Topic 5 - Waves and Particle Nature of Light

Question 1

How is visible light produced in a sodium gas discharge tube? (3 marks)

Answer 1

• Sodium atoms gain/absorb energy
• And electrons move to higher energy levels
• When sodium electrons drop to lower energy levels, they release photons

Question 2

How does a stationary wave form when a violin string is plucked? (3 marks)

Answer 2

• A wave on the string is reflected
• At the end/peg/bridge.
• Superposition/interference takes place.

Question 3

What are the similarities and differences between the stationary wave on a violin string and the sound waves transmitted through the air? (5 marks)

Answer 3

• Waves have the same frequency/period
• Waves have different speeds/wavelengths
• A sound wave has the same amplitude for all points, and a stationary wave does not
• Sound waves transfer energy, and stationary waves do not.
• Waves on a string are transverse, and sound waves are longitudinal.

Question 4

Why is Einstein’s particle model of electromagnetic radiation more successful than the wave model in explaining the photoelectric effect? (6 marks)

Answer 4

• There is a threshold frequency required to release electrons
• For waves, any frequency would be able to release electrons
• The release of electrons is instantaneous
• If the wave model were correct, energy would take time to build up before electrons were released
• The kinetic energy of released electrons is dependent on frequency
• If the wave model were correct, the kinetic energy of the released electrons would be dependent on the intensity.

Question 5

Explain the phenomenon observed by the student with the loudspeaker and metal plates with reference to interference. (6 marks)

Answer 5

• Waves diffract as they pass through the gaps
• At point X, the waves are in phase
• Leading to constructive interference/superposition
• When moving away from X, the path/phase difference changes
• Resulting in destructive interference/superposition when no sound is heard.
• This is where the waves are in antiphase.

Question 6

A student measures the stopping potential Vs for different light frequencies in a photoelectric effect experiment. When the ammeter reads zero, the maximum kinetic energy of emitted electrons is eVs. Explain how to determine Planck’s constant h using a graphical method. (5)

Answer 6

• hf = Φ + 1/2mv²ₘₐₓ
• Hence eVₛ = hf - Φ
• Compare with y - mx + c
• So plot a graph of Vₛ against f Or plot a graph of eVₛ against f
• Gradient = h/e Or gradient = h

Question 7

A student determined a lens focal length by measuring object distance u and image distance v for multiple positions. He claimed his initial estimate of f = 15 cm was confirmed by his graph. Comment on whether the student’s data is consistent with this focal length determination. (5)

Answer 7

• 1/f = 1/v + 1/u, re-arranged to make 1/v the subject
• Comparison with 𝑦 = 𝑚𝑥 + 𝑐
• So intercept equals 1/f
• Use the y intercept to calculate a value for f
• Comment on the agreement with the initial determination including an appropriate justification