Topic 5: Waves and Particle Nature of Light Flashcards
What is amplitude?
Amplitude is a wave’s max displacement
What is frequency?
Frequency is the number of complete oscillations passing through a point per second.
What is time period?
Time taken for one full oscillation
What is wavespeed?
The distance travelled by the wave per unit time
What is wavelength?
The length of one whole oscillation.
What are the properties of oscillations in longitudinal waves?
In longitudinal wave, oscillations are parallel to the direction of energy transfer.
They are
- Made of compressions and rarefractions
What are the properties of oscillations in transverse waves?
In transverse waves, oscillations are perpendicular to direction of transfer
What is phase of a wave?
Position of a certain point in a wave cycle.
What is phase difference in waves?
How much a particle/wave lags behind another particle/wave.
What is path difference?
Path difference is the difference in distance travelled by 2 waves.
What is superposition?
Superposition is when displacements of 2 waves combine as they pass each other.
The resultant displacement is the vector sum of both displacements.
What is a wavefront?
Wavefront is a surface which is used to represent the points of a wave which have the same phase.
What is coherence?
Coherent light source has the same frequency and wavelength.
What is a wavefront?
A wavefront is a surface used to represent points of a wave which have the same phase.
What is constructive interference?
Constructive interference is when 2 waves superpose and their amplitudes add together
When does constructive interference occur?
Occurs when two waves are in phase and superpose.
What is destructive interference?
Destructive interference is when two waves superpose and their amplitudes cancel out, decreasing amplitude of resulting wave.
What does it mean when 2 waves are in/out phase?
In phase means when 2 waves have
- the same frequency and wavelength.
- phase difference is a multiple of 2π
What does a standing/stationary wave do?
It stores energy.
How is a standing/stationary wave formed?
When 2 waves of same frequency/wavelength/amplitude are travelling in opposite directions and superpose.
E.g waves in a contained box, one reflects and interferes with initial wave
What happens where standing waves meet in phase/ anti-phase?
When waves meet in phase
- Constructive interference occurs
- Anti-nodes are formed; regions of max displacement
When waves meet out of phase
- Destructive interference occurs
- Nodes are formed; regions of 0 displacement
How to calculate speed of a transverse wave on a string?
v = √T/μ
T = tension in string
μ - mass/unit length
What is definition and formula for intensity?
Intensity is power/unit area
I=P/A
What is refraction?
Refraction is when a light ray enters a new medium with different density leading to a change in speed and change direction.
What does the refractive index of a material mean and what is formula?
Measurement of how much the material slows down light passing through it.
n = c/v
Higher refractive index = more optically dense
What happens when the new medium is more/less optically dense?
When medium is more dense;
- Light ray slows down; bends towards the normal
When medium is less dense
- Light ray speeds up; bends away from normal
What is the critical angle?
Critical angle is the angle of incidence causes angle of refraction to be exactly 90° and reflected along the boundary.
How can you calculate critical angle when one medium is air?
Use formula sinC = 1/n
C - Critical angle
n
What is total internal reflection?
Total internal reflection is when light ray in the medium is reflected at the same angle off the boundary.
When does total internal reflection occur?
Occurs when angle of incident > critical angle
What are the 2 types of lenses?
Converging - convex shaped lens; curved outwards; cause parallel light rays to converge to focal point
Divergent - concave shaped lens; curved inwards; light rays diverge from a point
What is the principle focus?
In converging
- where light rays meet
In diverging
- where image appears to come from
What is the focal length?
Focal length is the distance from the centre of the lens to the principle focus
What is the power of a lens?
Measure of lens’ ability to bend light.
Converging - positive value
Diverging - negative value
How can you work out the power of a lens?
Power = 1/focal length
How to draw ray diagrams?
- Draw a line parallel to principle axis then draw connecting line through focal point
- Draw a line through centre of lens
When is a real/virtual image produced in ray diagrams?
Real - when the rays meet at the other end of the principal axis
Virtual - when rays meet at same end of principal axis
When is a magnified/shrunk image produced in a ray diagram?
