5. Waves

Question 1

Model answer - Superposition

Answer 1

• Explain why there is more than one wave
• When the waves meet, superposition occurs
• Where they meet in-phase, constructive interference occurs creating max amplitudes
• This creates (fill in context to question)

Question 2

Model Answer - Single slit diffraction

Answer 2

ALL IN TERMS OF PATH DIFFERENCE

• Waves pass through narrow gap causing diffraction and waves spread out
• Light reaches wall from each part of the slit with differing path length and superposition occurs
• When waves meet and if their path difference is nλ, constructive interference occurs, creating bright region
• When waves meet and if their path difference is nλ/2, constructive interference occurs, creating dark region

Question 3

Model Answer - Standing waves

Answer 3

• Explain why there is more than one wave
• Two waves travelling in opposite direction and are coherent.
• When the waves meet, they superpose
• Where they meet in-phase, constructive interference occurs, creating antinodes with max amplitude,
• Where they meet anti-phase, destructive interference occurs creating nodes with 0 displacement
  (mention what effect each interference has)

Question 4

Model Answer - Absorption spectra

Answer 4

• Electrons exist in discrete energy levels
• When a photon’s energy is equal to difference between energy levels
• photon will be absorbed by electron
• As energy levels are discrete, only certain changes are possible.
• As E = hf only certain frequencies are absorbed.
• All other frequencies pass through forming a spectrum with dark lines corresponding to the absorbed frequencies

Question 5

Model Answer - Emissions spectra

Answer 5

• Electrons move to higher energy levels
• After some time they move back down to lower energy levels
• The energy from the move is given out as photon
• The energy of the photon is equal to difference in energy level
• As energy levels are discrete, only certain energy differences are possible
• As E=hf only certain frequencies are emitted

Question 6

Model Answer - Photoelectric effect

Answer 6

• Photons are discrete packets of EM energy
• One photon is absorbed by one electron
• If photon energy is greater/equal to work function, then the photon is absorbed and an electron is immediately emitted
• As E=hf, photon energy depends on frequency , therefore threshold frequency
• If light was a wave, energy would build up
• Any frequency would cause the emission of a photoelectron after enough time

Question 7

Definition - Unpolarised waves

Answer 7

• Oscillation of waves in many direction
• perpendicular to direction of propagation

Question 8

Definition - Polarised waves

Answer 8

• Oscillation of wave in only one direction
• perpendicular to direction of propagation

Question 9

What is meant by plane polarised light?

Answer 9

Polarised light is light where the oscillations are in a single plane

Question 10

Explain what the student observes as he gradually rotates the filter to 180° and then to 270°

Answer 10

• At 180, light is normal intensity…
• as when oscillations are parallel to filter, all light is transmitted
• At 270, screen is darkest
• as when oscillations are perpendicular to filter, all light is absorbed
• So as light filter is rotated from 180-270, there is gradual decrease in intensity
• as light is partially polarised between

Question 11

What is meant by unpolarised light?

Answer 11

Light waves oscillate in all directions perpendicular to propagation

Question 12

Model Answer - Rotating a polaroid filter in presence of polarised wave

Answer 12

• Mention that source is polarised and explain why
• Polarised wave oscillates in many directions perpendicular to the direction of propogation
• Polaroid filters only allow oscillations in one plane to pass through
• When the plane of polarisation of filter and plane of polarisation of waves are parallel, the wave is transmitted. (state effect)
• When the plane polarisation of filter and plane polarisation of waves are perpendicular, wave is absorbed (state effect)