Physics__Waves and the Particle Nature of Light Flashcards
Snells Law
n1 sin() = n2 sin()
What’s the refractive index of air?
about 1
What’s the grating equation and how can it be derived?
d sin() = n lambda
What’s the wavefront?
A line joining all points on a wave that are in phase
A polarising filter is placed in front of a laser. When the laser is switched on a red spot is seen on a screen. The filter is then rotated through 180*. As the filter is rotated, the intensity of the red spot falls to almost zero and then returns to original intensity. Explain these observation
Lasers emit plane polarised light.
Whenever the transmission axis of the filter is parallel to the plane of polarisation of the laser, light is transmitted.
Whenever the transmission axis of the filter is perpendicular to the plane of polarisation of the laser light, no light is transmitted.
Explain the difference between plane polarised and unpolarised light.
Unpolarised light - has oscillations in many planes.
Plane Polarised light - oscillations are in a single plane
Plane includes the direction of propagation
What’s diffraction?
The process of waves spreading when they pass through a gap or barrier
Describe longitudinal waves in terms of pressure variation and the displacement of molecules
Longitudinal waves have areas of high pressure called compressions, and low pressure called rarefactions.
Particles oscillate parallel to the direction of the propagation of the wave and direction of energy transfer
Describe transverse waves?
Particles oscillate in the direction of energy transfer but perpendicular to the direction of the propagation of the wave.
Transverse waves show areas of crests (peaks) and troughs.
What are the different ways of representing transverse waves on a graph?
Displacement-distance graphs, displacement-time graphs
What are the different ways of representing longitudinal waves on a graph?
Displacement-distance graphs
What are the different ways of representing standing waves on a graph?
Standing waves can be transverse or longitudinal so can be be plotted on a graph as thus
What’s coherence?
Interference is observable only if produced by a coherent source.
Waves are said to be coherent if they have:
A constant phase difference and the same frequency
What’s the principle of superposition
Waves from two sources meet and occupy the same region.
Total displacement is the vector sum of their individual displacements at that point
What’s interference?
When multiple waves combine to produce a resultant wave with a new amplitude
What’s constructive interference?
When waves from two sources superpose at a point in-phase, constructive interference occurs.
The resultant wave has a larger amplitude than any of the individual waves
What’s destructive interference?
When waves from two sources superpose at a point anti-phase, destructive interference occurs. Resultant wave has a smaller amplitude than individual waves
What is phase?
2 points on waves are in phase when they are at the same point in their wave cycle.
Angle between their wave cycles is the phase difference.
What is path difference
The difference in distance travelled by two waves from their sources to the point where they meet
How can we tell if waves are in-phase or anti-phase?
When path difference is an integer no. of wavelengths, then in-phase.
When path difference is an odd no. of half wavelengths, then antiphase.
Explain how a stationary wave is formed. Then describe the key features of the stationary wave?
When two progressive waves of equal frequency and amplitude travelling in opposite directions superpose.
Nodes where there is zero amplitude.
Antinodes where amplitude is maximum.
Destructive superposition at nodes/Constructive superposition at antinodes.
Energy is not transferred along string - instead it’s stored
What are nodes and antinodes?
Nodes: points of zero amplitude on standing wave.
Antinodes: points of maximum amplitude on standing wave
What is the fundamental frequency?
The lowest frequency that can produce a standing wave
What is the equation for the speed of a transverse wave on a string?
Where v is wave speed, T is tension, and M is mass per unit length
What is the focal length?
Distance between the optical centre of the lense and the focal point (principle focus).
f is measured in metres, it depends on how curved a lense is (the more curved, the shorter the length)
How to calculate the power of a lense?
f is focal length in metres
What is a real image?
It is formed by the convergence of rays of light (the rays meet).
Real images can be projected onto a screen
What’s a virtual image?
A virtual image is seen but not formed on a screen
The rays of light have not met, they have been perceived by the eye
e.g.: An image viewed through a magnifying glass is a virtual image
What is the equation for magnification
Where v is image height, and u is object height
Huygen’s construction
(You need to be able to draw this) Those point sources which pass through the gap create new wavelets on the other side, leading to the characteristic curved shape of the diffracted wave
What happens to a wave when it meets a slit?
Some energy is dissipated as a wave is diffracted through the gap, effecting the amplitude
What happens as a wave meets an obstacle?
The wave diffracts around the edges of the obstacle, and a “shadow” forms where no part of the wave reaches behind.
What factors effect diffraction?
Diffraction is most prominent when the size of the gap or obstacle is the same or smaller than the wavelength of the wave.
As the size of the gap or obstacle increases, the effect gradually gets less pronounced.
When the gap is much larger than the wavelength, the waves are no longer spread out.
How do diffraction experiments provide evidence for the wave nature of electrons?
Electron diffraction using an electron gun suggests that particles can demonstrate wave like properties.
Electrons are accelerated through vacuum towards a crystal lattice
Where they interact with the small gaps between atoms.
Electron diffraction produces the following pattern.
If electrons didn’t have a wave nature, then when they pass through a slit, they would be unaffected - a single bright region would be formed
What factors does the de Broglie wavelength depend on?
Mass and Velocity
How does pulse echo technique work?
Wave pulse transmitted
Transmitted and reflected at the boundary between 2 media
Returning wave echo is detected
Speed and time taken are used to calculate distance to object
What may limit the amount of info that can be obtained by pulse-echo technique
Wavelength of radiation (must be short to reduce diffraction)
Duration of pulse (must be shorter pulses so that reflections from nearby interfaces don’t reach transducer before pulse has ended - i.e. reduce interference)
Which 2 models are used to describe EM radiation
The wave model
The particle model
What provides evidence for the particle model?
The photoelectric effect
What is the photoelectric effect?
Photons of a certain frequency are fired at a metal plate
Photon transfers all energy to an electron.
If the frequency of the photon is high enough, the electrons will gain enough energy to overcome the metal’s work function
Electrons are then emitted from the surface of the metal.
One photon liberates one electron
What is the threshold frequency?
The minimum frequency of a photon that is required to liberate an electron from the surface of a particular metal.
If the intensity of light being shone on a metal increases, how does the energy of photoelectrons change?
Energy is unaffected, An increase in intensity means that there are more photons per area. Therefore more photoelectrons are emitted.
Why are photoelectrons emitted with a range of kinetic energies?
Electrons are at different depths within the metal.
So, require different amount of energy to be liberated.
After liberation, the excess energy from the photon is the kinetic energy of the photoelectron.
How to convert between eV and J
1eV = 1.6 x 10-19J
How do electrons transition between energy levels?
When electrons absorb radiation, they move to a higher energy level.
When electrons emit radiation, they move to a lower energy level.
Why can only certain frequencies of radiation be absorbed by an atom to cause an electron to transition energy levels?
Electrons exist in discrete energy levels.
The energy of the photon must be the exact amount of energy required to cover the difference between the two discrete energy levels (the bandgap)
What is evidence for the wave theory of EM radiation?
Young’s Double Slit experiment.
What is meant by a real image?
Light rays converge to a point where the image is formed
