Topic 5 - Wave & Particle nature of light

Question 1

Explain how standing waves are produced?

Answer 1

• Two waves travelling in opposite directions interfere
• Constructive interference if path difference nλ = antinodes (maximum amplitude)
• Destructive interference if path difference is (n + ½)λ or antiphase = nodes (minimum amplitude)

Question 2

What are coherent sources?

Answer 2

• Same wavelength
• Same frequency
• Fixed phase difference

Question 3

Explain how light waves travel

Answer 3

• Transverse electromagnetic wave
• Oscillates perpendicular to the direction of travel

Question 3

Explain how sound waves travel

Answer 3

• Longitudinal (series of compressions and rarefactions)
• Oscillated particles parallel to the direction of travel
• Molecules close together form high pressure

Question 4

Define Huygens’ construction

Answer 4

• As waves pass through slit they spread out
• Each point on the wavefront acts as a source of secondary waves (wavelets)
• Wavelets inerfere with each other

Question 5

What is a wavefront?

Answer 5

• Surface joining points of a wave that are in phase

Question 6

Explain why monochromatic light source is important in diffraction experiments

Answer 6

• Emits a very small range of frequencies
• So smaller variation at each diffraction angle
• Producing a sharper interference pattern

Question 7

Explain how a diffraction pattern occurs

Answer 7

• Waves pass through a narrow gap so spread out
• Wave reaches the surface (wall) with different path lengths
• Wavefront/bright fringe = constructive interference (superposition)
• Dark fringe = destructive interference (anti-phase)

Question 8

Explain how the absorption spectra is created

Answer 8

• Photon is absorbed by electron
• If the energy of photon is equal to the energy level difference is it excited to a higher energy level (frequency matches energy difference)
• The absorption spectra is created as the frequencies absorbed are missing from the continuous spectrum

Question 9

Explain how the emission spectra is created

Answer 9

• Photon is absorbed by electron
• If the energy of photon is equal to the energy level difference is it excited to a higher energy level (frequency matches energy difference)
• As the electron relaxes back down to the ground state, a photon is released (frequency determined by energy difference)
• Limited number of energy levels so only certain frequencies are emitted and observed on the spectrum

Question 10

Explain why light does not fit the wave model at all times

Answer 10

• If light was a wave, as the intensity increases, the energy transferred would increase and the electron would be emitted (no matter what the frequency)
  oThis is not the case as only certain (high) frequencies cause the electron to be emitted
• The energy of the emitted electrons would depend on the intensity
  o The intensity does not effect the ability for electrons to be emitted