Waves Flashcards
Observations of the photoelectric effect support the particle theory of light. State one such observation and explain how it supports the particle theory of light
Intensity of light does not increase speed of electron. One photon releases one electron. Energy of photon depends on frequency. E=hf.
State what is meant by an energy level.
Discrete energy of an atom
Explain how atoms can emit a line spectrum
Atom/electron gains energy/ excited and moves to a higher level. Electron can only move between fixed levels. Electron falls to a lower level by emitting energy in the form of photons. E= hf
Explain how a standing wave is produced.
Two waves travelling in opposite directions. Superposition occur. Nodes and antinodes produced.
Explain what is meant by antiphase and destructive interference.
Antiphase: one wave 180º/ π /half a cycle out of phase with another wave. Destructive interference is when two waves cancel each other out / produce
zero amplitude
Waves
transfer energy via oscillations, but cause no net movement of matter.
Mechanical Wave is
a wave where there needs to be a material medium. Sound waves transfer energy by repeated vibrations of air molecules.
Wave Equation
wave speed = frequency x wavelength. v = fλ
Define period
Time taken for one complete oscillation at one point on the wave
Define Frequency
Number of complete wave cycles per second.
Define Transverse Wave
Oscillations occur perpendicular to direction of movement of wave energy.
Define Longitudinal Wave
Oscillations occur parallel to direction of wave energy, in a series of compressions and rarefactions.
State why ultrasound is transmitted in pulses
One wave returns before the next is sent.
A complete wave cycle =
360/ 2π/ λ
Define Wave Superposition
Two or more waves pass through the same medium. The resultant wave is formed by a vector addition of their individual waves.
Define Constructive Interference
the superposition of two waves that are in phase - producing a larger amplitude resultant wave.
Define Deconstructive Interference
the superposition of two waves out of phase ( 180/π) - resultant wave in zero amplitude.
Define Wavefront
lines connecting points on the wave that are at exactly the same phase position
Define Standing Wave
A progressive wave is reflected at a boundary, causing superposition and nodes and antinodes are produced. Wave is coherent.
Define Coherent
Same frequency and constant phase relationship.
Define Node
Destructive interference occurs - amplitude is zero, therefore no displacement
Define Antinode
Construction interference - maximum amplitude at this point.
Define Diffraction
The spreading out of wave energy after passing an obstacle/gap.
Diffraction happens the most when
gap size is the same as the wavelength
Explain interference in single slit diffraction
Waves meet in phase producing a larger amplitude wave, which appears as a light spot. Destructive interference creates dark spots (node).
What is a diffraction grating?
Device used to create multiple diffraction patterns. Splits are parallel and have a fixed distance.
Diffraction Grating Equations
d=1/number slits per metre
nλ = d sin θ
n is the order = 0 at central maximum.
Define Refraction
Change in wave speed and direction when a wave enters another medium. Frequency is constant, change is speed causes change in wavelength. V=fλ
What evidence is there for light behaving as a wave?
Diffraction, interference and polarisation
What evidence is there for light behaving as a particle?
Photoelectric effect
What evidence is there for electron behaving as a wave?
Diffraction, interference
What evidence is there for electron behaving as a particle?
Ionisation
Define Work Function
The minimum energy need by an electron to escape the surface of the metal.
Define Threshold Frequency
Minimum frequency of EM radiation that causes the emission of photoelectrons from the metal.
Define Ground State
Lowest energy level for a system
Define excitation
An energy state for a system that is higher energy than ground state.
For TIR to occur:
- the ray is attempting to emerge from the more dense material
- the angle between ray and normal > critical angle.
Convex Lense causes
the rays to converge. Can project an image onto a screen
Concave lense causes
cause rays to diverge creating an inverted virtual image. Focal point is behind the lens. -ve focal length.
Magnification =
image size/object size
Polarisation is only possible with:
Transverse Waves, as longitudinal waves are already oscillating in one direction
Define Polarised
All oscillations occur in one plane.
Explain why the minima never have a zero value? June 2009 Q20
because there is not complete cancellation/overall displacement is not zero/ not total destructive interference. Because the waves have different amplitude, energy loss due to reflection
Describe how these atoms emit visible light.
There are electron transitions between energy levels in the atoms, when electrons return to a lower level they emit energy in the
form of photons
Explain what is meant by the particle nature of light.
Light consists of particles called photons, which are discrete packets of energy
A student looks at the sunlight reflected off a puddle of water. She puts a polarising Polaroid filter in front of her eye. As she rotates the filter the puddle appears darker
then lighter. Explain this.
Reflected light is polarised. Polarised light oscillates in one plane/direction. Polaroid filter only allows oscillations in one plane to pass through. When planes are parallel puddle appears light. When perpendicular puddle
appears dark.
Compare the properties of the two sound waves necessary to produce complete cancellation of the two waves that reach the ear.
Waves must have same frequency or wavelength, amplitude and be π radians apart.
State what is meant by the de Broglie wavelength.
The wavelength (associated) with a particle/electron with a given momentum. λ = h/p
Explain how the results of photoelectric effect support a particle theory, but not a wave theory of light
E=hf or quanta of energy. 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.
Explain how the photoelectric effect supports the particle model of light and not the wave model of light.
Number of electrons emitted depends on intensity. E=hf. One photon releases one electron. Electron release requires work function energy. Wave energy depends on intensity.
State the principle of superposition of waves
Two or more waves overlap. The total displacement at a point is the sum of the individual displacements.
State what is meant by the critical angle.
The incidence angle for which the angle of refraction is 90 degrees/transmitted along the boundary.
Explain how the spectral lines are created with specific wavelengths.
- Electrons are found in discrete energy levels.
- Electrons are excited and when electrons fall to lower levels they emit energy as photons, with equal energy to the difference in levels.
- Photon energy = hf
- Only specific energy changes are possible, so only specific wavelengths are possible.
An experiment investigating the nature of electrons involves firing a beam of electrons
through a thin sheet of crystalline material towards a fluorescent screen.
Explain what the results of this experiment demonstrate about the nature of electrons.
Electrons diffract and from and interference pattern. Electrons must behave as waves, as diffraction is wave behaviour.
Explain what is meant by vertically polarised
unpolarised light has oscillations in all planes. Vertically polarised light has oscillations in a vertical plane only.
Describe how infrasound travels through the air.
Oscillations of air particles occur parallel to direction of energy transfer, in a series of rarefactions and compressions.
Explain why there is a threshold frequency.
One photon is absorbed by one electron. Photon energy increases with frequency - E=hf. There is a minimum energy needed for emission of a electron. So emission of electrons only occur if frequency of light is greater than the threshold frequency.
Explain what is meant by a photon
Packet of electromagnetic radiation.
