Waves Flashcards
Describe a longitudinal wave
The direction of oscillation is parallel to the direction of travel Made of compressions (high pressure) and rarefactions (low pressure) Molecules move back and forth from equilibrium position
Draw a labelled wave
Define Wavefront
A line or surface joining points which are at the same state of oscillation, i.e. in phase
Define Coherence
Two waves are coherent if the phase difference between them is constant. For this to be the case they must have the same frequency.
Define superposition
When two or more waves of the same type meet at a point, the resultant displacement of the waves is equal to the vector sum of their individual displacements at that point.
Define destructive interference
When the superposition of two or more waves leads to zero amplitude because the waves are exactly out of phase
Define constructive interference
When the superposition of two waves that are perfectly in phase - this causes the amplitudes to double.
Define Phase
Phase is the position of a point in time on a waveform cycle
Give the equation linking path difference and phase difference
ΔX=λ⋅Δϕ/2π
Here, ΔX is path difference, Δϕ is phase difference.
Describe how standing waves are formed
Superposition/interference between waves travelling in opposite directions (from open end and wave reflected at closed end)
Waves have same frequency /wavelength
Nodes and antinodes are produced
At node the waves are in antiphase, so there is destructive interference
At the antinode they are in phase so there is constructive interference
At an antinode there is maximum amplitude
At a node there is zero amplitude
Define Nodes and Antinodes
At node the waves are in antiphase, so there is destructive interference so there is zero amplitude
At the antinode they are in phase so there is constructive interference so there is maximum amplitude
Define Amplitude
The maximum displacement of a wave from the position of equilibrium
Define frequency
Number of oscillations per second
Define Time Period
The time taken for one oscillation
Define Wavelength
The distance between two successive points of maximum or minimum displacement
Give the wave equation
v = fλ
Define Refraction
Refraction is the bending of light as it moves from one medium to another
Give the equation for refraction index
n1 sin θ1 = n2 sin θ2
Since the refractive index of air is approximately 1
n2 = sin θ1 / sin θ2
n = sin i / sin r = c/v
Draw a ray digram for refraction
Draw a wavefront diagram for refraction
Wave Particle Duality Model Answer
Particle theory
Reference to E=hf or quanta of energy /packets of energy/photons
Increased f means more energy of photon
Release of electron requires minimum energy /work function
One photon releases one electron
Greater energy of photon means greater KE of electrons
More intense light means more photons, therefore more electrons
Wave theory
Wave energy depends on intensity
More intense light should give greater K.E of electrons
Energy is spread over the whole wave
If exposed for long enough photons eventually released, doesn’t happen.
Double Slit
The waves superpose Or diffraction at the double slits
Where they are in phase Or
when path difference is a whole number of wavelengths constructive interference takes place
Where they are in antiphase / when path difference is an odd number of half wavelengths destructive interference takes place
Bright bands are when waves are in phase / when path difference is nλ / constructive interference
Or reverse for dark bands
Give the equation for the energy of the photon
E = hf
Model Answer for Total Internal Reflection
. The direction of travel of light is denser to rarer medium
Angle of incidence is greater than the critical angle
light is reflected not refracted/no light emerges
Define Critical Angle
The angle beyond which total internal reflection (of the light) occurs
Give the equation used to find the critical angle
n1sinθ1 = n2sinθ2
Not light passes through therefore θ2 = 90 => sinθ2 = 1
n1sin θ1 = n2
sin θ1 = n2/n1
Since n2 is usually air/vacuum then n2 = 1 (dense to less dense so must be n2)
sinθ1 = 1/n1 in some cases
Describe how to measure critical angle
Draw a vertical line
Place glass block perpendicular to line
Shine ray box directly up line
Rotate raybox until TIR occurs
Mark where the light enters the block
Mark position where light should exit block
Join the two positions
Measure angle between vertical line and the line just drawn
Draw a Huygen’s Diagram
Explain the double slit experiment
When in phase constructive interference/superposition occurs Or when path difference is nλ constructive interference/superposition occurs
When in antiphase destructive interference/superposition occurs Or when path difference is (n + ½ ) λ destructive interference/superposition occurs
Light band forms when in phase Or path difference is nλ Or constructive Or Dark band forms when in antiphase Or path difference is (n + ½ ) λ Or destructive
Explain the photoelectric effect
Only one photon can transfer energy to a single electron Or a photon transfers all of its energy to a single electron
Photon energy depends on frequency Or photon energy = hf
Therefore photons must have a frequency greater than or equal to the minimum frequency (threshold frequency) in order to provide sufficient energy Or photon energy must be greater than work function
Explain Threshold Frequency
Idea that one photon is absorbed by one electron
Photon energy given by E = hf Or photon energy increases with frequency
The idea that there is a minimum energy needed for emission of a (photo)electron (So) emission of electrons only occurs if frequency of light greater than the threshold frequency Or threshold frequency is the minimum frequency for the emission of (photo)electrons
Define Work Function
The (minimum) energy required to remove one/an electron from the surface of the metal
Give the equation involving work function
hf = Work Function + KE
Explain why the photoelectric effect gives evidence for the particle theory of light
- Increasing the intensity (of light) increases the number of electrons emitted(per sec) Or number of electrons emitted(per sec) depends on the intensity (of light)
- One photon releases one electron
- Intensity determines number of photons
OR
- Increasing the intensity (of light) does not increase the energy/speed of the electrons
- One photon releases one electron
- Energy of photon determined by/depends on frequency (not intensity) Or E = hf
OR
- Below a certain frequency / threshold frequency no electrons emitted Or above a certain wavelength no electrons emitted
- Energy of photons increases with / depends on frequency Or E = hf
- Each photon needs a minimum amount of energy / work function Or One photon releases one electron
OR
- Electron emission starts at once (even for low intensity)
- One photon releases one electron
- Wave theory would allow energy to build up
OR
- Increasing the frequency (of light) increases the energy/speed of the electrons Or Increasing the frequency (of light) increases the stopping potential
- Energy of photon determined by/depends on frequency Or E = hf
- One photon releases one electron Or Wave theory would allow energy to build up