Magnification - when the image is taller than the object
Shrunk - when the image is shorter than object
What is a real/virtual image?
Real - Image that can be projected onto a screen as light rays reach image location
Virtual - Image that can’t be projected onto a screen.
How can you use the distances of images in a ray diagram to calculate power?
1/u + 1/v = 1/f
When does polarisation occur?
Polarisation occurs when particles are only allowed to oscillate in one of the directions perpendicular to direction of wave propagation.
Which directions can a transverse wave be oscillated in?
Transverse waves can be polarised horizontally, vertically or any direction in between.
Why can longitudinal waves not be polarised?
Longitudinal waves oscillate parallel to direction of wave transfer; polarisation requires wave to oscillate perpendicular to propagation.
How can light be polarised?
Light can be polarised by passing it through polarising filter.
What is diffraction?
Diffraction is the spreading out of waves when they pass a gap.
What is Huygens’ construction?
Huygens’ construction states that every point on a wavefront is a point source to secondary wavelets.
What does the amount of diffraction depend on?
Diffraction increases as the wavelength gets closer to length of gap. Greatest amount of diffraction occurs when gap = wavelength
What is a diffraction grating?
Diffraction grating is a slide containing equally spaced slits close together.
What happens when a monochromatic light/laser is lit through a diffraction grating?
Pattern of narrow bright fringes produce on a screen.
What is the diffraction grating equation?
dsin = nλ
d - distance between slits
θ - angle to normal made by maximum
n - order
λ - wavelength
What is an interference in waves?
Interference is a boundary between 2 materials.
What 2 things happen at an interference?
Transmission
- When they pass into the next material
Reflection
- Hits the boundary but does not pass through, reflects and stays in original medium
How to know if transmission or reflection will occur?
Transmission occurs when densities of materials are similar
Reflection occurs when densities of materials are different
What is the pulse-echo technique used with?
Pusle-echo technique is used with ultrasound waves (>20kHz) for imagining of objects
Why are ultrasound waves used in pulse-echo technique?
Non-invasive
- no ionising radiation; safer to perform x-rays
High freq
- Above human-hearing; high freq = better resolution
What does the pulse-echo technique rely on?
It relies on the fact that waves are reflected when they meet boundaries of different material.
How does pulse-echo technique work?
- Short pulses of ultrasound waves transmitted into target.
- Pulse travels inside target until some of pulse is reflected back; amount of reflection depends on difference in densities; higher diff, higher reflection
- Reflected waves are detected
-
Intensities of reflected waves used to determine structure of target;
time taken for reflected waves to return is used to determine position in the target.
What determines the position of objects in the target in pulse-echo technique?
Position is determined by the time taken for reflected wave to return.
What determines the structure of the target?
Structure is determined by the intensities of reflected waves.
What 2 things will decrease the resolution in pulse-echo technique?
- If duration of pulses is too long; they will likely overlap; amount of information obtained will decrease
- If wavelength of waves used increases, less fine details, less information obtained
What does the photon model state?
EM waves travel in discrete packets called photons.
What does the wave model state?
Wave model states EM radiation can be described as transverse waves.
How to calculate photon energy?
E = hf
What is the photoelectric effect?
When photoelectrons are emitted from the surface of a metal after light above a certain frequency is shone on it.
What is the threshold frequency?
Threshold frequency is the minimum frequency of light required for a electron to be emitted from the surface of a metal.
How are photoelectron emitted?
Photoelectrons are emitted as they absorb a photon and gain enough energy to leave the surface.
What is the work function of a metal?
Work function of a metal is the minimum energy required for electrons to be emitted.
Photoelectric equation
E = hf = ϕ +KEₘₐₓ
What does increasing the intensity of light do to photoelectric emission?
It increases photons released; increases number of photoelectrons released
What happens to electrons which are deeper in the metal during photoelectric emission?
They lose some energy through collision with metal lattice; will have lower kinetic energy
