = c / v
What does critical angle =.
sin (C) = 1/n
When does TIR occur?
- High to Low n
- Angle of incidence greater than critical angle
What is focal length
- Focal length of a lens is the distance between midpoint of the lens on the principle axis and the focal point
What is the equation for the power of lenses?
- Pt = P1 + P2 + P3
- P = 1 / f
What should you talk about when talking about images?
- Real or virtual
- Upright or inverted
- Magnified or diminished
What is the equation starting with 1 /f.
- 1/f = 1/u + 1/v
What is the magnification equation?
- m = v/u = image height / object height
What is plane polarisation?
- If light passes through polarising filter oscillations in all directions but one will be abosorbed
What are the photon equations and when are they used?
- E = hf
- hf = work function + 1/2mv2
- Used when a wave is behaving like a particle
What is threshold frequency?
- Minimum frequency needed to emit one photo electron
What focal point do coverging lenses have?
- Real focal point
What focal point do diverging lenses have?
- Virtual focal point
- It negative
Which way does light refract moving from high to low refractive index?
- Refracts away from the normal
Which way does light refract moving from low to high refractive index?
- Refracts towards the normal
When can something be plane polarised?
- Only if they are transverse waves
What happens when a wave is plane polarised?
- Osillate in one plane
- Oscillate perpendicular to direction of travel
- The plane of polarisation includes direction of energy transfer
When does diffraction occur?
- It occurs when a wavefront is disturbed by an obstacle or passes through an aperture
When does the semi-circular pattern occur in diffraction?
- When the wavelength of a wave has the same order of magnitude as the width of the gap the wavefront spreads out in a semi circular shape
What is the diffraction grating equations?
- n*λ = d*sin(x)
- d = 1/line spacing
What is a diffraction grating?
- A diffraction grating is a surface with thousands of equally spaced microscopic gaps
What is the rule of reflection?
- Angle of incidence equals angle of reflection
What is pulse echo technique?
- Method for measuring the speed of a wave or distance from the reflecting surface
What is the equation for pulse echo technique?
- x = vt / 2
- Distance = velocity * time / 2
What must be the case for pulse echo technique to work?
- The bang of the gun needs to be much shorter than time taken for wave to return
- Sufficient reolution if λ is less than 1/2 the object being scanned
What is the evidence for particle theory?
- Photoelectric effect
- Light is made of photons
What is the evidence for wave theory?
- Diffraction
- Interference
- Polarisation
What is Huygens construction?
- Every point on the wavefront is a source of secondary spherical wavelets that spread out with the wave velocity
- Round an obstacle or through a gap, each end of the wavefront spills over into the geometric shadow
State the De Broglie equation and when it should be used
- Wavelength = Planks constant / Momentum
- λ = h / p
- Only used for particles (e.g electrons) behaving like waves
How do you convert J to eV
- divide by 1.6*10^-19
How do you convert eV to J?
- Multiply by 1.6*10^-19
Why does Particle theory explain the photoelectric effect but wave theory doesn’t?
- There is a threshold frequency below which photoemission doesn’t occur
- E = hf
- Instant emission of electrons
- Number of photoelectrons emitted are proportional to intensity of radiation
- One photon is absorbed by one electron so if the intensity goes up you have more photons therefore more photoelectrons
- Intensity has no effect on the KE of the photoelectrons
- The frequency is the only thing which effects the KE of the photoelectron
What does intensity equal?
- Intensity = Number of photons
What is the difference between a real and virtual image?
- Real can be projected on a screen
- Virtual cannot be projected on a screen
What is the distance between adjacent nodes or antinodes?
- 1/2 a wavelength
What forms emission spectra of light?
- Electrons falling from an exited state to a lower energy level and releasing energy in the form of a photon
What causes an absorbtion spectra?
- When a gas absorbs certain frequencies of light leaving dark lines in the spectra
- Electrons move up in the gas and move to an exited state
What happens when there is enough energy provided to electron to escape the atom?
- The electron becomes exited and gains enough energy to escape the electrostatic attraction of the nucleus
- The incident radiation frequency is above the threshold frequency so the atom is ionised and the electron escapes
How do you find the threshold frequency?
- Give the electron enough energy so that hf0 = work function and therefore KEmax is zero
Explain the emission spectra?
- Electrons exist in discrete energy levels
- Stable atoms are in lowest energy level called ground state
- To exite atom the electron must gain energy to move to a higher energy level. The atom is now in an exited state
- An exited atom cannot stay that way so electron drops down energy levels emitting energy in the form of a photon
Where can the energy come from in the emission spectra?
- Heat
- Incoming photons
- Collisions with fast moving particles
- Electricity
Define a wavefront
- A line on a wave where all points are in phase
